It is clear that discards from commercial fisheries are a key food resource for many seabird species around the world. But predicting the response of seabird communities to changes in discard rates is problematic and requires historical data to elucidate the confounding effects of other, more 'natural' ecological processes. In the North Sea, declining stocks, changes in technical measures, changes in population structure and the establishment of a recovery programme for cod (Gadus morhua) will alter the amount of fish discarded. This region also supports internationally important populations of seabirds, some of which feed extensively, but facultatively, on discards, in particular on undersized haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus). Here we use long-term data sets from the northern North Sea to show that there is a direct link between discard availability and discard use by a generalist predator and scavenger--the great skua (Stercorarius skua). Reduced rates of discarding, particularly when coupled with reduced availability of small shoaling pelagic fish such as sandeel (Ammodytes marinus), result in an increase in predation by great skuas on other birds. This switching of prey by a facultative scavenger presents a potentially serious threat to some seabird communities.
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Thls paper provides the first proof that shortfound that, although chicks are fed daily for the first tailed shearwaters Puffinus tenuirostris breeding in Australia
The survival rates of breeding adult Great Skuas Catharacta skua were examined at Foula, the largest colony in the world, where numbers have been declining since the late 1970s. Resightings of colour‐ringed breeding adults over a 12‐year period were analysed using Cormack‐Jolly Seber models to estimate survival rates. Annual survival rates averaged 0.89 but varied among years between 0.82 and 0.93, with annual variations being temporally associated with variations in sandeel abundance during the breeding season. Most birds appeared to die outside the breeding season and so it is possible that nutritional stress and reproductive costs of breeding in years of poor food supply affect survivorship on migration or in the wintering range. Survival rates of adult Great Skuas were affected by their age according to a quadratic equation, with survival increasing significantly with age from 0.73 in 5‐year‐old‐birds to between 0.85 and 0.96 in birds from 7 to 22 years old, with a sharp decline to between 0.75 and 0.87 in birds over 22 years old. Year effects were evident when controlling for age, indicating that annual variations in survival rates are not explained by changes in age‐composition of the marked population among years.
Between March 1986 and March 1987, the stomach contents of 236 little penguins on Penguin Island, Western Australia, were obtained using an emetic. The 1392 prey items identified included 16 fish species, one squid and one prawn, but four fishes comprised most of the birds' diet. Hyperlophus vittatus was taken throughout the year, Sardinops neopilchardus and Hyporhamphus melanochir mainly in winter and Spratelloides robustus during spring/summer. The penguins are largely opportunistic in their foraging and their diet appears to be similar to the fish species caught locally by commercial bait fishermen.
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