Small biased body size of salmon fry preyed upon by piscivorous fish in riverine and marine habitats

Abstract

Predator–prey interactions must be considered when aiming to enhance populations by releasing artificially reared individuals into natural environments. Released individuals create an abundance of prey for predators, and a basic hypothesis of predator–prey interactions suggests that predators select large prey due to the high caloric content. An alternative hypothesis is that small individuals are vulnerable to predation due to their poor predator avoidance. This study tested these hypotheses using stocked chum (Oncorhynchus keta) and masu (Oncorhynchus masou) salmon fry and piscivorous salmonids in marine and riverine habitats in Hokkaido, Japan. Stomach contents were sampled from predators, and fork length of prey fry was measured. Then, their fork length was compared with whole stocked fry (range of mean fork length (±SD): 45.8 ± 2.55 to 49.2 ± 2.76 mm) for each habitat. As a result, prey fry were ∼3%–6% smaller than whole stocked fry, even under a prey-abundant condition (i.e., just after hatchery reared salmon fry were stocked). Piscivorous salmonids pursue schooling fry, and small fry may be easily caught due to their slow speed in avoiding predators.