Functional response of juvenile pink and chum salmon: effects of consumer size and two types of zooplankton prey

Abstract

Feeding rate experiments were conducted for pink salmon Oncorhynchus gorbuscha fry [mean fork length (LF) 39 mm], juveniles (103–104 mm LF) and juvenile chum salmon Oncorhynchus keta (106–107 mm LF). Fishes were presented with small copepod (Tisbi sp.) or larger mysid shrimp (Mysidopsis bahia) prey at varying densities ranging from 1 to 235 prey l−1 in feeding rate experiments conducted at water temperatures ranging from 10·5 to 12·0° C under high light levels and low turbidity conditions. Juvenile pink and chum salmon demonstrated a type II functional response to mysid and copepod prey. Mysid prey was readily selected by both species whereas the smaller bodied copepod prey was not. When offered copepods, pink salmon fry fed at a higher maximum consumption rate (2·5 copepods min−1) than larger juvenile pink salmon (0·4 copepods min−1), whereas larger juvenile chum salmon exhibited the highest feeding rate (3·8 copepods min−1). When feeding on mysids, the maximum feeding rate for larger juvenile pink (12·3 mysids min−1) and chum (11·5 mysids min−1) salmon were similar in magnitude, and higher than feeding rates on copepods. Functional response models parameterized for specific sizes of juvenile salmon and zooplankton prey provide an important tool for linking feeding rates to ambient foraging conditions in marine environments, and can enable mechanistic predictions for how feeding and growth should respond to spatial‐temporal variability in biological and physical conditions during early marine life stages.