Consumption of juvenile chum salmon by a seabird species during early sea life

Abstract

Juvenile mortality, a key factor determining the stock sizes of salmonids, may be increased by predation both in rivers and along shores. Information on predation at sea, however, is scarce. We analysed the diet of a pelagic seabird, the rhinoceros auklet Cerorhinca monocerata, breeding on an isolated island off eastern Hokkaido, Japan, and estimated their consumption of juvenile chum salmon Oncorhynchus keta. The auklets brought juvenile chum salmon and Japanese anchovy Engraulis japonicus for their chicks and, in addition, fed on squid themselves. The fork length of most of the juvenile chum salmon was over 80 mm, indicating that they were migrating to offshore nursery area, the Sea of Okhotsk. Using a simple bioenergetics model, the consumption of juvenile chum salmon by auklets during the first half of chick-rearing period (late June to middle July) was 316.6 t in 2014 (296.4 t by adults, 20.2 t by chicks) and 11.7 t in 2015 (0 t by adults, 11.7 t by chicks). The otoliths of six out of 66 juvenile salmon found in the diet for chicks had thermal markings and indicated that these fish were released from the rivers along Pacific coast of Hokkaido. Our study first demonstrates the extent of the auklets' consumption contributing to the reduction of the survival in the open coastal area of juvenile chum salmon with the known stock origins.