Indicator: CO-17 Change in species and trophic structure of fish species caught

Data

An indicator of change in trophic structure of commercial catches across all Australian fisheries would require commercial fishers to keep detailed records of the species composition of mixed catches and of maximum size of targeted predators caught. No such recording is currently undertaken and consequently no national data are available. However, several studies of trophic impacts of fishing, from both Australia and overseas, show the seriousness of this issue and of our lack of comprehensive Australian data.

The history of Australia’s South-east fishery shows that, since the fishery commenced in 1915, fishing has greatly altered the biodiversity of the continental shelf and deeper water ecosystems. While flathead remains the main target species in the fishery, the mix of other exploited species has changed substantially. The spawning biomass of tiger flathead in this area, for example, has been reduced from about 27 000 tons in 1915 to about 7 000 tons in 2004; and leatherjacket and latchet have now almost disappeared from catches in the fishery.

Source: Klaer, N. L 2001, 'Steam trawl catches from south-eastern Australia from 1918 to 1957: trends in catch rates and species composition', Marine & Freshwater Research, vol. 52, pp. 399-410.

Since the 1970s, the catch rates of sharks and rays in the fishery have also declined dramatically, and populations of most of these species are now at very low levels.

Source: Graham KJ, Andrew NL and Hodgson KE 2001, 'Changes in relative abundance of sharks and rays on Australian South East Fishery trawl grounds after twenty years of fishing', Marine & Freshwater Research, vol. 52, pp. 549-61.

A qualitative analysis of the ecological risks from fishing in 14 Commonwealth-managed fisheries identified the sharks and rays as a group that have a higher level of risk from the impacts of fishing than the other groups considered. These species are mainly predators and, although climate change and habitat destruction may also have important impacts, fishing impacts are likely to be very important.

Source: Hobday A, Smith ADM & Stobutzki I 2004, Ecological risk assessment for Australian Commonwealth fisheries, final report stage 1, hazard identification and preliminary risk assessment, CSIRO, 174pp.

Using a meta-analytic approach, Myers and Worm estimate that, globally, large predatory fish biomass today is only about 10% of pre-industrial levels. They conclude that declines of large predators in coastal regions have extended throughout the global ocean, with potentially serious consequences for ecosystems.

Source: Myers, R.A. and Worm, B 2003, 'Rapid worldwide depletion of predatory fish communities', Nature, vol. 423, pp. 280-283, viewed 24 May 2006, http://www.mindfully.org/Water
/2003/Predatory-Fish-Depletion15may03.htm.

Petrie Choi and Leggett provide evidence of a trophic cascade in the large eastern Scotian Shelf ecosystem off Nova Scotia, Canada. The cascade involved four trophic levels and nutrients and was driven by changes in the abundance of large predators (primarily cod) of fish and macroinvertebrates. In addition to cod, several other commercially exploited species declined. The transition occurred during the mid-1980s and early 1990s and resulted in the virtual elimination of the ecosystem-structuring role of the large-bodied predators that had dominated for centuries. The abundance of small pelagic fishes and benthic macroinvertebrates, once among the primary prey of the benthic fish community, increased markedly following the benthic fish collapse. There were also conspicuous indirect effects resulting from removal of the top predator.

Source: Frank, K.T, Petrie, B, Choi, J.S. and Leggett, W.C 2005, 'Trophic cascades in a formerly cod-dominated ecosystem', Science, vol. 308, pp. 1621-1623, viewed 24 May 2006, http://www.sciencemag.org/cgi/content/full/308/5728/1621.

Baum and Myers model data from fishery research cruises in the Gulf of Mexico in the 1950s and compare it with data from science observers on longline vessels in the 1990s. Results suggest that it may be particularly easy for baselines of incidentally harvested species to shift because they are usually poorly monitored. The study provides first estimates of baseline abundances for pelagic sharks in the open ocean, demonstrating that these species were enormously more abundant than today. The mean weight of each of the modelled species is also estimated to be much lower than in the 1950s. The average size of oceanic whitetip sharks is near the size at maturity and the mean size of other species is now below the size at maturity

Source: Baum, J.K. and Myers, R.A 2004, 'Shifting baselines and the decline of pelagic sharks in the Gulf of Mexico', Ecology Letters, vol. 7, pp. 135-145, viewed 24 May 2006, http://fmap.dal.ca/ramweb
/papers-total/Baum_Myers_2004.pdf.

A UK study reviews 39 documented trophic cascades from 21 locations around the world, including two from Australia. However, very little information is provided, noting only that: “Complex cascade effects have been noted for many years from kelp forests and other subtidal algal assemblages around the world, including the North Western Atlantic, Alaska, California, South Africa, New Zealand and Australia (Schiel & Foster 1986). Almost all of these documented trophic cascades involve a predator, sea urchins and macroalgae/ kelp, although the urchin species and predator may vary amongst sites.”

Source: J.K. Pinnegar, N.V.C. Polunin, P.Francour, F. Badalamenti, R. Chemello, M.-L. Harmelin-Vivien, B. Hereu, M. Milazzo, M. Zabala, G. D'anna and C. Pipitone 2000, 'Trophic cascades in benthic marine ecosystems: lessons for fisheries and protected-area management', Environmental Conservation, vol. 27 (2), pp. 179-200, viewed 24 May 2006, http://www.unice.fr/LEML/Pages/Pub_LEML/Pinnegar_et_al_2000.pdf.

Coleman and Williams, in a study of species that are “marine ecosystems engineers”, note some ecological impacts of fishing that go beyond direct changes to the food chain. They observe that two high level predators of the deep and more remote ocean off the Californian coast, tilefish and grouper, are associated with discrete habitat features. Their characteristic longevity and slow growth suggest that they are vulnerable to overexploitation and would need prolonged periods for recovery if overfished. Because these fish species are likely to have multiple ecological roles, their loss could have “effects that extend beyond their own demise to resonate throughout the ecosystem”.

Source: Coleman, F.C. and Williams, S.L. 2002, 'Overexploiting marine ecosystem engineers: potential consequences for biodiversity', Trends in Ecology and Evolution, vol. 17, pp. 40-44, viewed 24 May 2006, http://72.14.203.104/search?q=cache:TX5SjDcc_acJ
:www.bio.fsu.edu/mote/colemanTREE_01.02.pdf++%22overexploiting+marine+ecosystem+engineers:+potential+
consequences+for+biodiversity%22&hl=en&gl=au&ct=clnk&cd=1.

What the data mean

To show whether changes in trophic structure are occurring within commercial or recreational fisheries, detailed data on Australian fish catches would need to be collected by each fishery. At present no data that would shed light on this issue are collected. However, the range of studies quoted above indicate that trophic structure has changed in at least one Australian fishery and that fishing in general has significant potential to change trophic structure.

Data Limitations

No continental data on changes in size and mix of fish caught and bycatch of commercial and recreational fisheries are currently available.

Issues for which this is an indicator and why

Coasts and Oceans — Direct pressure of human activities on coasts and oceans - Pressure of fishing 

Change in trophic structure in commercially exploited species could be indicative of significant ecological change. Although any such change may not be entirely attributable to the impacts of fishing, fishing (including illegal, recreational and Indigenous) is the most probable cause of this kind of change because larger (and generally more predatory fish) are the most sought after in terms of food value and are also more vulnerable to modern netting techniques.

Other indicators for this issue:

Biodiversity — Pressures on biodiversity - Pressures on marine biodiversity: pressures of fishing 

Change in trophic structure in commercially exploited species could be indicative of significant ecological change, resulting from the pressures of fishing on marine biodiversity. Although any such change may not be entirely attributable to the impacts of fishing, fishing (including illegal, recreational and Indigenous) is the most probable cause of this kind of change because larger (and generally more predatory fish) are the most sought after in terms of food value and are also more vulnerable to modern netting techniques.

Other indicators for this issue:

Biodiversity — Utilisation and value of biodiversity - Harvesting and trade in wildlife 

Harvesting of wild fish is the principal commercial harvesting of wild animals in Australia.

Other indicators for this issue:

Australian Antarctic Territory — Environment - Human Pressures on the environment 

Change in trophic structure in commercially exploited Antarctic species could be indicative of significant ecological change, resulting from the pressures of fishing on marine biodiversity. Although any such change may not be entirely attributable to the impacts of fishing, fishing is the most probable cause of this kind of change because larger (and generally more predatory fish) are the most sought after in terms of food value and are also more vulnerable to modern netting techniques.

Other indicators for this issue:

Further Information

Source: Scheffer, M, Carpenter, S. and de Young, B. 2005, Cascading effects of overfishing marine ecosystems. Trends.