Thursday
February 13, 2003
4:30-5:30 pm
102 Fishery Sciences
(auditorium)
Social follows talk
Robert FurnessProfessor of Biology, University of GlasgowHow Many Fish Should We Leave in the Sea for Top Predators? |
|||
When we talk about ‘top predators’ of conservation concern in marine food webs we tend to think of charismatic marine mammals, sea turtles, sharks and seabirds. In fact, some of these animals are members of relatively low trophic levels. Examples include the plankton-eating baleen whales, basking sharks, prions and many Southern Ocean penguins. These animals are often large enough to be able to avoid most predators so may indeed be top predators of very short food chains. However, many marine mammals and seabirds are predominantly fish-eaters. Fisheries can affect populations of piscivorous marine mammals and seabirds in a number of ways. There can be direct impacts due to bycatch or entanglement of animals in lost equipment. There can be indirect effects due to changes in fish stock abundance caused by fisheries. This last topic is the focus of this talk.
These indirect effects of fisheries on piscivorous marine mammals and seabirds can be of a variety of types: 1. Fisheries that harvest the food-fish of marine mammals or seabirds may reduce stocks, and this may lead to declines in predator populations dependent on these stocks for their food. 2. Some fisheries that harvest predatory fish may have quite a different effect; reducing predation impact on food fish by depleting stocks of predatory fish may permit marine mammals and seabirds to increase as a consequence of reduced competition for this food. 3. Other fisheries that harvest predatory fish may have adverse effects on seabirds. This may occur if the seabirds can only catch the food fish when they are driven to the sea surface by large underwater predators, as seen in tropical oceans where surface-feeding seabirds depend on tuna and dolphins to drive small fish to the surface. In this case, any increase in food fish abundance is less important than the reduction in food availability at the surface due to smaller numbers of underwater predators. 4. Fisheries on predatory fish may increase the food supply to scavenging seabirds by discarding at the sea surface offal, and unwanted fish caught from depths that these seabirds cannot reach. This makes available to scavenging seabirds a food supply that they cannot access naturally.
Understanding the ecological relationships between fisheries and top predators is important if fishery management is to be modified to minimize these impacts, since a lack of scientific knowledge can only be compensated by precautionary management. By definition, precautionary management to solve a conservation concern is likely to involve a greater reduction in fishery harvest than may be necessary if the science underpinning management is known. One obvious question is how much food do the marine mammals and seabirds need? Bioenergetics models show that in several quite different marine ecosystems around the world, annual consumption by seabirds is often only 2-5% of their food-fish stock, while marine mammals may take 5-10%. It would be quite wrong for at least two reasons to infer from this that fisheries could remove up to 80% or more of the fish before seabirds and mammals would be short of food. Firstly, seabirds and marine mammals are aggregated central place foragers for at least a major part of their annual cycle. So local changes in prey abundance may be more important than changes across an entire stock range. Secondly, rather than the amount of food being the critical parameter, the predators require a sufficient prey density to allow them to forage profitably. Unfortunately, this critical threshold prey density is more difficult to assess. We know that many predators select ‘patches’ where prey density is particularly high, and prey patchiness tends to increase with mean prey density, so depletion of prey stocks may rapidly reduce numbers of suitable prey aggregations. Empirical data show that seabirds and marine mammals often do have critical thresholds of prey abundance below which their foraging becomes unprofitable. One challenge for fisheries management is to avoid depletion of prey fields in the local areas of critical habitat for seabirds and marine mammals. As a case study of the complexity of interactions I will present a detailed example of the North Sea, where there have been very intense fisheries for predatory fish (primarily gadoids) and for industrial fish (sandeels for reduction to fish meal), alongside large populations of seals and seabirds of conservation importance. These fisheries not only alter food-fish abundances, but also provide enormous quantities of offal and discards that distort seabird community composition, with further interactions resulting when fishery harvest rates or discard rates change. These problems have led several ’green’ groups to call for fishery closures in the North Sea. But studies of these interactions can help fisheries managers to maintain harvests while minimizing impacts on top predators. In this particular case, local depletion of sandeels close to major seabird colonies was seen to be a conservation issue; restricting local catch of sandeels within sensitive areas when seabirds were exhibiting low productivity permitted the sandeel fishery to avoid this impact while not reducing the overall TAC for the fishery. Seabird breeding success recovered immediately after this restriction was established.
Furness, R.W. 2002. Management implications of interactions between fisheries and sandeel-dependent seabirds and seals in the North Sea. ICES J. mar. Sci. 59: 261-269.
Furness, R.W. and Tasker, M.L. 2000. Seabird-fishery interactions: quantifying the sensitivity of seabirds to reductions in sandeel abundance and identification of key areas for sensitive seabirds in the North Sea. Mar. Ecol. Prog. Ser. 202: 253-264.
Heubeck, M., Mellor, R.M., Harvey, P.V., Mainwood, A.R. and Riddington, R. 1999. Estimating the population size and rate of decline of kittiwakes breeding at Shetland, 1981-97. Bird Study 46: 48-61.
Oro, D. and Furness, R.W. 2002. Influences of food availability and predation on survival of kittiwakes. Ecology 83: 2516-2528.
Phillips, R.A., Caldow, R.W.G. and Furness, R.W. 1996. The influence of food availability on the breeding effort and reproductive success of Arctic skuas. Ibis 138: 410-419.
Piatt, J.F. and Methven, D.A. 1992. Threshold foraging behavior of baleen whales. Mar. Ecol. Prog. Ser. 84: 205-210.
Bob Furness is Professor of Seabird and Fishing Interactions at the University of Glasgow in Scotland. He has been studying seabirds in Shetland every summer for 30 years, with a particular interest in skuas, but has travelled widely as a seabird ecologist to work on islands from the sub-Antarctic to the tropics. He has a large and productive research team currently comprising 7 postdocs and 6 postgrads. Bob has supervised 40 Ph.D. students, mostly on topics in seabird ecology, but also on contaminants (especially mercury) in marine food webs, on birds as monitors of environmental change, on bioenergetics, and on diet and animal migration studies using stable isotopes and fatty acid signatures. Bob was recently seconded to industry for three years to develop a ‘seabird-friendly’ management plan for the North Sea sandeel fishery, a million ton/yr fishery considered to be in potential conflict with conservation of seabirds and seals. This work led to the first FMP in Europe that uses seabird productivity as a modifier of regional fishing effort, thereby managing to avoid conflict with conservation interests without any reduction in the overall TAC of sandeels. Bob chairs the ICES Working Group on Seabird Ecology and the IAPEME, an international panel advising the Danish Government on monitoring ecological impacts of the new marine wind farm technology rapidly becoming established off European coasts to reduce CO2 emissions. He was appointed by the Scottish Executive to advise on the selection of sites for conservation in Scotland and has served as Chair of the research committee of the British Trust for Ornithology. He is the coordinator of a large European research project ‘DISCBIRD’ that is currently evaluating the consequences of changes in discarding rates in European fisheries for seabird populations. This work includes satellite tracking of scavenging seabirds, application of data loggers, and forensic techniques to identify diets of individual birds.
1. Furness, R.W. 2002. Management implications of interactions between fisheries and sandeel-dependent seabirds and seals in the North Sea. ICES J. Mar. Sci. 59, 261-269.
3. Oro, D. and Furness, R.W. 2002. Influences of food availability and predation on survival of kittiwakes. Ecology 83, 2516-2528.