Management implications of interactions between fisheries and sandeel-dependent seabirds and seals in the North Sea

Robert W. Furness 0 0 Robert W. Furness: Institute of Biomedical and Life Sciences , Graham Kerr Building , University of Glasgow , Glasgow G12 8QQ , UK The lesser sandeel, Ammodytes marinus, is a key food for many seabirds and seals, and is also the target of the largest single-species fishery in the North Sea. Despite claims that sandeel fishing has harmed dependent predator populations, census data show that most seabirds and grey seals increased in numbers as the fishery grew and reached peak harvest. Generally high breeding success of black-legged kittiwakes at North Sea colonies also suggests that sandeel abundance has remained good for breeding seabirds at the broad scale, though local and small-scale effects of sandeel fishing should not be overlooked. VPA and CPUE data suggest that abundance increased as the fishery grew. A negative correlation between sandeel recruitment and total stock size preceding spawning suggests that there is now resource competition (bottom-up control). Bioenergetics modelling indicates that predatory fish take far more sandeel than taken by the industrial fishery or wildlife. Effects of decreases in predatory fish stocks have been greater than increases in the take by seabirds and seals and by the fishery. Thus, overall, there appears to have been a reduction in mortality during the last 30 years. Changes in predatory fish abundances, especially mackerel and whiting, may influence sandeel stocks more than changes in industrial fishery, at least at the scale of the North Sea as a whole. These interactions imply that seabird and seal food supply in terms of sandeel may be strongly dependent on decisions regarding management of stocks of mackerel and gadoids. The overwhelming influence of predation on ''food-fish'' by predatory fish may be a feature of many marine food webs worldwide, where ''fishing down the food web'' has occurred, and this has clear management implications if wildlife and fisheries are to coexist. 1054-3139/02/040261+09 $35.00/0 - The North Sea has been intensively fished over many decades (Greenstreet et al., 1999a,b). There are detailed data to describe the fisheries and fish stocks (Greenstreet et al., 1999a,b; ICES, 2000), and populations, diets, and breeding success of both seabirds (Lloyd et al., 1991; Furness and Tasker, 1999; Upton et al., 2000) and seals (Hammond et al., 1994; Brown and Pierce, 1998; Pomeroy et al., 2000). Because of detailed data, the history of intensive fishing, and the large populations of seabirds and seals, the North Sea is an ideal area for the investigation of interactions between fisheries and wildlife. This topic has been the focus of much recent attention (Furness, 1990, 1999; Harris and Wanless, 1997; Rindorf et al., 2000), such that the North Sea example features as a text book case study on fisherywildlife interactions (Jennings et al., 2001). Certainly, the North Sea provides an outstanding example of fishing down the food web (Pauly et al., 1998). Concern about overexploitation of large predatory fish such as cod, Gadus morhua, was expressed as long ago as the 1880s (Daan et al., 1990). Since the 1950s there has been a large increase in the harvest of sandeel (predominantly the lesser sandeel, Ammodytes marinus, but several different species as well as different sub-stocks are managed as a single unit in this fishery). Growth of sandeel stocks followed major reductions in the stocks of cod, haddock, Melanogrammus aeglefinus, whiting, Merlangius merlangus, herring, Clupea harengus, and mackerel, Scomber scombrus, and one hypothesis suggests that sandeel increased as a result of the reduced predation and competition with other fish (Sherman et al., 1981). Fishery landings increased during the 20th century, but with an increasing proportion of the catch comprising industrial fish rather than human consumption fish. The North Sea fish community shows the effect of intensive harvesting as reductions in the size of the individual fish in the community, and a reduction in the proportion of large predatory fish (Greenstreet et al., 1999a,b; Jennings et al., 2001). A recent review of the effects of aquaculture on world fish supplies (Naylor et al., 2000), suggested that the growth of aquaculture posed a threat to worldwide marine food chains through its demand for aquafeeds. These are manufactured predominantly from fishmeal and fish oils derived from industrial fisheries. The authors claimed that large-scale industrial fisheries are incompatible with thriving populations of marine predators, and stated The impact of pelagic fisheries depletion is thought to reduce available food supplies for marine predators. . . . In the North Sea, for example, overexploitation of many capelin [sic], sandeel and Norway pout stocks, mainly for reduction to fishmeal, has been implicated in the declines of certain stocks of other wild fish . . . and in the distribution, population sizes and reproductive success of various seal and seabird colonies. This is both factually inaccurate, and highly misleading. As pointed out by Furness and Tasker (2000) and Yodzis (2001), North Sea seabirds have not shown declines in numbers or dramatic reductions in breeding success that can be attributed to overexploitation of food-fish by industrial fisheries. This lack of a damaging effect contrasts with various predictions that wildlife will suffer through competition for food-fish (Yodzis, 1994, 2001), and so could be considered counter-intuitive. I review the evidence for interactions between industrial fisheries, stocks of industrial fish, and populations of top predators in the North Sea (seabirds, seals, and those larger fish that feed extensively on industrial fish stocks) in trying to explain the apparently counterintuitive observation that a very large industrial fishery has developed in the North Sea alongside growing populations of sandeel-dependent seabirds and seals. Finally, I consider fisheries management and wildlife conservation implications of these relationships among North Sea fisheries, fish stocks and wildlife, and comment on the extent to which the observed scenario may be typical of heavily fished shelf-sea ecosystems elsewhere in the world. The sandeel fishery is the largest industrial fishery in the North Sea, and the principal fishery of concern as potentially in competition with wildlife, because sandeel provide an important part of the diet of many top predators. The other major industrial fishery is for Norway pout, Trisopterus esmarkii, but Norway pout is either not eaten or forms only a tiny part of the diet of seabirds and seals in the North Sea (Hammond et al., 1994; Furness and Tasker, 1997; ICES, 1997; Brown and Pierce, 1998; Tasker et al., 2000). Therefore any effects of the pout fishery on these top predators are likely to be very much less than those of the sandeel fishery. For this reason, the pout fishery will not be discussed. The spatial scale of interactions Sandee (...truncated)