Download Grey seal diet composition and prey

CSG 15
DEPARTMENT for ENVIRONMENT, FOOD and RURAL AFFAIRS
Research and Development
Final Project Report
(Not to be used for LINK projects)
Two hard copies of this form should be returned to:
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Project title
Grey Seal diet Composition and Fish Consumption in the North Sea
DEFRA project code
MF0319
Contractor organisation
and location
Sea Mammal Research Unit, Gatty Marine LaboratoryUniversity of St
Andrews, St Andrews, Fife KY16 8LB
Total DEFRA project costs
£ 201,111.00
01/07/01
Project start date
Project end date
31/05/05
Executive summary (maximum 2 sides A4)
Grey seal diet composition and prey consumption in the North Sea
P.S. Hammond & K. Grellier
Sea Mammal Research Unit, Gatty Marine Laboratory
University of St Andrews, St Andrews, Fife, KY16 8LB, Scotland, UK
Background and objectives
Essential information for assessing the impact of grey seals on fisheries includes which species of fish are taken and how
much is consumed. Direct observation of seal diet is not possible so reliable information has previously come from the
analysis of hard prey remains recovered from scats collected at haul-out sites. However, for most areas, the most recent
information available dates back to 1985. Since then, there have been declines in stocks of most commercially exploited
fish species that are major grey seal prey items in the North Sea, and the grey seal population has more than trebled.
Updated estimates of grey seal diet are needed to inform policy related to the impact of the still increasing grey seal
population on declining fish stocks. As part of this work, some aspects of the methodology required improvement.
The objectives of this project were:
1. To obtain improved estimates of digestion coefficients and rates of complete digestion for otoliths of major prey of
grey seals.
2. To sample grey seal diet seasonally for one year at all major haul-outs in the North Sea.
3. To determine grey seal diet composition and estimate consumption of commercial fish species by grey seals in the
North Sea in 2002.
4. To investigate seasonal and regional variation in grey seal diet in the North Sea in 2002.
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Project
title
Grey Seal diet Composition and Fish Consumption in the
North Sea
DEFRA
project code
MF0319
5. To relate changes in grey seal diet composition and consumption between 1985 and 2002 to changes in the abundance
of fish prey.
Methods
Diet composition and prey consumption were estimated using scat sampling methods. Scats were collected on a monthly
or quarterly basis throughout 2002 along Britain’s North Sea coast. Fish otoliths and cephalopod beaks recovered from
scats were identified and measured. Each otolith was graded for the amount of digestion.
At the Gatty Marine Laboratory captive seal facility, initial trials to determine experimental protocols were conducted.
Then, 86 feeding trials with seven grey seals and 18 prey species were conducted to derive estimates of species- and
grade-specific digestion coefficients (to account for partial digestion) and recovery rates (to account for complete
digestion).
The methods used to estimate diet composition and prey consumption by grey seals broadly followed those used in
previous analyses of grey seal diet by the Sea Mammal Research Unit. For diet composition, measurements of fish
otoliths and cephalopod beaks recovered from scats were used to estimate the weight of prey associated with each
structure, which were summed over species and expressed as percentages in the diet by weight. For consumptions, the
amount of prey in the scat samples was converted to energy, equated to estimated energy requirements for the population
in the region, converted back to weight, and expressed as tonnes consumed per annum. These methods were used to
reanalyse the data from 1985. Estimates of diet were then compared with estimates of stock biomass for 1985 and 2002
from ICES assessments.
Findings of the research
The experiments resulted in revised estimates of digestion coefficients to account for partial digestion of otoliths as they
pass through the gut of a seal and the first estimates of recovery rates to account for complete digestion of otoliths by grey
seals. These data significantly improve the analyses and afford high confidence in the reliability of the results.
Comprehensive coverage regionally and seasonally was achieved in the collection of scats in 2002. This adds to our
confidence in the reliability of the results and allows a direct comparison with the reanalysed data from 1985.
We found marked changes in grey seal diet composition between 1985 and 2002. The core species (sandeels, cod and
other gadoids) were similar in both time periods, but the proportions they contributed were different both regionally and
seasonally. At Donna Nook, benthic prey (dragonet and seascorpions) were more important and sandeel less important in
2002 than in 1985. Much less cod and much more whiting were consumed in 2002 compared with 1985. In the East Coast
region, the general changes were less pronounced; the percentage of gadoids in the diet was lower and the percentage of
sandeel was higher in 2002 compared with 1983-88. Within the gadoids, however, the percentage of cod in the diet
overall declined almost 5-fold, and the percentage of haddock increased by an order of magnitude. In Orkney, the overall
change in diet between 1985 and 2002 was dominated by an increase in the percentage of gadoids and a decrease in the
percentage of sandeel. There was a particularly large increase in the percentages of cod and haddock taken in the first
quarter of the year.
Estimates of annual consumption of commercially important fish prey by grey seals increased markedly from 39,000
tonnes in 1985 to 116,000 tonnes in 2002, in line with the increase in population size. The estimated amount of sandeel
consumed increased from 29,000 t in 1985 to 69,000 t in 2002, and estimated consumption of cod increased from 4,100 t
to 8,300 t.
Per capita prey consumption was 4.7 kg.d-1 (1.72 tonnes.yr-1). Consumption per seal decreased between 1985 and 2002
for cod (by ~30%) and sandeel (by ~15%), remained about the same for whiting, approximately trebled for plaice, and
approximately quadrupled for haddock.
Grey seal predation was not significant in 1985; estimated prey consumption was less than 1% of estimated stock size for
all species. In 2002, consumptions relative to stock size of most prey species were several times higher but only for cod
(3.7%) sandeel (2.7 %) and plaice (1.5 %) were the percentages greater than 1%. These relative changes between 1985
and 2002 are caused by a combination of three factors: an overall increased consumption of prey by grey seals (driven by
an almost threefold increase in seal numbers); changes in diet composition; and declines in most assessed fish stocks. We
conclude that grey seal predation on commercially exploited fish stocks in ICES Sub-Area IV was very much higher in
2002 than in 1985.
The results show that sandeel, cod, other gadoids and plaice are the most important prey of grey seals in the North Sea.
Sandeel continue to be consumed in large quantities. The amount of cod consumed per seal declined slightly between
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Project
title
Grey Seal diet Composition and Fish Consumption in the
North Sea
DEFRA
project code
MF0319
1985 and 2002 but the stock declined much more. The amounts of haddock and plaice consumed per seal increased
markedly between 1985 and 2002 in the face of stock declines.
The question of the impact that grey seals may have on fish stocks and, therefore, fish catches is an important one in light
of the results presented here. Might grey seals limit the ability of cod, especially, and other gadoid stocks to recover in the
North Sea? Alternatively, might declines in fish stocks impact grey seal population growth? We are unable to answer
these questions but, to help address them, a model of grey seal interactions with their prey would be a useful tool. Defra
has previously supported the development of such models; they should be further developed and parameterised with
results from this project to address this and other management-related questions.
Grey seal diet should be reassessed in the relatively near future. In addition, the diet of the large harbour seal populations
in these areas should be systematically assessed as a priority.
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title
Grey Seal diet Composition and Fish Consumption in the
North Sea
DEFRA
project code
MF0319
Scientific report (maximum 20 sides A4)
Grey seal diet composition and prey consumption in the North Sea
P.S. Hammond & K. Grellier
Sea Mammal Research Unit, Gatty Marine Laboratory
University of St Andrews, St Andrews, Fife, KY16 8LB, Scotland, UK
1. Introduction
Essential information for assessing the impact of grey seals (Halichoerus grypus) on fisheries includes which species of
fish are taken and how much is consumed. Direct observation of seal diet is not possible so reliable information has
previously come from the analysis of hard prey remains recovered from scats collected at haul-out sites (Prime &
Hammond 1987). However, for most areas, the most recent information available dates back to 1985 (Prime & Hammond
1990; Hammond & Prime 1990; Hammond et al. 1994a,b). Since then, the grey seal population has almost trebled in the
North Sea (http://www.smru.st-and.ac.uk/CurrentResearch.htm/scos.htm) and there have been declines in the stocks of
most commercially exploited fish species that are major prey of grey seals in this area.
Before this project, it was not possible to give up-to-date information on what or how much fish grey seals are eating.
Work on developing a predictive model to assess interactions between grey seals and fish stocks was funded by Defra
under projects MF0309, MF0311 and MF0320. Up-to-date diet information was needed to maximise the value of this
work.
In this project we reassessed grey seal diet in the North Sea through a comprehensive programme of sample collection in
calendar year 2002 and a detailed analysis of the data. The focus was on estimating diet composition expressed as
percentage of each species in the diet by weight, and on estimating the amount of each species consumed in a calendar
year. Previous work has shown substantial variation in diet regionally and seasonally, so scats were collected and
estimates of diet composition and consumption made for each quarter of 2002 in each region. For the first time, data are
available for Shetland; the collection of samples from Shetland was funded by SEERAD/SNH under a parallel project.
The coefficients used to account for partial digestion of fish otoliths and cephalopod beaks as they pass through the gut of
grey seals required revision. In this project, we updated digestion coefficients for the major prey species by conducting an
extensive set of feeding experiments at the Gatty Marine Laboratory. In addition, and most importantly, we estimated
species-specific rates of complete digestion for the first time.
The value of the results for 2002 is greatly enhanced by comparing them to estimates of grey seal diet from data collected
in 1985. We compare our results for 2002 with estimates of diet composition and prey consumption from a complete
reanalysis of the 1985 data, using the new experimental data. These results are then put in the context of changes in fish
stocks by comparing them with estimates of stock sizes from ICES assessments for the North Sea (ICES Sub-Area IV) in
1985 and 2002.
2. Scientific objectives as set out in the proposal
1. To obtain improved estimates of digestion coefficients and rates of complete digestion for otoliths of major prey of
grey seals.
2. To sample grey seal diet seasonally for one year at all major haul-outs in the North Sea.
3. To determine grey seal diet composition and estimate consumption of commercial fish species by grey seals in the
North Sea in 2002.
4. To investigate seasonal and regional variation in grey seal diet in the North Sea in 2002.
5. To relate changes in grey seal diet composition and consumption between 1985 and 2002 to changes in the abundance
of fish prey.
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Project
title
Grey Seal diet Composition and Fish Consumption in the
North Sea
DEFRA
project code
MF0319
3. Materials and Methods
3.1 Data collection
Scat sampling was carried out on a monthly or quarterly basis throughout 2002 along Britain’s North Sea coast, including
Orkney and Shetland (Fig. 1). Grey seal haul-out sites were visited either on foot, by boat or by helicopter within 3 hours
of low water. Fresh scat samples produced by individual grey seals were collected, placed in separate plastic bags and
stored at -20C.
Scat sampling in Shetland was funded by a separate contract with SEERAD/SNH. The data are included here to give a
complete picture of the North Sea.
3.2 Laboratory analysis
Scat samples were defrosted and hard parts (fish otoliths and cephalopod beaks) were extracted using a nest of sieves of
mesh sizes 1 mm, 600 m and 335 m, running water and a soft brush, taking care not to break any hard parts. Otoliths
were stored dry; beaks were stored in 70 % ethanol to prevent them from drying out. Otoliths and beaks recovered from
scats were identified to species using a reference collection and two identification guides (Härkönen 1986; Leopold et al.
2001). Where they could not be identified to species with 100 % certainty, they were recorded at order level (e.g. as
“unidentified gadoid”) or as “unknown species”.
Otolith lengths and widths were measured to the nearest 0.01 mm using digital callipers (Mitutoyo) under a binocular
microscope (PZO MST130). The callipers were zeroed between measurements and were frequently cleaned. Either lower
rostral or lower hood length measurements were taken from cephalopod beaks. Small cephalopod beaks were
photographed using a microscope (Zeiss Axioskop 2 plus) and digital camera (Zeiss AxioCam) and measured to the
nearest 0.01 mm using image measurement software (AxioVision 3.1). Large cephalopod beaks were measured using a
binocular microscope equipped with an eyepiece graticule. Broken otoliths and beaks were counted and measured only if
the widest/longest part of the otolith, or the lower rostral length of the beak, was complete.
For scats containing more than 30 otoliths or beaks of the same species, a random sub-sample was measured and
calculations from these extrapolated to the total number in the sample. For scats containing more than 30 but fewer than
120 otoliths or beaks, 30 were measured. For scats containing more than 120, 25 % were measured. Each recovered
otolith was also examined to assess and record the amount by which it had been digested, which was classified based on
its external morphological features. Pristine otoliths were classified as grade 1, moderately digested otoliths as grade 2
and considerably digested otoliths as grade 3 after Leopold et al. (2001). The amount by which beaks had been digested
was not classified.
3.3 Experimental estimation of digestion correction factors
Application of digestion correction factors to measurements and counts of fish otoliths and cephalopod beaks recovered
from seal scats is required to estimate accurately the size and quantity of prey consumed. To obtain robust estimates of
these digestion correction factors we carried out extensive digestion experiments with captive grey seals at the Gatty
Marine Laboratory captive seal facility. We first conducted a set of six preliminary experiments with two seals and
concluded that the best methodology was to feed whole fish of known size as single species meals to seals housed
individually in a purpose-built enclosure (for details see Grellier and Hammond 2005, Annex 1). Following this protocol,
we then carried out 86 feeding trials with seven grey seals and 18 prey species to derive estimates of digestion
coefficients (to account for partial digestion) and recovery rates (to account for complete digestion) (for details see
Grellier and Hammond 2006, Annex 2).
Species- and grade-specific digestion coefficients were calculated (as the ratio of mean undigested otolith or beak size to
mean digested otolith or beak size) for each seal for each trial for both length and width (otoliths) and for lower rostral
length (beaks). The variance of each digestion coefficient was also calculated. The digestion coefficients and variances
from each trial were combined to give single values for each prey species.
To account for otoliths or beaks that are completely digested, a recovery rate was calculated as the proportion of otoliths
eaten that was recovered at the end of each feeding trial. A value of 1 thus means that all otoliths eaten were recovered, a
value of 0 that no otoliths were recovered. The theoretical variance of recovery rate was calculated. Recovery rates and
variances from each trial were combined to give single values for each prey species.
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Project
title
Grey Seal diet Composition and Fish Consumption in the
North Sea
DEFRA
project code
MF0319
3.4 Estimation of diet composition and prey consumption
The methods used to estimate diet composition and amounts of fish consumed by grey seals broadly followed those used
in previous analyses of seal diet by the Sea Mammal Research Unit (Prime & Hammond 1987; Prime & Hammond 1990;
Hammond & Prime 1990; Hammond et al. 1994a; Hammond et al. 1994b; Hammond & Fedak 1994; Hammond &
Rothery 1996, Hall et al. 1998). For diet composition, the measurements of fish otoliths and cephalopod beaks recovered
from scats were used to estimate the weight of prey associated with each structure, which were summed over species and
expressed as percentages in the diet by weight. For consumptions, the amount of prey in the scat samples was converted
to energy, equated to estimated energy requirements for the population in the area, converted back to weight and
expressed as tonnes consumed per annum. All calculations were made using purpose-written Fortran programs.
3.4.1 Diet composition
For each prey species (or higher taxon) represented in the data, a decision was made on which otolith or beak
measurement to use in calculations, based on: the availability of experimental data; the accuracy and precision of the
estimated experimental digestion coefficients (Grellier & Hammond 2006); and the availability of measurements from
recovered hard parts. The decision for otoliths was one of (a) length preferred, (b) width preferred, (c) length only, or (d)
width only. For cephalopods it was either (a) lower rostral length only, or (b) lower hood length only.
Measurements of partially digested otolith/beak size were then converted to estimates of undigested otolith/beak size
using newly obtained experimentally derived species-specific (for 1985) and grade-specific (for 2002) digestion
coefficients (Grellier & Hammond 2006). For many species, there were insufficient experimental data to obtain gradespecific digestion coefficients for grades 1 and 2. In these cases, we used a digestion coefficient of 1.0 for grade 1
otoliths, and grade 2 digestion coefficients estimated for groups of species (gadoids, flatfish, etc). For species for which
no experimental data were available, either group-specific values (gadoids, flatfish, etc) or the values for the closest
matching species were used. In the latter cases, the prey species were always minor contributors to the diet.
Estimates of undigested otolith/beak size were then converted to estimates of fish/cephalopod weight using derived
allometric equations from the literature. We used the equations from Leopold et al. (2001) where available, and data from
other sources (Brown & Pierce 1998, Clarke 1986; Härkönen 1986; Santos et al. 2001) for the approximately 10% of
cases where they were not. For prey species for which no equations were available we used equations for the closest
matching species. These latter species were all minor prey. For unidentified gadoid otoliths, the relationship between
otolith size and fish weight for cod was used. For unidentified flatfish, the relationship for plaice was used. Analysis using
alternative relationships showed that the results were insensitive to these choices.
In scats where only a sub-sample of the otoliths identified for a species had been measured (usually for sandeel but
occasionally for other species), the fish weight represented by each unmeasured otolith was assumed equal to the mean
weight of all measured otoliths of that species in that scat. This was also done for broken otoliths without an appropriate
measurement. If there were no measured otoliths of that species in that scat, the mean fish weight of that species over all
scats was used.
For cephalopods, because only one part of the beak was measured (but each fish has two otoliths), estimated weights
were doubled.
The weights estimated for each species in the group of scats being analysed were then adjusted by the new experimentally
derived species-specific recovery rates (see above; Grellier & Hammond 2006) to account for species-specific differences
in rates of complete digestion. For species for which no experimental data were available, either group-specific (e.g.
gadoids, flatfish) values or the values for the closest matching species were used.
3.4.2 Prey consumption
In each region and for each season (quarter), estimated weights for each species were converted to units of energy using
energy density values of fish from the literature (Murray & Burt, 1977) and species-specific energy values were expressed
as a proportion of the total energy represented by the group of scats being analysed. This proportion was then multiplied
by the estimated number of seals in the region (see below), the average daily energy requirement for a grey seal (Sparling
& Smout 2003) and the number of days in the season represented to give estimates of the energy consumed of each prey
species by the population of seals in the region. These values were then divided by the species-specific energy density
values to give estimates of weight consumed. The estimated value of average grey seal daily energy requirement
calculated by Sparling & Smout (2003) of 5,497 Kcals was almost identical to the value of 5,530 Kcals previously used
from Fedak & Hiby (1985).
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Project
title
Grey Seal diet Composition and Fish Consumption in the
North Sea
DEFRA
project code
MF0319
We had no data for quarter 2 in Shetland, so we assumed the amount of prey consumed in this quarter was equal to the
average of the two adjacent quarters.
Estimates of consumption of each prey species were summed across quarters and regions to give values for the whole of
the North Sea appropriate for comparison with estimates of prey stock sizes from ICES assessments of Sub-Area IV.
3.4.2.1 Grey seal population size
Estimates of the number of grey seals in each region in 2002 were derived from new population estimates calculated by
Thomas & Harwood (2005), who fitted a range of models to the data on grey seal pup production to generate regional
estimates of population size. Models allowed density dependent population growth through either decreasing juvenile
survival or decreasing fecundity with increasing population size. For each density dependent mechanism, two models
were fitted: one in which the rate of decline in survival or fecundity was a simple linear function of population size, and a
second extended model incorporating an additional parameter to allow this rate to be non-linear. Results showed that all
the models received approximately equal support from the data, but that the resulting population estimates varied widely.
There is thus considerable model uncertainty in current estimates of grey seal population size.
In our calculations we used population estimates from the simple linear density dependent survival model, which were
considered by the NERC Special Committee on Seals (SCOS) more likely to be a reflection of true population size; these
are given in Table 1. Because of the model uncertainty, we also explored the effect of using population estimates from the
extended non-linear density dependent survival model on estimates of prey consumption; these estimates are therefore
also given in Table 1. A discussion of the effect of this uncertainty is given in section 5.1.3 below.
The Thomas & Harwood (2005) estimates are based on data from regularly monitored pupping sites only. To these
estimates we added estimates of the number of seals associated with sites that are not monitored regularly based on the
most recent pup count multiplied by the ratio of estimated population size to pup count for the appropriate region from
Thomas & Harwood (2005). Table 1 gives the estimates of population size used for each region.
Note that the estimates in Table 1 are calculated from pup counts from surveys in quarter 4 each year. We assumed that
they applied to all four quarters of the year in each region. There are no systematic data on the distribution of grey seals
outwith the pupping season in the North Sea that can be used to assess the robustness of this assumption. However, there
are data on counts of grey seals hauled out ashore made during surveys of harbour seals in August for the Inner and Outer
Hebrides (http://www.smru.st-and.ac.uk/CurrentResearch.htm/scos.htm) and we did a simple analysis to investigate the
sensitivity of estimates of consumption in that region to this assumption (see Hammond & Harris 2006). We assumed that
the total number of grey seals in the Hebrides region was distributed in quarters 1-3 according to the proportions observed
in the August surveys. Resulting estimates of consumption of the main prey species were very similar to those assuming
seals were distributed throughout the year as in quarter 4. We do not know that the same will be true in the North Sea but
we assume it will and that our assumption is not an important source of bias.
3.4.3 Estimates of variability
Variances of estimates of diet composition and consumption were estimated using the method described by Hammond &
Rothery (1996). There are two elements to this: (a) sampling error estimated using non-parametric bootstrap resampling
with a scat as the sampling unit; and (b) “measurement error” estimated using parametric resampling of the coefficients
describing the relationships used to obtain estimates of fish/cephalopod consumption from otolith/beak measurements.
Measurement error includes variability associated with (a) estimating undigested otolith/beak size from partially digested
measurements via species- or grade-specific digestion coefficients; (b) estimating fish/cephalopod weight from estimated
undigested otolith/beak size via species-specific allometric relationships; (c) accounting for complete digestion of
otoliths/beaks using estimated recovery rates; and (d) estimating consumption using estimated daily energy requirements
and population size of grey seals.
Estimates of the variability associated with experimentally derived estimates of digestion coefficients and recovery rates
were taken from Grellier & Hammond (2006). Estimates of variability associated with otolith size - fish weight
relationships were taken from Leopold et al. (2001). Estimates of the variability associated with seal population estimates
(Thomas & Harwood 2005) were obtained from Len Thomas (pers. comm.). We assumed the estimate of energy
requirement had a coefficient of variation of 10%.
For estimates of diet composition and prey consumption within each region/season, 95% confidence limits were estimated
as the 2.5%-ile and 97.5%-ile of the bootstrapped distributions. To estimate confidence limits of prey consumption
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Project
title
Grey Seal diet Composition and Fish Consumption in the
North Sea
DEFRA
project code
MF0319
summed across seasons and/or regions, bootstrapped estimates of consumption from each replicate were summed and the
percentiles taken from the distribution of summed values.
3.4.4 Length of consumed fish
Equations relating fish length to otolith size from Leopold et al. (2001) were used to generate frequency distributions of
estimated fish length for the major species in the diet.
3.5 Reanalysis of 1985 data
Data collected in 1985 in Orkney, the Farne Islands and Isle of May, and Donna Nook were reanalysed with the same
methods as used for the 2002 data. This was important because differences in results from those generated by previous
analyses would be expected because of the use of the new experimental data on digestion and new relationships between
otolith size and fish weight (as described above in section 3.4.1; Grellier & Hammond 2006). In particular, the application
of species-specific recovery rates to prey remains was expected to make a substantial difference to previous results. For
the east coast sites of Farne Islands and Isle of May, data were available for 1983 to 1988. We used all these data, instead
of just the 1985 data, because sampling in 1985 had poor seasonal coverage and low numbers of scats.
In previous analyses of the 1985 data, the otolith measurement of choice was thickness but, except for sandeel, otolith
length and width were also measured where possible. Thus, for species other than sandeel, data were available in most
cases to apply the new experimental relationships based on otolith length and/or width (see above; Grellier & Hammond
2006). For otoliths for which only thickness had been measured, we estimated length and width from length-thickness or
width-thickness relationships fitted to the available data on partially digested otoliths.
In 1985, otoliths were not graded for the amount of digestion so species-specific digestion coefficients were applied in all
cases. We cannot know how different the 1985 results would be if these otoliths had been graded. However, we did
investigate the sensitivity of estimates of diet composition in Orkney in 2002 to using species-specific rather than gradespecific digestion coefficients. Overall, the relative amount of sandeel in the diet increased by around 4% and the relative
amount of cod decreased by around 5% when species-specific digestion coefficients were used. The amounts of other
gadoid prey also decreased, except for saithe, which increased. The relative contribution of flatfish also generally
decreased. Overall then, we would expect only slight differences in the results for 1985 if we had been able to use gradespecific digestion coefficients; less sandeel and more of most of the other main prey species.
Other than these specific points, the 1985 data were analysed in exactly the same way as the 2002 data.
4. Results
4.1 Estimates of digestion coefficients and rates of complete digestion
Eighty-six feeding trials with seven adult female grey seals and 18 prey species were successfully completed. In total,
13,845 otoliths and beaks were eaten by the seals during the feeding trials; 9,650 (69.7 %) of these otoliths and beaks
were recovered from scats. Results are described in detail in Grellier & Hammond 2006).
4.1.1 Recovery rates (rates of complete digestion)
Rates of complete digestion, or recovery rates, are summarised in Table 2. Recovery rates were greatest for squid beaks
(0.942) and then large gadoid (0.935), Trisopterus spp. (0.920), flatfish (0.806) and sandeel (0.350) otoliths. Recovery
rates of otoliths of the benthic species (dragonet and short-spined seascorpion) were low (0.306 and 0.356, respectively)
while those of the pelagic species (herring and mackerel) were more variable (0.349 and 0.719 respectively).
Mean recovery rates for each prey species/type are not equivalent to the total number of otoliths or beaks recovered
divided by the total number eaten because, rather than pooling the data, recovery rates and variances from each trial were
sequentially combined to give single values for each seal for each prey species/size combination, each prey species/size
combination, each prey species and, finally, single values for each prey type (all Trisopterus, all large gadoids, all flatfish
and flounder-plaice).
4.1.2 Digestion coefficients (rates of partial digestion)
Digestion coefficients calculated using otolith length (OL) were different to those calculated using otolith width (OW),
but the pattern of values was similar.
Digestion coefficients varied among prey species (Table 3). Mean gadoid and sandeel digestion coefficients were
relatively large (1.49 and 1.56 for OL, 1.32 and 1.65 for OW, respectively) compared to those for flatfish (1.25 for OL,
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Project
title
Grey Seal diet Composition and Fish Consumption in the
North Sea
DEFRA
project code
MF0319
1.31 for OW) and Trisopterus spp. (1.21 for OL, 1.18 for OW). Squid beaks were relatively undigested (mean lower
rostral length digestion coefficient 1.02).
Of the 9,650 otoliths recovered from feeding trials, 0.8 % were assigned a grade 1 (pristine), 8.5 % a grade 2 (moderately
digested) and 90.7 % a grade 3 (considerably digested).
Grade-specific digestion coefficients are shown in Table 4. Because relatively few grade 1 and grade 2 otoliths were
recovered from feeding trials, it was only possible to calculate digestion coefficients for all three grades for one prey
species, herring. Two grade-specific digestion coefficients were calculated for six prey species, and a single gradespecific coefficient for five species (six species when using otolith width). For three species, measurements from grade 1
and grade 2 otoliths were pooled, and a “non-grade 3” digestion coefficient was calculated in addition to one for grade 3.
The large differences in partial and complete digestion rates found among prey species reinforce the importance of
obtaining robust estimates of these quantities.
4.2 Diet sampling in 2002
Grey seal diet was sampled seasonally for one year (2002) at all major haul-outs in the North Sea. Figure 1 shows the
locations of haul-out sites that were visited; there were three sites in Shetland, 17 in Orkney, three in the Moray Firth and
the others are listed individually. In total, 1,740 scats containing hard parts were processed and 107,613 otoliths and beaks
were recovered (Table 5). The greatest number of scats was collected in Orkney (n = 711) while the best seasonal
coverage was obtained in the Donna Nook and East Coast regions. The number of scats collected was smaller in quarters
2 and 3 in Orkney and Shetland. This was not effort-related but primarily due to the behaviour of the seals, which tended
to be hauled out at the water’s edge, partially in the water, at these sites at these times of year. The only region/season not
sampled was Shetland in Quarter 2, when bad weather prevented access.
The number of otoliths of the main prey recovered from scats is detailed by region and season in Table 6, and summarised
in Table 7. The prey species listed are those which contributed >5 % of the diet by weight in any region or season, and
those for which ICES stock assessment data were available. Sandeel otoliths were the most common hard parts recovered,
followed by whiting, dragonet, short-spined seascorpion and haddock otoliths.
4.3 Diet composition in 2002
Seasonal variation in grey seal diet in the North Sea in 2002 (expressed as the percentage of each species in the diet, by
weight) is given for each species in each region and season in Table 8 and summarised by prey type (e.g. gadoids, flatfish
etc.) in Table 9.
At Donna Nook, benthic prey (mainly dragonets and seascorpions) contributed most to the diet in all quarters (from 36 %
in quarter 4 to 73 % in quarter 3). Gadoids (primarily cod and whiting) were also important and contributed between 9 %
in quarter 3 and 31 % in quarter 4. Sandeel contributed a maximum of 32 % in quarter 2 but much less than this in other
quarters. Flatfish (primarily sole and plaice) contributed between 7 and 16 % in each quarter.
In the East Coast region, in contrast, sandeel was the most important prey type found, contributing more than half the diet
in each quarter. Gadoids (mainly cod and haddock) were also important, contributing 12-15% in quarters 2 and 3 and 2835 % in quarters 1 and 4.
In Orkney, the diet was again composed of more than 50% sandeel throughout the year and gadoids were the next most
common prey, especially in quarter 1.
In Shetland, the diet was greatly dominated by sandeel, especially in quarters 1 and 3. Gadoids made up most of the rest
of the diet, especially in quarter 4.
4.4 Prey consumption in 2002
Table 10 gives the estimates of prey consumption by grey seals by region, season and species for 2002. The pattern of
relative consumption of different species follows that of their percentage contribution to the diet by weight in each region
and quarter, but in regions with more seals more fish is consumed. Estimated prey consumption is dominated by the
figures for the Orkney region because this region contained 80 % of the North Sea grey seal population in 2002.
Estimated consumption of the key prey species (those making a significant contribution to the diet in at least one quarter
in a region, and those for which ICES stock assessments are available) is given for the whole North Sea for 2002 in Table
11. Of the commercially important species, the main consumption was of sandeel (69,000 t), cod (8,300 t), haddock
(6,500 t), plaice (5,200 t), whiting (2,500 t) and saithe (2,000 t). Grey seals also consumed significant quantities of shortCSG 15 (1/00)
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Grey Seal diet Composition and Fish Consumption in the
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MF0319
spined seascorpion (6,000 t) and dragonet (1,900 t). Overall, grey seals were estimated to have consumed 116,000 tonnes
(95% CI: 100,00-142,000 tonnes) of prey in the North Sea in 2002.
If the seal population estimates from the extended non-linear density dependent survival model are used (Table 1), the
estimates of prey consumption are approximately 40% larger.
4.4.1 Size of prey consumed by seals
Estimated length distributions of the main prey species consumed in 2002 are given in Figure 2. Because these lengths are
estimated, the tails of the distributions are likely a result of the associated error and these should not be over-interpreted.
The distributions show that grey seals mostly consumed fish less than about 30 cm in length. For sandeel (Fig 2a), 84%
were less than 20 cm (mean = 16.5 cm). For plaice (Fig 2b), 72% were less than 25 cm (mean = 19.0 cm). For the
gadoids, 86% of cod (Fig 2c) were less than 50 cm (mean = 35.4 cm), 68% of whiting (Fig 2d) were less than 24 cm
(mean = 21.0 cm), and 96% of haddock (Fig 2e) were less than 40 cm (mean = 30.3 cm). Analysis of the age and maturity
of fish at these lengths remains to be undertaken but a cursory inspection indicates that most of the cod and plaice would
have been immature fish, but most of the sandeel and a considerable proportion of the whiting and haddock would have
been mature.
4.5 Reanalysis of grey seal diet in 1985
Grey seal diet was previously sampled seasonally for one year in 1985 at all major haul-outs in the North Sea, except
Shetland. The locations were Donna Nook, East Coast (Farne Islands and Isle of May) and Orkney (Figure 1). Because
coverage was poor and the number of scats collected small in 1985 in the East Coast region, data were combined from
collections in 1983-88 and assumed to be representative of 1985. In total, 1,455 scats containing hard parts were
processed and 90,934 otoliths and beaks were recovered (Table 12). The greatest number of scats was collected in Orkney
(n = 859) while the best seasonal coverage was obtained at Donna Nook. As in 2002, the number of scats collected was
relatively small in quarters 2 and 3 in Orkney. The only region/season not sampled was quarter 3 on the East Coast.
The number of otoliths of the main prey recovered from scats is detailed by region and season in Table 13, and
summarised in Table 14. The prey species listed are those which contributed >5 % of the diet by weight in any region or
season, and those for which ICES stock assessment data were available. Sandeel otoliths were the most common hard
parts recovered, followed by whiting and cod otoliths.
4.5.1 Diet composition in 1985
Seasonal variation in grey seal diet in the North Sea in 1985 (1983-88 for the East Coast region), expressed as the
percentage of each species in the diet, by weight, is given for each species in each region and season in Table 15 and
summarised by prey type (e.g. gadoids, flatfish etc.) in Table 16.
At Donna Nook, sandeel was the main species in the diet but there was significant seasonal variation. In quarters 1 and 3,
sandeel dominated but in quarter 2, flatfish (primarily sole) were the main prey and in quarter 4, gadoids (mainly cod) and
benthic prey (mainly short-spined seascorpion) were equally as important as sandeel. In the East Coast region, the diet
was dominated by cod and sandeel. Cod dominated in quarter 1, sandeel in quarter 4. Benthic prey contributed
significantly in quarter 1. In Orkney, the diet was strongly dominated by sandeel. Cod was the main gadoid species.
The 1985 results presented here differ from those published previously (Hammond & Prime 1990; Prime & Hammond
1990; Hammond et al. 1994a). This is because (a) the data were re-analysed using the new species- and grade-specific
digestion coefficients and recovery rates for fish otoliths and cephalopod beaks from the digestion experiments carried out
as part of this contract, and (b) new allometric equations were used to estimate fish weight from otolith size (Leopold et
al. 2001). The greatest effect is from the use of recovery rates to account for species-specific differences in rates of
complete digestion of otoliths. These new estimates for 1985 should be considered as the best estimates for 1985 and the
previous results disregarded.
4.5.2 Prey consumption in 1985
Table 17 gives the estimates of prey consumption by grey seals by region, season and species for 1985 (1983-88 for the
east coast region). The pattern of relative consumption of different species follows that of their percentage contribution to
the diet by weight in each region and quarter, but in regions with more seals more fish is consumed. Estimated prey
consumption is dominated by the figures for Orkney because this region contained 80 % of the North Sea grey seal
population in 1985, excluding Shetland (for which there are no diet data in 1985).
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Grey Seal diet Composition and Fish Consumption in the
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MF0319
Consumption of the key prey species (those making a significant contribution to the diet in at least one quarter in a region,
and those for which ICES stock assessments are available) is given for the whole North Sea for 1985 in Table 18. Of the
commercially important species, the main consumption was of sandeel (29,000 t) and cod (4,150 t). Relatively small
amounts of other species were taken. Overall, grey seals were estimated to have consumed 38,700 tonnes (95% CI:
34,400-44,400 tonnes) of prey in 1985.
If the seal population estimates from the non-linear density dependent survival model are used from Table 1, the estimates
of prey consumption are approximately 25% larger.
4.5.3 Size of prey consumed by seals
Estimated length distributions of the main prey species consumed in 1985 are given in Figure 3. As before, because these
lengths are estimated, the tails of the distributions are likely a result of the associated error and these should not be overinterpreted.
For sandeel (Fig 3a), 89% were less than 24 cm (mean = 19.7 cm). For plaice (Fig 3b), >72% were less than 25 cm (mean
= 21.0 cm). For the gadoids, 92% of cod (Fig 3c) were less than 50 cm (mean = 37.1 cm), 83% of whiting (Fig 3d) were
less than 24 cm (mean = 19.3 cm), and 89% of haddock (Fig 3e) were less than 40 cm (mean = 30.5 cm). As for 2002
data, analysis of the age and maturity of fish at these lengths remains to be undertaken but a crude inspection indicates
that most of the cod, whiting and plaice would have been immature fish, but most of the sandeel and a considerable
proportion of the haddock would have been mature.
4.6 Comparison of diet in 1985 and 2002
4.6.1 Diet composition
We found marked changes in grey seal diet composition in 1985 and 2002 (Tables 8 and 9, compared with Tables 15 and
16). The core species (sandeels, cod and other gadoids) were similar in both time periods, but the proportions they
contributed were different both regionally and seasonally.
At Donna Nook, benthic prey (dragonet and seascorpions) were more important and sandeel less important in 2002 than
in 1985. The proportion of gadoids in the diet was about the same in each year, but much less cod and much more whiting
were consumed in 2002 compared with 1985. Less flatfish were consumed in 2002 than in 1985, primarily as a result of
fewer sole being consumed in Quarter 2.
In the East Coast region, the general changes were less pronounced; the percentage of gadoids in the diet was lower and
the percentage of sandeel was higher in 2002 compared with 1983-88. Within the gadoids, however, the percentage of cod
in the diet overall declined almost 5-fold, and the percentage of haddock increased by an order of magnitude. The main
seasonal difference was in the first quarter of the year when sandeel dominated the diet in 2002 but were almost absent in
1983-88.
In Orkney, the overall change in diet between 1985 and 2002 was dominated by an increase in the percentage of gadoids
taken (~20% in 2002 vs ~10% in 1985) and a decrease in the percentage of sandeel taken (~60% in 2002 vs ~80% in
1985). A strong feature of this difference was an approximate three-fold increase in the percentage of gadoids taken in the
first three quarters of the year in 2002 compared with 1985, primarily due to the large increase in the percentages of cod
and haddock taken in the first quarter of the year.
4.6.2 Prey consumption
Estimates of annual consumption of commercially important fish prey by grey seals increased substantially between 1985
and 2002 (Table 19). Overall, grey seals in the North Sea were estimated to have consumed 39,000 tonnes of prey in 1985
(excluding Shetland) and 116,000 tonnes of prey in 2002 (including Shetland) in line with the almost threefold increase in
seal population size (Table 1). If estimates of population size from the extended non-linear density dependent model are
used (Table 1), estimated annual prey consumption is 49,000 t in 1985 and 161,000 t in 2002.
The estimated amount of sandeel consumed increased two and a half times from 29,000 t in 1985 to 69,000 t in 2002, and
estimated consumption of cod doubled from 4,100 t to 8,300 t, both less than the increase in grey seal population size.
Compared to 1985, an order of magnitude more haddock (6,500 t in 2002; 600 t in 1985); and plaice (5,200 t in 2002; 600
t in 1985) was consumed in 2002. The estimated amount of whiting, saithe, ling and herring consumed increased
approximately in line with grey seal population size. Of the non-commercially exploited fish, there were large increases in
the estimated consumption of seascorpions (6,600 t in 2002; 1,000 t in 1985) and dragonets (1,900 t in 2002; 300 t in
1985).
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DEFRA
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MF0319
Another way of illustrating the changes in consumption between 1985 and 2002 is to compare the average amount of fish
consumed per seal in each of the two years (Table 20). For the North Sea as a whole, total prey consumption by the
average seal increased slightly between 1985 and 2002 from 4.5 to 4.7 kg.d-1; from 1.63 to 1.72 tonnes.yr-1. Per capita
prey consumption is expected to remain the same because the analytical method equates information on diet to the
estimated energy requirement, which is assumed the same in 1985 and 2002. The slight increase is because a smaller
proportion of energy rich sandeel were consumed in 2002 than 1985.
For the two main species in the diet, consumption per seal decreased between 1985 and 2002: for cod by about 30 %
(0.48 to 0.34kg.d-1 and 175 to 125 kg.yr-1); and for sandeel by about 15 % (3.3 to 2.8 kg.d-1 and 1,218 to 1,209 kg.yr-1).
Consumption per seal remained about the same for whiting, approximately trebled for plaice, and approximately
quadrupled for haddock.
4.6.3 Size of prey consumed by seals
There were differences in the sizes of fish consumed in 2002 compared to 1985 for all major prey species except haddock.
Sandeel were markedly smaller in 2002 (mean length 16.5 cm, Fig 2a) than in 1985 (mean length 19.7 cm, Fig 3a). Plaice
were also smaller in 2002 (mean length 19.0 cm) than in 1985 (mean length 21.0 cm). In 2002, there were relatively more
plaice in the range 25-35 cm and relatively fewer in the range 35-45 cm (Figs 2b and 3b). Cod were also smaller in 2002
than 1985 (mean length 35.4 cm vs 37.1 cm) with relatively more 20-30 cm fish consumed in 2002 and relatively more
40-50 cm fish consumed in 1985 (Figs 2c and 3c). Whiting consumed by grey seals were larger in 2002 (mean length 21.0
cm) than in 1985 (19.3 cm) apparently because of relatively more fish in the 20-30 cm range in 2002 (Figs 2d and 3d).
Analysis of the age and maturity of fish at these lengths remains to be undertaken but a crude inspection indicates that in
both 1985 and 2002 most of the cod and plaice consumed would have been immature fish while most of the sandeel and a
considerable proportion of the haddock would have been mature. Most of the whiting consumed in 1985 would have been
immature fish while a considerable proportion of those consumed in 2002 would have been mature.
More detailed analysis of these data are planned in collaboration with scientists at CEFAS and FRS.
4.6.4 Prey consumption by grey seals compared with fish stock sizes in 1985 and 2002
Table 21 shows, for 1985 and 2002, the estimated consumption of fish prey by grey seals compared to estimates of total
stock biomass (TSB) in ICES Sub-Area IV for those species for which ICES has conducted stock assessments
(http://www.ices.dk/products/CMdocs/2005/ACFM/ACFM0705.pdf).
Table 21 also shows the catch/landings of
assessed species.
In 1985, grey seal predation was small compared to TSB for all species. In 2002, relative consumptions of most prey
species were several times higher, but only for cod (3.7%) sandeel (2.7 %) and plaice (1.5 %) are the percentages greater
than 1%.
Table 22 compares relative changes in estimated prey consumption, total stock biomass (TSB) and catch/landings of the
main commercially exploited species in the North Sea, between 1985 and 2002. The greatest increases in consumption
relative to grey seal population size are for Norway pout, haddock and plaice. Sandeel and cod consumption declined
slightly relative to population size.
These relative changes between 1985 and 2002 can be considered more robust. They are caused by a combination of three
factors: an overall increased consumption of prey by grey seals (driven by an almost threefold increase in seal numbers);
changes in diet composition, as described above; and declines in most assessed fish stocks.
We can conclude that grey seal predation on commercially exploited fish stocks in Sub-Area IV was very much higher in
2002 than in 1985.
5. Discussion
5.1 The results and their reliability
The results confirm that sandeel and gadoids are the primary prey of grey seals, as found previously (Prime & Hammond
1990; Hammond & Prime 1990; Hammond et al. 1994a; b) and in the Hebrides in 2002 (Hammond & Harris 2006).
Benthic species and flatfish also continue to feature as relatively minor contributors. Further work to explore the seasonal
and regional variation in grey seal diet is in progress.
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5.1.1 Assumptions made in analysis of hard remains from scats
Scat analysis makes a number of assumptions, the violation of which can lead to biased results, as noted by a number of
authors (DaSilva & Neilsen 1985; Murie & Lavigne 1985; Jobling & Breiby 1986; Jobling 1987; Pierce & Boyle 1991).
Below, we address each of these assumptions.
Scat analysis (like any other sampling method) assumes that the data are representative of the population to which the
results are extrapolated. This means firstly, that the faeces recovered from haul-out sites should be representative of the
population diet. Because diet can vary seasonally and regionally, we stratified our analysis accordingly to avoid bias as a
result of this, although simple comparisons of results from stratified and pooled data suggest that any bias is likely to be
small. If significant foraging occurred sufficiently far from haul-out sites that prey remains would be defaecated at sea,
and if the prey consumed far offshore were different, the prey remains found in scats on haul-out sites would not be
representative. However, telemetry studies, funded partly by Defra (projects MF0503, MF0309, MF0311), have shown
that around Britain grey seals typically forage within about 40 kilometres of haul-out sites and return rapidly to land after
foraging (McConnell et al. 1999; Matthiopoulos et al. 2004). Furthermore, analysis of data on passage times of prey
remains through seal guts (Grellier & Hammond 2006) combined with the telemetry data and information on prey
distribution from analyses of fish survey data shows that no bias in diet composition is expected from sampling scats on
haul-out sites for British grey seals (Smout 2006).
Scat analysis also assumes that all prey consumed have recoverable remains. In particular, the analysis of otoliths relies
on seals eating the whole prey including the heads of fish. Fishermen often state that seals do not eat the heads of large
fish and that scat analysis is biased towards smaller prey. These statements are based on observations of “rogue” seals
around fishing nets knocking the heads off large fish or just taking the guts of fish caught in the net. But feeding
behaviour around fishing nets is unlikely to be representative of feeding on free-ranging fish in normal circumstances.
Observations made during the Defra-funded digestion experiments with the (short-term) captive seals indicate that
observing what seals eat at the surface is not necessarily representative of what they actually consume. The three
experimental seals that were offered large fish (~1 kg cod) consumed then in different ways. One individual swallowed
the fish whole. Another ripped the fish into smaller pieces that she then consumed; only parts of the backbone were left
uneaten. The third ate all the bodies and later consumed all the heads. Whether or not seals always eat the heads of all fish
during normal feeding is unknowable. However, many otoliths from large fish were found in the scats collected in 2002
and in 1985, so we know that seals do eat the heads of a considerable number of large fish. Further analysis of the data
presented here will investigate estimated size distributions of fish taken by grey seals and compare these with the size
structure in the prey populations.
Large fish eat small fish and it is likely that the otoliths of some small fish recovered in seal scats are actually from the
stomachs of larger fish - so-called secondary prey. Calculations based on the otoliths found in scats and the observed
stomach contents of large fish show clearly that, even in the most extreme circumstances, the contribution of secondary
prey to the estimates of diet composition is minimal - much less than 1%.
Some species that could in theory be potential prey do not possess otoliths or beaks; for example, crustacea are sometimes
found in scats. However, there is no evidence that grey seals eat crustaceans and we assume these are secondary prey.
Reconstruction of seal diet using otoliths and beaks is, of course, subject to bias if the effects of partial and complete
digestion are not taken into account. Our methods use comprehensive and robust estimates of digestion coefficients and
recovery rates to eliminate any biases resulting from these effects (see section 5.1.2). There is no need to consider other
hard remains from prey, such as skeletal bones, to try to improve estimates of diet composition, as has been proposed by
others (Zeppelin et al. 2004; Tollit et al. 2003; Tollit et al. 2004).
5.1.2 Experimental work on digestion
The species- and grade-specific digestion coefficients and species-specific recovery rates were obtained under strict
experimental protocols designed to minimise bias and variance. The results are therefore not only comprehensive, they
are as robust as could be obtained within the limitations of the time allowed by the project. They are the most reliable for
any species anywhere to date.
The protocol of feeding whole prey to captive seals is unbiased, whereas the usual practice of using a carrier species to
deliver otoliths to the seal’s gut is biased (Grellier & Hammond 2005). This important finding extended the length of the
experiments but has resulted in a set of results that we can be confident are robust. The use of grade-specific digestion
coefficients reduces bias and variance in the analysis of seal diet. Most importantly, the availability of otolith and beak
recovery rates for the first time means that estimates of diet could be corrected to account for species-specific differences
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in the complete digestion of hard parts, thus eliminating an important source of bias. In summary, the experimental work
has significantly improved our ability to apply scat analysis methods for estimating seal diet and has given us a high level
of confidence in our results on diet composition and prey consumption.
5.1.3 Remaining sources of uncertainty
The comprehensive coverage of our scat sampling, seasonally and regionally, in the North Sea during 2002 forms the
basis of a set of reliable estimates of diet composition and prey consumption. These estimates will form an important
benchmark for the future. The methods used during this project to analyse the measurements of otoliths and beaks
recovered from the scats are the most comprehensive in use anywhere. They take full advantage of the new experimental
data to minimise bias in the results and they generate realistic estimates of variance around the estimates of diet
composition and prey consumption. Nevertheless, there remains uncertainty in the results, which has not been
incorporated in the estimated confidence intervals, in the form of potential bias.
To estimate consumption, we equate estimated diet composition to the estimated energy requirement of the seal
population in question. To be free from bias, this means that our estimates of seal energy requirements and seal
population size (discussed above) should be unbiased. The energy requirement we used was an annual average (Sparling
& Smout 2003) but grey seal energy expenditure and intake varies seasonally as part of the annual cycle of pupping and
moulting. Seasonal values of energy requirements have not yet been estimated and it is not known how much they will
vary. The general effect is expected to reduce the influence of estimates of prey consumption in quarters 1 and 4 and
increase the influence of estimates in quarters 2 and 3. A cursory examination of the 2002 results for the region with the
highest seal numbers (Orkney) indicates that the overall likely impact of this would be a decrease in the estimated
consumption of gadoids and benthic prey, an increase in flatfish and a slight decrease in sandeel. In any case, these
variations are likely to be small.
The greatest uncertainty in the estimates of prey consumption is model uncertainty in the estimation of grey seal
population size. The 2005 meeting of the NERC Special Committee on Seals (SCOS; http://www.smru.stand.ac.uk/CurrentResearch.htm/scos.htm) noted this uncertainty and recommended as a high priority that work be
undertaken to reduce it. In the mean time, SCOS concluded that the estimate from the simple linear density dependent
survival model was more likely to be a reflection of true population size. We note, however, that Thomas & Harwood
(2005) consider that the results from the extended non-linear density dependent survival model better reflect the observed
patterns of variation in pup production.
This model uncertainty in estimates of population size is important because of the size of the effect on estimates of grey
seal prey consumption. The seal population estimates used here to derive consumption estimates are the lowest of those
presented by Thomas & Harwood (2005).If population estimates from the extended non-linear density-dependent survival
model are used, estimates of prey consumption are approximately 25% larger in 1985 and 40% larger in 2002. It is
therefore much more likely that our estimates of prey consumption are underestimated than overestimated.
Another source of uncertainty that impacts all results is selection of the otolith measurement used in the calculations. We
used a rigorous procedure to determine whether otolith length of width should be used (see section 3.3.1) and for most
species the choice of measurement has a negligible effect on results. However, for a small number of species, notably cod,
the choice of measurement makes a considerable difference. Our protocol selected otolith length for cod but if we had
used otolith width, estimates of the proportion of cod in the diet would have been much higher and the proportion of other
species consequently lower. This uncertainty is not reflected in the results presented here.
There is also uncertainty in the use of grade-specific digestion coefficients. Using grade-specific digestion coefficients is
definitely more appropriate than not using them but the extent to which they account for any non-linearity in the
relationship between digestion rate and otolith size is uncertain. It is likely that if the most digested category (grade 3)
were sub-divided, some improvement could be made. There is also uncertainty associated with the use of grade-specific
digestion coefficients in 2002 but not in 1985 (because otoliths were not graded then). However, as described above in
section 3.5, examination of diet estimates for Orkney in 2002 calculated with species-specific digestion coefficients
indicates that differences are relatively small.
Other sources of uncertainty not taken into account are seasonal differences in length-weight relationships and seasonal
and age-related differences in the energy densities of fish. These considerations are, however, minor compared to
uncertainty in population size estimation.
There is also uncertainty in how the results should be interpreted. Estimates of fish stock biomass in Sub-Area IV from
ICES assessments are subject to uncertainty and comparisons of estimated consumption by grey seals with estimates of
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TSB are therefore also uncertain. It is clear, however, that most fish stocks in Sub-Area IV have declined and the
increased estimated consumptions between 1985 and 2002 are of considerable interest.
5.2 Implications of the findings
One of the main results from this project of direct relevance to Defra is that grey seals continue to consume large
quantities of sandeel, cod and other gadoids despite declines in the stocks. In 1985, the amounts taken by grey seals were
small compared to stock sizes. In 2002, the amounts consumed relative to stock size were much higher and potentially
important for cod and sandeel. Grey seal predation on commercially important fish stocks in the North Sea is clearly
much greater now than it was in 1985.
Ecologically, the results show that sandeel, cod, other gadoids and plaice are the most important prey of grey seals in the
North Sea. Sandeel continue to be consumed in large quantities. The amount of cod consumed per seal declined slightly
between 1985 and 2002 but the stock declined much more. The amounts of haddock and plaice consumed per seal
increased markedly between 1985 and 2002 in the face of stock declines.
The question of the impact that grey seals may have on fish stocks and, therefore, fish catches is an important one in light
of the results presented here. Might grey seals limit the ability of cod, especially, and other gadoid stocks to recover in the
North Sea? The other side of this question is whether declines in stocks of the main prey of grey seals might impact grey
seal population growth.
We are currently unable to answer these questions. Simple comparisons between estimates of prey consumption by grey
seals and estimates of fish stock size do not allow an assessment of the impact of seals on fish stocks and fisheries
because of the complexity of the ecosystem in which these species coexist. In particular, we cannot use these results to
infer grey seal impact on a fish stock without information on the predation rates of other predators.
A valuable tool to help address these questions would be a dynamic model of grey seal interactions with their prey
incorporating a multi-species functional response. Defra has previously supported the development of such models under
projects MF0309, MF0311 and MF0320, and this work has continued at SMRU as part of other projects (Smout 2005,
http://www.rrz.uni-hamburg.de/BECAUSE/). These models could be further developed and parameterised with results
from this project to address this and other related management questions. Results from such modelling will not provide
clear-cut quantitative management advice but should improve understanding of seal-fish population interactions and allow
the provision of better scientific advice.
6. Future work
There has been insufficient time to fully explore the data and results obtained in this project. We will continue work on
this, focussing on a more detailed analysis of regional and seasonal variation in diet composition, and on an exploration of
the size distribution of consumed prey compared with the size structure of prey populations.
These new results will be valuable for the work of the ICES Working Group on Multispecies Assessment methods. We
will work with members of the Working Group to enable our new diet estimates for grey seals to be incorporated into
developing multispecies models.
Outside this project, these results will allow the models developed under Defra projects MF0309, MF0311 and MF0320
to be more fully utilised and developed to explore management related questions. This could include, in particular,
predicting future consumption of cod and other prey by grey seals using multi-species functional responses and exploring
whether grey seals may affect the ability of North Sea and Scottish west coast cod and other stocks to recover.
Grey
seal
pup
production
in
the
North
Sea
is
still
increasing
(http://www.smru.stand.ac.uk/CurrentResearch.htm/scos.htm) and the amount of fish that grey seals consume will thus also continue to
increase in the future. The diet is likely to change as the abundance of fish prey changes, as it did between 1985 and 2002.
It will therefore be important to reassess grey seal diet in the relatively near future.
As well as the 70-90,000 grey seals in the North Sea and another 40-50,000 west of Scotland, there are also large
populations of common or harbour seals (Phoca vitulina), as presented and discussed by the NERC Special Committee on
Seals (http://www.smru.st-and.ac.uk/CurrentResearch.htm/scos.htm). Up-to-date diet information for harbour seals is
very limited but earlier studies have shown them to consume broadly the same prey as grey seals, including sandeel,
gadoids and flatfish. In assessments of the impact of pinniped predation on commercially important fish stocks, it will be
important to include estimates of harbour seal consumption. We recommend that harbour seal diet be assessed in the
North Sea (and elsewhere around Britain) in the near future.
CSG 15 (1/00)
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Project
title
Grey Seal diet Composition and Fish Consumption in the
North Sea
DEFRA
project code
MF0319
7. References
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Project
title
Grey Seal diet Composition and Fish Consumption in the
North Sea
DEFRA
project code
MF0319
Murray, J. and Burt, J.R. 1977. The composition of fish. Torry Advisory Note No. 38. MAFF, Torry Research Station,
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