Download Abundance and distribution of the larval stages of the mesopelagic

ICES CM 2004/K:76
Abundance and distribution of the larval stages of the mesopelagic fish Maurolicus
muelleri (Gmelin, 1788) in relation to the hydrography off the west coast of Ireland.
Silvana Acevedo1, Julie M. Fives2 and Christian Mohn1
1 Earth and Ocean Science Department, National University of Ireland, Galway.
2 The Zoology Department, National University of Ireland, Galway.
ABSTRACT
The distribution and abundance of the pearl-side, Maurolicus muelleri larvae were
investigated as part of the EU-funded project ‘Ichthyoplankton-based indices of abundances
of Spring-spawning commercial fish population in Western European waters’ (INDICES).
The larvae were found to be the second most numerous species recorded and occurred mainly
west of the Irish shelf break. Spawning occurred from March to July with a peak in early
summer. Greatest larval abundances occurred in June-July, with the highest concentrations
recorded north of the study area. Larval distribution patterns were investigated in relation to
hydrographic conditions.
Key words: west of Ireland; larvae; Maurolicus muelleri, distribution.
INTRODUCTION
The present work is part of an EU-funded project, ‘Ichthyoplankton-based indices of
abundances of Spring-spawning fish populations in Western European waters’ (INDICES),
the primary objective of which was to build up a time series of abundance indices of the early
life history of commercial fish species, with a view to the management of seven target species
and the assessment of other non-target species. The target species were mackerel, Scomber
scombrus; horse mackerel, Trachurus trachurus; sardine, Sardina pilchardus; hake,
Merluccius merluccius; blue whiting, Micromesistius poutassou; anchovy, Engraulis
encrasicholus and megrim, Lepidorhombus whiffiagonis.
Mauroliucs
muelleri is a
mesopelagic and pseudoceanic species (Hulley, 1981) and in this study was the second most
abundant species recorded. It also had been among the most numerous species recorded by
O’Brien & Fives (1995) off the west coast of Ireland and by Horstman & Fives (1994) in the
Celtic Sea. The pear-lside is a very abundant species in slope areas (GjØsaeter & Kawaguchi,
1980) and its stages constitute an important link in the food chain (Okiyama, 1971). Apart
from its ecological importance it is considered to have ‘minor commercial importance’
(Rainer & Sampang, 2004; GjØsaeter & Kawaguchi, 1980). Because the species occurred in
high numbers in the study area, and due to its economic potential, the distribution and
abundance of its developmental stages were assessed in relation to recorded environmental
factors.
MATERIALS AND METHODS
The plankton samples examined were taken off the west coast of Ireland within the area 48º to
60ºN and 08º15' to 15º45'W (Fig. 1). The samples were collected in 1998, as part of the
International Council for the Exploration of the Sea (ICES) Triennial Mackerel Egg Survey,
by the research vessels Walther Herwig, Tridens, Scotia, Celtic Voyager and G.O. Sars. The
plankton samples were taken, using a variant of the Gulf III (Gehringer, 1952) (r.v. Walther
Herwig, Tridens, Scotia, Celtic Voyager) and Bongo samplers (r.v. G.O. Sars), at the center
of rectangles which were defined by the 0.5° latitude and 0.5° longitude. Both samplers
(nosecone aperture diameter 200mm) were towed at speed of 5 knots (Gulf III) and 2-3 knots
(Bongo) in double-oblique tows from the surface to within 3m of the bottom in shallower
water or to a maximum depth of 200m in deeper water. Calibrated flow-meters were used to
calculate the volume of water filtered on each deployment. The sea surface temperature,
salinity, sample depth and bottom depth were recorded at each station. The samples were
preserved in 4% buffered formaldehyde solution. All fish larvae were removed and identified
to species level where possible. The standard length (from the snout to the end of the urostyle)
of each fish larva was recorded. Length-frequency histograms were constructed in which the
larval lengths were divided into three-size groupings:
i) length at hatching to yolk-sac absorption,
ii) length at yolk-sac absorption to post-larva with recognisable features (fins, pigmentation,
spines, etc.) and
iii) late post-larva with additional observable characters (fins, fin-ray, pigmentation, urostyle
development, etc.).
The number of fish larvae from each sample was standardized to that number found beneath a
10m2 of sea surface, using the formula described by Smith & Richardson (1977).
The percentage of fish larvae from each sample was calculated for each of 3 periods and for
the full period sampled. Abundance and distribution maps for Maurolicus muelleri were
constructed. A logistic linear regression model was used to describe associations between the
abundance of larvae and sea surface temperature and salinity.
RESULTS
Surface temperature showed a seasonal pattern during the sampled period, with values
increasing seaward. Surface salinities showed no seasonal pattern during the sampled period,
no data was available north of 50˚ in June-July (Fig. 1). The log linear model of larval M.
muelleri abundance from each sample and the sea surface temperature and salinity, showed
that there was no significant relation between these variables. A total of 472,409 fish larvae
(individuals/10m2) were recorded with the maximum occurring in June-July (Fig. 2). Of the
target species, the mackerel, Scomber scombrus was the most abundantly recorded species,
whereas the horse mackerel Trachurus trachurus was the third and the pilchard Sardina
pilchardus was the fifth most abundantly recorded species (Table 1). Of the non-target
species, Maurolicus muelleri was the second most abundantly recorded species (Table 1).
Table 1. Percentage abundance of larvae of the ten most numerous species recorded in the 1998
survey.
Species
Scomber scombrus
Maurolicus muelleri
Trachurus trachurus
Benthosema glaciale
Sardina pilchardus
Clupea harengus
Callionymus sp.
Micromesistius poutassou
Ammodytidae sp.
April-May (%)
27.48
9.34
7.80
7.79
4.75
0.10
4.78
5.54
3.49
May-June (%)
38.39
11.93
11.04
6.53
3.79
1.50
2.52
0.19
2.65
June-July (%)
15.54
20.48
15.00
3.53
6.36
6.17
1.10
0.45
Oveall (%)
25.82
13.21
12.80
6.03
5.80
3.50
2.86
2.12
2.07
Maurolicus muelleri larvae were distributed mostly west of the shelf break, with greatest
abundances occurring generally north of 55˚ (Fig. 3).
In March-April, M. muelleri larvae were not very widespread or abundant and most of the
larvae occurred west of the shelf in samples taken between 52º15’N
and 54º N. The
maximum number occurred northeast of the Porcupine Bank. The majority of larvae were
greater than 6.0mm in length (Fig. 4).
In the May-June period a total of 10,119 larvae/10m2 were recorded, with a maximum of
1613 individuals/10m2 occurring west of the shelf break, northwest of Ireland (Fig. 3). The
larvae ranged in size from 2.5mm to over 6.0mm with the majority in the 4.1 to 6.0mm range
(Fig. 4).
In June-July the greatest numbers (45,799 individuals/10m2) of M. muelleri were recorded,
with a maximum of 4352 individuals/10m2 again occurring west of the shelf break, off the
northwest coast of Ireland (Fig. 3). The majority (85%) of these larvae were larger than
4.1mm (Fig. 4).
DISCUSSION
The fact that the vast majority of the larval stages of Maurolicus muelleri were recorded west
of the shelf break and on the shelf break is not surprising as the adults are mesopelagic and
are normally found at 200m depths, only migrating into the upper 100m to feed at night
(Whitehead et al., 1989). In each study period, all the larval stages remained west of the shelf
break, unlike other species, for example Scomber scombrus and Merluccius merluccius where
the larger larvae were transported onto the shelf, an area of potentially greater food sources
(Fives et al., 2001). But unlike these species, M. muelleri releases eggs at depths of
approximately 200m and the larvae rise towards surface waters as they develop, but the post
larvae then sink gradually with development (Whitehead et al., 1989).
The size of the larvae recorded indicate that spawning was occurring in March, as over 56%
of the larvae recorded in April-May were already larger than 6.0mm. A further burst of
spawning was indicated by the larger number (23%) of newly hatched larvae recorded in
May-June. Spawning was still occurring throughout June as evidenced by the June-July
records, where 15% of larvae were newly hatched and 45% were in the early stages of
development. In each study period, the distribution and abundance of all
size classes
indicated that the majority of the spawning population must have been located in the
northwest of the study area, west of the shelf break. This was unexpected for the adults of this
species, whose distribution extends south to the northwest African coast (Whitehead et al.,
1989). Mean sea-surface temperatures for the northeast Atlantic along the shelf edge in 1998
were slightly higher than in previous years (Reid et al., 2001) and this may explain the
preference for the more northerly spawning positions. Reid et al. (2001) showed that changes
in the abundance of some species in the North Sea after 1989 and 1998, may be strongly
related to changes in the oceanic input and water temperature. Also, indications are that the
species spawned over an extended area in March 1998, whereas in 1980 and 1983 there was
very little evidence of such an early and extensive spawning (O’Brien & Fives, 1995).
Nevertheless, high abundances of the larval stages of Maurolicus muelleri were recorded in
May-June 1980 and 1983 (O’Brien & Fives, 1995). The developmental stages of M. muelleri
already constitute important links in the food chain at the sea and it is quite possible that this
very abundant and widespread species may become a viable commercial prospect as the
present commercial species become over fish.
60
April-May
35.2
55
11.5
35.2
35.4
50
35.6
11.5
35.8
60
35.2
35
May-June
55
35.4
34.8
35
35.2
50
35.4
35.6
60
June-July
12.5
55
50
35.5
35.5
35.4
-15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5
Temperature (ºC)
Salinity (ppm)
Figure 1. Sea surface temperature and sea surface salinity for the period April to July 1998.
60
April-May 1998
Total 16,3842 ind./10m2
Max. 16873 ind./10m2
55
50
60
May-June 1998
Total 84963 ind./10m2
Max. 3555 ind./10m2
55
50
60
June-July 1998
Total 22,3604 ind./10m2
Max. 14860 ind./10m2
55
50
16
14
12
10
8
6
4
0
1-100
101-500
501-2000
2000-4000
> 4000
Maximum
Figure 2. Synoptic results for total number of fish larvae (individuals/10m2).
60
April-May 1998
Total 1580 ind./10m2
Max. 305 ind./10m2
55
50
60
April-May 1998
Degrees North
Total 10144 ind./10m2
Max. 1613 ind./10m2
55
50
60
April-May 1998
Total 45799 ind./10m2
Max. 4352 ind./10m2
55
50
16
14
12
10
8
6
4
Degrees West
0
1 -10
11-100
101-500
501-1000
1001-3000
> 3000
Maximum
Figure 3. Distribution and abundance of Maurolicus muelleri larvae (individuals/10m2) in
the study area over the three sampling periods.
April-May 1998
100
59.62
%
50
34.62
5.77
0
2.5-4.0
>6.0
May-June 1998
100
%
4.1-6.0
46.66
50
30.10
23.24
0
2.5-4.0
4.1-6.0
>6.0
100
June-July 1998
%
50
44.99
40.38
4.1-6.0
>6.0
14.63
0
2.5-4.0
SIZE (mm)
Figure 4. The percentage abundance of the developmental stages of Maurolicus muelleri
for each sampling period.
ACKNOWLEDGEMENTS
The results on which this paper is based were obtained within the European Union INDICES
project (contract DG-XIV-97017). We gratefully acknowledge the provision of plankton
samples by The National Marine Institute, Ireland; The Scottish Office, Agriculture and
Fisheries Department, Marine Laboratory, Aberdeen, Scotland, The Federal Research Centre
for Fisheries, Institute for Sea Fisheries, Hamburg, Germany; DLO Netherlands Institute for
Fisheries Research (RIVODLO), Ijmuiden, The Netherlands; and the Institute for Marine
Research, Bergen, Norway.
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