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

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 (O. 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): 45.82.55-49.22.76 mm) for each habitat. As a result, prey fry were ca. 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.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author