Dietary changes of seabirds indicate shifts in pelagic food webs

Long-term monitoring of prey deliveries to chicks by gannets and guillemots have provided useful indices of annual variability in the timing of the inshore movements, relative abundance, sizes and reproductive conditions of pelagic fishes and cephalopods in the Northwest Atlantic. From 1977 through 1994, prey harvests by gannets tended to shift from warm-water, long-distance migrants (mackerel, squid, saury) to cold-water residents (capelin). Annual variation in prey harvests by gannets and guillemots indicated 3- to 4-week delays in the inshore migration, reproductive maturation and spawning by capelin during cold sea surface events in the 1990s. Cold surface water anomalies appear to have influenced pelagic food webs by inhibiting the movements of migrant, warm-water pelagic fishes and cephalopods into inshore regions in the Northwest Atlantic. These oceanographic influences may have amplified the effects of the overfishing of short-finned squid in the region in the late 1970s/early 1980s. We contrast recent effects of cold water perturbations on pelagic fishes with contentions about environmental influences on the demise of demersal cod in the Northwest Atlantic.