Interactions Between Commercial Fishing and Walleye Pollock

by Chris Wilson, Anne Hollowed, Michiyo Shima¹, Paul Walline, and Sarah Stienessen

Example echograms illustrating echosign layers attributed to pollock (A) and capelin (B) from the August 2001 fishery interaction study.

Scientists from the Alaska Fisheries Science Center (AFSC) initiated a multiyear field experiment  in August 2000 near Kodiak Island in the Gulf of Alaska (GOA) to characterize the effects of commercial fishing activity on the distribution and abundance of walleye pollock (Theragra chalcogramma) over short spatiotemporal scales.  The work forms part of a larger research effort designed to determine whether commercial fishing activities significantly impact the availability of walleye pollock and other forage fish species, such as capelin (Mallotus villosus) to endangered Steller sea lions (Eumetopias jubatus).

The impetus for the work was a need to understand possible mechanisms that produced the precipitous decline in the western stock of Steller sea lions, which began in the 1970s.  One of several explanations offered for the declining number of Steller sea lions was that large-scale commercial fisheries, such as those for walleye pollock and Atka mackerel (Pleurogrammus monopterygius), compete with sea lion populations by reducing the availability of potential prey in relatively localized areas.  The home range of a foraging Steller sea lion could be considered a localized area.  A reduction in prey availability may result from a reduction in prey abundance or a disruption in the spatial patterns of the sea lion prey.  The spatiotemporal extent of the perturbation to the prey field could determine the impact on the foraging success of the sea lion.  For example, fishing removals may cause a decline in the abundance of a prey species within a localized area, but recovery to prefishery levels may be so quick that impacts to predator foraging success would be negligible.  Alternatively, disturbances from fishing operations may elicit longer-term behavioral responses by prey species that might affect spatial patterns and impact Steller sea lion foraging behaviors. For example, disturbed fish might move deeper in the water column to form smaller, denser aggregations. This change may adversely impact the foraging behavior of Steller sea lions.   Unfortunately, no data exist to answer two important questions regarding interactions between commercial fishing, Steller sea lions, and their prey.  First, do commercial fishing activities affect the distribution and abundance of potential Steller sea lion prey species significantly?  And secondly, if the fishery induces perturbations in prey spatial patterns or abundance, how do these perturbations impact Steller sea lion foraging success?

The primary goal of the AFSC study was to develop a field program to investigate the first question of whether measurable changes in spatial patterns and abundance occurred in walleye pollock at scales relevant to sea lion foraging.  This article reports on preliminary results from the first 2 years of the field study to examine the spatiotemporal characteristics of pollock and other sea lion forage species before, during, and after a commercial fishing season.  Future directions for this type of fishery-interaction research are also discussed.

• Introduction
• Study Area/Season
• Field Methods
• Data Analysis
• Results and Discussion
• August 2001 Fishery
• Geographical Distribution and Abundance
• Vertical Distribution
• Patch Analysis
• Summary
• Future Research

Continued>>> 

¹School of Aquatic and Fishery Science, University of Washington

AFSC Quarterly
Research Reports
Jan-March 2002