What’s Eating Juvenile Salmon?

A new study on predation mortality of juvenile salmon from the Columbia River Basin suggests that several conditions near the mouth of the Columbia River are useful indicators of potential juvenile salmon mortality that could be helpful in salmon management.

The study by Beth Phillips, titled “Characterizing juvenile salmon predation risk during early marine residence,” was published online at www.journals.plos.org. Phillips is a post-doctoral research associate with the Northwest Fisheries Science Center in Seattle.

“While predation mortality is often assessed using direct observations of prey consumption, potential predation can be predicted from co-occurring predator and prey densities under varying environmental conditions,” Phillips wrote in an abstract on the study.

Her research team opted to estimate smolt predation risk based on observations of piscivorous seabirds and local densities of alternative prey fish, including northern anchovy in Oregon and Washington coastal waters during May and June of 2010-2012. They evaluated predation risk relative to availability of alternative prey and physical factors, including turbidity and Columbia River area plume and compared risk to returns of adult salmon. River plumes are the region where the most intense river-sea-land interaction occurs. They are characterized by complex material transport and biogeochemical processes.

The presence of seabirds and smolts consistently occurring at sampling stations throughout most of the study area indicate that juvenile salmon are regularly exposed to avian predators during their early marine residence. The study found that predation risk for juvenile coho, yearling Chinook and subyearling Chinook salmon was on average also present.

Predation risk was greater in turbid waters and less as water clarity rose. Juvenile coho and yearling Chinook salmon predation risk was lower when river plume surface areas were greater that 15,000 km2 while the opposite was estimated for subyearling Chinook salmon, the study found.