Processes affecting the productivity of capelin and pollock in the Gulf of

The over-arching goal of this project was to investigate the processes affecting the productivity of capelin and walleye pollock, important forage fishes in the Gulf of Alaska (GOA). The first objective was to investigate the key physical processes by comparing the distribution of fish with oceanographic properties and the distribution of prey. The second objective was to investigate the potential for interspecific competition by comparing measures of foraging success between capelin and juvenile pollock. The field study to address these objectives was conducted in Barnabus Trough off Kodiak Island during September 2005 on the NOAA Vessel Miller Freeman. The distribution of fish was assessed with fisheries acoustics and mid-water trawls. Fish were collected from trawls for stomach contents. Physical and biological data were collected with conductivity-temperature-depth (CTD) probes and zooplankton tows. Our results support the linkage between oceanography, prey, fish diet and fish distribution. We found that age-0 pollock were distributed in cool, high-salinity waters offshore of a mid-trough front, coincident with the distribution of the bulk of their preferred prey, euphausiids. In contrast to pollock, we found that capelin were distributed throughout the trough, as was the distribution of their dominant prey, copepods. Although capelin and pollock had different diets in 2005, in a previous year’s survey (2004) they were both found inshore of the mid-trough front foraging on euphausiids, suggesting the potential for competition. We found that sympatric capelin (occurring with pollock) often had reduced foraging success compared to allopatric capelin (occurring alone). Our results also suggested that age-0 pollock were the superior competitor of the two species and that the exclusion of capelin from foraging on euphausiids can have negative consequences for growth and potentially survival.