The utilization of cold-water zooplankton as prey for chum salmon fry (Oncorhynchus keta) in Yamada Bay, Iwate, Pacific coast of northern Japan

Abstract

The food availability of chum salmon fry is one of the principal factors affecting survival in their coastal residence period. The aim of this study was to better understand the quality and quantity of available food resources and the feeding habits of salmon fry in nearshore waters. We examined stomach contents of salmon fry and the zooplankton community structure of the water column during salmon fry’s coastal residence period (April and May) over four years in Yamada Bay, a coastal embayment of the Pacific Ocean in northern Japan. The mean water temperatures during study period were low in 2014 and 2013 (6.7 and 8.3 °C, respectively),and high in 2016 and 2015 (11.1 and 9.4 °C, respectively). Neocalanus plumchrus and Themisto japonica, typical cold-water zooplankton dominated the stomach contents of chum fry (except for 2016), however, they were not necessarily the most dominant in the water column, suggesting that chum fry selectively preyed upon these cold-water zooplankters. On the other hand, chum fry preyed primarily on decapod larvae in 2016 the warmest water year. Condition factor of chum fry were higher in cold-water years than those of warm-water years. Because these two cold-water zooplankton species are typically abundant in the Oyashio Current, the magnitude of Oyashio inflow may affect prey availability for chum salmon fry and their nutritional status. The relationship between the food environment at the coastal residence period of chum fry and the adult return rate after three years was also discussed.