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Ecosystem Monitoring and Assessment Program

Use of Shelf, Slope, and Basin Habitat by Age-0 Walleye Pollock and Pacific Cod in the Gulf of Alaska

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6 age-0 pollock
Figure 1. Age-0 walleye pollock captured in a surface trawl. Photo by John Eiler.

Walleye pollock and Pacific cod  are widely distributed across the Gulf of Alaska (GOA) and are commercially and ecologically important species. Recruitment estimates based on the latest stock assessments for both species are highly variable. Prior to the mid-1980s, pollock recruitment in the GOA was correlated with larval mortality, which is believed to have been largely influenced by environmental conditions. However, this relationship deteriorated following a major environmental shift in the mid-1970s, and it appears that the critical stage for recruitment of pollock is now largely dependent on conditions experienced during the post larval, juvenile life stage.  AFSC scientists are in the process of developing a new, long-term monitoring survey designed to assess the health, abundance, and distribution of age-0 pollock and P. cod and other marine species with the goal of providing information to aid stock assessment and resource management efforts in the GOA.

Age-0 pollock (Fig. 1) and P. cod were collected in surface trawls with other species during July and August 2012 as part of the North Pacific Research Board’s GOA Project. This survey sampled stations along a series of 128-km long transects located west of the Alaska panhandle and 300-km long to the east of Kodiak Island, Alaska (Fig. 2) and was designed to compare processes occurring in two distinctly different regions of the GOA. Age-0 pollock and P. cod inhabiting continental slope waters of the GOA during summer acquired higher energy stores than those occupying the shelf and basin, suggesting that individuals remaining over the slope were better conditioned to survive winter. Individuals inhabiting slope waters in the eastern GOA on average contained higher energy stores relative to those over the shelf and basin as well, although statistically significant differences in somatic energy density were not detected.

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Figure 2. Abundance of catch during the summer 2012 Gulf of Alaska survey for (A) Pacific cod and (B) walleye Pollock.

Increased energy reserves and overwinter survival of age-0 pollock and P. cod in the Bering Sea have been directly related to cool ocean conditions caused by late ice retreat which support the production of large, energy-rich Calanus spp. copepods during late summer. Mechanistic explanations of how climate and temperature may affect copepod production in the GOA are limiting; however, zooplankton guilds are believed to be segregated according to ocean (offshore) habitat in the northern GOA.

Previous investigations report that the ocean zooplankton community contains larger copepod species such as Neocalanus christatus and Eucalanus bungii whereas the shelf community contains smaller species such as Calanus marshallae and Pseudocalanus spp., and the transition zone separating these communities commonly crosses the shelf break, allowing for the incursion of oceanic species up onto the shelf. Thus, age-0 pollock and P. cod located over the slope may have more opportunities to forage on larger, more energy-rich copepods.

P. cod were longer and heavier than pollock across all habitats in the GOA, and their life history strategy appears to favor the allocation of energy toward increasing body size. Larger fish are better conditioned to avoid starvation, have an improved immune response, and a greater thermal tolerance. Age-0 pollock inhabiting shelf and slope waters of the GOA are likely allocating energy toward storage during summer months in order to limit foraging explorations and exposure to predators during winter.

Pollock and P. cod were equally abundant in the central GOA and overlapped substantially in their distribution. Pollock were also widely distributed and abundant in the eastern GOA where P. cod were seldom encountered. Density-dependent interactions may occur in areas with spatial overlap of juvenile fish occupying similar guilds, and given the large dietary and spatial overlap, density dependence may be an issue in the central GOA during summer months.

By Jamal Moss, Marilyn Zaleski, and Ron Heintz

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