Will Reduced Discarding Help or Harm Seabird Populations?

Will Reduced Discarding Help or Harm Seabird Populations?

Robert W. Furness

Will Reduced Discarding Help or Harm Seabird Populations?This is part of Ecosystem Approaches for Fisheries Management
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About 27 million t of fish was discarded worldwide in 1993. Particularly high discarding levels occur in heavily exploited mixed fisheries where possibility for gear selectivity is constrained. Clearly reduction of discarding is desirable. The FAO (Food and Agriculture Organization of the United Nations) estimates that a reduction of at least 60% in bycatch is possible by the year 2000. However, discards now form a major food supply for scavenging seabirds. In the North Sea, discard consumption by seabirds varies, being highest in winter, highest in the northwest North Sea, and high for offal (94-100%) and roundfish (70-92%) but low for flatfish (10-35%), elasmobranchs (12%), and benthic invertebrates (3-17%). Larger scavenging seabirds steal fish from smaller seabirds and obtain higher intake rates. Technical measures to reduce discarding are likely to increase competition, reducing scavenging success for smaller seabirds. Technical measures that increase the mean size of discards (such as increased net mesh size) will particularly reduce food supply to the smaller scavenging seabirds. Severe effects on breeding success or population size of scavenging seabirds following reductions in discarding have already been documented.

Discards form more than half the breeding season diet of skuas. Skua colony sizes in the North Sea are many times larger than those in the sub-Antarctic where skuas feed predominantly by predation on seabirds. The increase in skua numbers in the North Sea seems to have been made possible by the provision of discards. Reducing discards available to seabirds, leading to switching diet, may have a severe impact on other seabird populations, since skuas are likely to switch to predation on other seabirds. The large populations of skuas in the North Sea might extirpate kittiwakes and other seabirds before their own populations fall to sustainable levels.

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