The ability of gadoids to take advantage of a short-term high availability of forage fish: the example of spawning aggregations in Barents Sea capelin
During 11 March to 4 April 2002, the distribution of Barents Sea capelin Mallotus villosus along the coast of Finnmark, northern Norway, was covered four times by combining acoustic survey with trawling, synoptically and simultaneously sampling capelin and its main fish predators; cod Gadus morhua, haddock Melanogrammus aeglefinus and saithe Pollachius virens. The surveys demonstrated how these gadoid predators were able to exploit such a short-term abundance of forage fish. The predator aggregation as well as the stomach fullness and proportion of capelin in their diet followed the capelin spawning migration, increasing in areas and periods with increasing capelin abundance. Capelin clearly constituted most of the biomass in stomachs of cod (97%), haddock (87%) and saithe (96%). The stomach fullness was highest in cod and lowest in haddock, although in areas with low capelin abundance, saithe had more capelin in their stomachs. The total length (LT) of capelin in predator stomachs increased with predator LT, but the proportion of capelin in the diet was not influenced by predator LT. The capelin in predator stomachs was significantly smaller than capelin in the trawl hauls, also when compared within the same sex, indicating feeding selectivity towards weaker individuals. Female capelin, being significantly smaller than the males, predominated in the diet of haddock, whereas in cod and saithe the sex ratio was more equal. Male capelin predominated in the predator diet during the pre-spawning period, whereas the females predominated as the spawning commenced. During the overall study period, most of the female capelin in predator stomachs was in a pre-spawning or a spawning stage, whereas the majority of the males appeared to be spent. Regardless of sex, the percentage of spent, relative to pre-spawning or spawning capelin in the diet of the predators, followed the capelin spawning dynamics, increasing with time as the spawning progressed.