Effects of the Sitka Eddy on juvenile pink salmon in the eastern Gulf of Alaska

A fisheries oceanographic survey has collected physical and biological data from the eastern Gulf of Alaska during the month of July since 2010. The Sitka Eddy is a mesoscale feature that can impact physical and biological characteristics of the eastern Gulf of Alaska, and it can occur during July. Herein, the historical presence of a Sitka Eddy during July in this region was examined between 1993 and 2015 using satellite-derived altimetry, and interannual distributions of juvenile pink salmon (Oncorhynchus gorbuscha) were compared between 2010 and 2015. Biological characteristics of juvenile pink salmon and oceanographic conditions were compared between 2010 (strong eddy) and 2012 (weak eddy), and a further analysis across the Sitka Eddy was conducted for 2010. The Sitka Eddy occurs regularly in the eastern Gulf of Alaska, but strong events that occurred during July and likely to impact migrating salmon were only evident in 13% of the 23 years examined. Juvenile pink salmon catch distribution appeared to be deflected offshore by the Sitka Eddy in July of 2010, compared to nearshore distributions in 2011 through 2015. In 2010, temperatures were warmer, chlorophyll-a had a greater range, and juvenile pink salmon were distributed farther offshore as compared to 2012. Juvenile pink salmon diets were dominated by euphausiids in 2010 and large copepods in 2012. Additionally, Fulton's condition factor (K), insulin-like growth factor I (IGF-I), and whole body energy content (WBEC) were all lower in 2010. In 2010, temperatures reached a station maximum of 12.60 °C on the southeastern edge of the Sitka Eddy, and chlorophyll-a concentrations reached a local minimum of 0.077 µg/l in the center of the eddy. Juvenile pink salmon from the eastern edge of the Sitka Eddy had significantly elevated values of K and IGF-I (p < 0.05), but WBEC did not vary significantly across the Sitka Eddy. Fish north of Cross Sound, which was outside of the Sitka Eddy, exhibited significantly lower K-values (p < 0.05) and relatively lower values of IGF-I and WBEC as compared to conspecifics south of Cross Sound. While the Sitka Eddy may act as an oasis offshore, when it impinges onto the shelf, it can squelch normal coastal production. By recognizing interannual variation in eddy magnitude, location, and impact to physical and biological characteristics, we can better understand and interpret interannual differences in the Gulf of Alaska ecosystem.