The Prey and Diving Depths of Seabirds on Hornoy, North Norway after a Decrease in the Barents Sea Capelin Stocks

Abstract

Following a supposed decrease in food availability during the breeding season as a result of a collapse in the Barents Sea capelin Mallotus villosus stock in 1986/87, it was hypothesized that there would be an increase in food competition and partitioning between the seabird species breeding in the region. A comparison of the diets collected from seven seabird species breeding on Hornoy in 1983 and 1989 showed that, despite a much lower capelin stock they actually caught a higher proportion of capelin in 1989 but fewer sandeels Ammodytes sp. It seems that the seabirds on Hornoy were more dependent on the sandeels than initially anticipated, and that sandeels have declined in abundance. We propose that the seabirds on Hornoy may also exploit a separate, late-spawning fjordic stock of capelin during the chick-rearing period. The median maximum diving depths attained by Common and Brunnich''s Guillemots Uria aalge and U. lomvia were ca. 50 m, while Razorbills Alca torda, Puffins Fratercula arctica and Shags Phalacrocorax aristotelis reached 25-30 m (median). These depths reflected the distribution of prey more than the birds'' diving abilities.