Transported zooplankton from the Okhotsk Sea facilitate feeding and growth of juvenile walleye pollock on a continental shelf along the Oyashio Current, western subarctic Pacific

The advective transport of zooplankton provides an important source of energy on continental shelves. On a continental shelf along the Oyashio Current, western subarctic Pacific (42°30′N, 144°00′E), the extant water mass is replaced biannually by inflows of water masses from the highly productive Okhotsk Sea. These inflows and accompanying zooplankton are hypothesized to enhance biological production on the shelf. In this study, seasonal feeding and growth of a juvenile planktivorous fish, walleye pollock (Gadus chalcogrammus), on the shelf were examined in relation to water mass exchanges from September 2003 to June 2005. Zooplankton were sampled from February to April in 2004 and 2005, and in April 2014 and 2015. Juvenile pollock frequently consumed an Arctic copepod, Calanus glacialis, and a boreal euphausiid, Thysanoessa inermis, after January when the shelf was occupied by cold water masses that originated from the southwestern Okhotsk Sea. Juvenile pollock whose diet contained higher contributions of these energy-rich prey items exhibited positive growth in February–March 2005, whereas fish with lower proportions of these prey exhibited little growth during this period in 2004. The winter distributions of the copepod and euphausiid extended from a continental slope area in the southwestern rim of the Okhotsk Sea to the shelf along the Oyashio Current. Together, these results suggest the transport of zooplankton from the Okhotsk Sea likely influences the biological production of the coastal Oyashio ecosystem by enhancing the availability of energy-rich prey for juvenile pollock.