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Indicator Fact Sheet
(FISH4) Fisheries impact habitats and ecosystems
Author: Zoe Trent and Steve Nixon, WRc
EEA project manager: Anita Künitzer
version 17.10.03
Key message
/ Fishing is causing a change in the ecosystem composition of the north-east Atlantic Ocean,
and the Mediterranean and Black Sea suggesting that fish stocks are being exploited at
unsustainable rates
Figure 1: Mean trophic level of fisheries landings for the north-east Atlantic and the
Mediterranean and Black Sea, 1950 to 1998.
3.8
Mediterranean and Black Sea
Atlantic, north-east
3.7
3.6
3.5
M
ea 3.4
n
Tr
op 3.3
hic
Le 3.2
vel
3.1
3.0
2.9
2.8
1950
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
Year
NB: Trophic level is the position in a food chain or web that an organism occupies and is a way of describing the feeding
hierarchy. For example, primary producers (organisms that get their energy directly from the sun) such as marine
phytoplankton are considered to occupy the lowest trophic level. Herbivorous zooplankton (tiny animals that feed on
plants) are considered to occupy the next trophic level. Fish which eat zooplankton occupy the next trophic level and so
on. The assigning of trophic levels is, in fact, a complex process involving modelling the food web and examining the
diets of each species.
Source: Adapted from Pauly et al. (1998) using Fishbase (Pauly and Christensen, 1997)
Results and assessment
Policy relevance: target or objective for the indicator
Fishing is extremely important economically but has a major impact on marine ecosystems. In
Europe, fishing is controlled by the common fisheries policy (CFP) (1983) which aims to
conserve fish stocks. However, the CFP has so far failed to achieve sustainable fishing.
The CFP is currently being reviewed and it is recognised that there is a need for an ecosystembased approach. An ecosystem-based approach for fisheries management will also be
necessary to protect the vulnerable marine wildlife and habitats as required by legislation to
preserve biodiversity and habitats (e.g. the Habitats Directive) (Pope and Symes, 2000). An
indicator, based on the impact on non-target species will therefore be important in the future to
assess whether sustainable fishing is achieved.
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In addition to this, the Council Decision (98/414/EC) of 8 June 1998 ratified the agreement for
the implementation of the provisions of the United Nations Convention on the Law of the Sea of
10 December 1982 relating to the conservation and management of straddling stocks and
highly migratory fish stocks.
There is also specific legislation to protect cetaceans (marine mammals that give birth at sea
such as dolphins, porpoises and whales), which are often victims of bycatch.
Policy context
The common fisheries policy aims to achieve sustainable fishing. It is recognised that the CFP
needs revising so that it has an eco-system based approach. Marpol — prevents dumping of
waste at sea. Dumping of waste fishing gear in particular can cause high mortality in non-target
species through ghost fishing and ingestion of fragments. The Convention on the Conservation
of Migratory Species of Wild Animals (The Bonn Convention) aims for favourable conservation
status of migratory species. Annex I is a list of endangered migratory species and includes a
number of cetaceans and Annex II list species with unfavourable conservation status and also
includes a number of cetaceans. The FAO Code of Conduct for Responsible Fisheries sets
international standards of behaviour for responsible fishing practices aiming to ensure
conservation, management and development of living aquatic resources, with due respect for
the ecosystem and biodiversity.
Environmental context: (scientific soundness and choice and definition of the indicator)
Fishing gear is designed to maximise yields of target species and minimise cost of effort but
they also trap non-target species and damage the marine environment. Non-target organisms
effected include benthos, birds, marine mammals, marine reptiles (turtles), plants and nontarget fish. The effects on non-target species can either be direct (e.g. accidental entrapment) or
indirect (e.g. through the alteration of energy transfers through trophic levels thus reducing
abundance and/or modifying relative size composition). Ecosystem effects include habitat
destruction, changes in diversity and community structure, and trophic interactions (Jennings
and Kaiser, 1998).
Although studies have been carried out to examine the effect on non-target populations through
direct mortality (e.g. Pope et al., 2000), there is no comprehensive data set for assessing the
problem in the EEA area. Most data exist on target populations but this can be used as an
indicator of biological diversity, ecological processes (Pope and Symes, 2000) and to assess
general changes in community structures. The alteration in average trophic level of target
populations has therefore been used for this indicator to try and detect community change.
Assessment
Capture fisheries tend to target the more valuable larger fish that are at higher trophic levels
such as piscivores (fish which feed on other fish). However, as overfishing reduces the
populations of these fish, the landings of fish lower down the food web such as zooplanktivores
(fish which feed on microscopic animals known as zooplankton) make up a larger proportion of
the overall catch. This is generally indicative of a negative impact on the whole ecosystem
caused by fishing and has been called ‘fishing down marine food webs’. This phenomenon was
first demonstrated by Pauly et al (1998) and is evident in the FAO fishing areas which are
mainly fished by European countries (as well as other fishing areas globally).
This can be seen on Figure 1 which shows that the mean trophic levels in both the ‘North East
Atlantic’ and ‘Mediterranean and Black Sea’ fishing areas have declined since 1950. It seems
that fundamental changes in the structure of these marine ecosystems has occurred and it is
likely that this is due to fishing.
The hypothesis that top-down removal of predators is affecting production at lower levels is
however only one explanation for declining mean trophic levels as Caddy and Garibaldi (2000)
have pointed out. Fishing down food webs may be a result of economic consideration, as
increased marine productivity and technological improvement of fishing gear have led to
increased levels of planktivore landings. In addition, shifts in the composition of communities
can also result from environmental change (Jennings and Kaiser, 1998).
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Nevertheless, Pauly et al’s (1998) modified approach on fishing down food webs (FDFW)
remains a powerful tool for fisheries management (CIESM’s workshop series, 2000). FDFW
leads at first to increasing catches, then to a phase transition associated with stagnating or
declining catches indicating unsustainable exploitation rates.
Numerous other studies also show that fishing is currently not sustainable both in terms of
stocks of target species and the effect on non-target populations. For example, Jennings et al
(1999) showed that there has been a shift towards small fish with fast life histories in a study on
the North Sea indicating that fishing has a greater effect on slower growing species with late
maturity.
References
Ball, B., Munday, B., I. Tuck (2000) ‘Effects of otter trawling on the benthos and environment in
muddy sediments’ Effects of fishing on non-target species and habitats, Kaiser, M. J., de Groot,
S. J (eds), Blackwell Science, Oxford, England, 399 pp.
Boudouresque, C. F., Avon, M.and Gravez, V., eds. (1991), Les especes marines a proteger en
Mediterranee, GIS Posidonie, 448 pp.
CIESM Workshop Series No 12 (2000), ‘Fishing down the Mediterranean food webs’, Kerkyra
(Greece) 23–26 July 2000, Briad, F., (eds), Monaco, 99 pp.
Caddy, J. F and Garibabldi, L. (2000) Apparent changes in the trophic composition of world
marine harvests: the perspective from the FAO capture database. Ocean and coastal
management, 43: pp. 615–655.
Frid, C. L. J., Harwood, K. G., Hall. S. J., Hall, J. A. (2000) Long-term changes in the benthic
communities of North Sea fishing grounds, ICES J. Mar. Sci. 57 (5): pp. 1303–1309.
Fonteyne, R., (2000) Physical impact of beam trawls on sea bed sediments In Effects of fishing
on non-target species and habitats, Kaiser, M. J., de Groot, S. J (eds) Blackwell Science,
Oxford, England, 399 pp.
Jennings, S., Greenstreet, S. P. R. and Reynolds, J. D. (1999) Structural change in an exploited
fish community: a consequence of differential fishing effects on species with contrasting life
histories, 68, pp. 617–627
Jennings, S. and Kaiser, M. J. (1998), The effects of fishing on marine ecosystems advances in
marine biology, Volume 34, pp. 201–350
Jennings, S., Dinmore, T. A., Duplisea, E., Warr, K. J. and Lancaster, J. E. (2001) ‘Trawling can
modify benthic processes’, Journal of animal ecology, 70, pp. 459–475
Jennings, S., Pinnegar, J. K., Polunin, N. V. C. and Warr, K. J. (2001a) ‘Impacts of trawling
disturbance on the trophic structure of benthic invertebrate communities’, Marine Ecology
Progress Series 213: pp. 127–142
Kaiser, M. J. (2000) ‘The implications of the effects of fishing on non-target species and
habitats’, Effects of fishing on non-target species and habitats, Kaiser, M. J., de Groot, S. J.
(eds) Blackwell Science, Oxford, England, 399 pp.
Pauly, D. and Christensen, V. (1997), ‘Trophic levels of fishes’, Box 16, p. 127, Froese, R. and
Pauly, D. (eds) Fishbase 97: concepts, design and data sources, Iclarm, Manila.
Pauly, D., Christensen, V., Dalsgaard, J., Froese, R. and Torres Jr. F. (1998) Fishing down
marine food webs, Science 279: pp. 860–863.
Pauly, D., Froese, R., and Christensen V.,(1998) ‘Response to Caddy et al.:How pervasive is
‘‘Fishing down marine food webs?’’’ Science 282: pp. 1384–1386.
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Pauly, D., Christensen, V., Froese, R. and Palomares, M. L. (2000) ‘Fishing down aquatic food
webs’, American Scientist 88: pp. 46–51.
Pope, J. G and Symes, D. (2000) ‘An ecosystem-based approach to the common fisheries
policy: Defining the goals’, English Nature.
Pope, J. G., MacDonald, D. S., Daan, N., Reynolds, J. D. and Jennings, S. (2000) ‘Gauging the
impact of fishing mortality on non-target species’, ICES, Journal of Marine Science, 57, pp. 689–
696
Simboura, N., Zenetos, A., Pancucci-Papadopoulou, M-A., Thessalou-Legaki, M., and
Papaspyrou, S. (1998), ‘A baseline study on benthic species in two neighbouring gulfs, with and
without access to bottom trawling’, Marine ecology, 19 (4): pp. 293–309
Zühlke, R. (2001), ‘Monitoring biodiversity of epibenthos and demersal fish in the North Sea and
Skagerrak, EC Project 98/021’, Monitoring report 2001 to the Commission of the European
Community
Data
Spreadsheet:
Trophic Level Decline.xls
Metadata
Technical information
1. Data source: FAO and Fishbase.
2. Description of data: FAO landings data. Fishbase — data on trophic levels of species was
used. Trophic levels were assigned by Pauly et al (1998) as describe in their paper.
3. Geographical coverage: Landings data is global.
4. Temporal coverage: FAO data: 1950–98.
5. Methodology and frequency of data collection: FAO data is collected and collated annually.
6. Methodology of data manipulation, including making ‘early estimates’.
Quality information
7. Strength and weakness (at data level):
•
Catch data: Despite being a large dataset, the FAO data has a number of problems. For
example, before 1984 species produced in aquaculture are also included although this is
generally a low tonnage of the overall value (Caddy et al, 2000). In addition, some countries
report landings of individual species and others lump multispecies catches together. The
data is also nominal and is not recorded as total biomass.
• Trophic levels: Considered to be fairly correct as discussed in Pauly et al (2000)
8. Reliability, accuracy, robustness, uncertainty (at data level): Variable
9. Overall scoring (give 1 to 3 points: 1 = no major problems, 3 = major reservations) (for the
main indicator):
Relevancy: 2
Accuracy: 2
Comparability over time: 2
Comparability over space: 2
Further work required
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Fishbase could be used to compare the resilience of by-catch with that of the target species and
thus identify those species (with lower resilience) that are likely to be threatened by fishing (Dr
Rainer Froese, personal communication).
A comprehensive examination of the direct effects of different types of fishing gear on nontarget species and on habitats would also be useful. This could then be used along with data on
changing gear type in different regions to look at the likely extent of ecosystem damage.
Population studies on key non-target species would also be appropriate alongside fishing effort
data. The method of measuring fishing effort should be standardised to make studies
comparable.
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