Productivity of Pacific Herring (Clupea harengus pallasi) in the Eastern Bering Sea under Various Patterns of Exploitation

Abstract

Pacific herring (Clupea harengus pallasi) is a major food source for western Alaska native people and has been commercially exploited in the eastern Bering Sea since the early 1900's. Commercial harvests were small and localized in coastal waters until foreign factory fleets located and developed a fishery on wintering herring concentrations in the early 1960's. Harvests peaked near 150 000 t in the early 1970's and then declined along with catch per unit effort. Foreign harvests were eliminated following establishment of the United States 200 mile Fishery Conservation Zone. In recent years a fishery has developed in State of Alaska coastal waters which harvests herring for sac roe (ovaries) during the spring spawning period. Proposals have been put forth by trawl fishermen to reestablish a food and bait fishery within Federal waters. Development of offshore mixed stock fisheries has been opposed by inshore commercial and subsistence users who fear that stocks will be overexploited. While both State and Federal managers have agreed to give subsistence users and inshore domestic commercial fishermen top priority, they have been unable to agree upon plans for dealing with potential offshore commercial harvests. In this paper we present results of a computer model that we developed to examine effects of various fishing patterns upon herring productivity and yield. Within our model, maximum sustainable yield (MSY) is achieved at an exploitation rate (E) of 0.3 (i.e. harvest of 30% of total spawning biomass). However, since stocks still appear to be below MSY biomass and since productivity and yield drop sharply at E values greater than 0.3, we suggest that an E of 0.2 be maintained under current conditions. This will result in a potential loss in yield of only 7% from an E of 0.3, but will allow a 52% increase in spawning biomass. Four fishing patterns in which both discrete and mixed stock fishery removals were allowed to occur were also examined. During years in which inshore fisheries fail to harvest 20% of available spawning biomass, an offshore allocation of up to 10 000 t could be permitted with minimal risk to damaging the reproductive potential of small spawning stocks. However, results indicated that mixed stock fisheries should be restricted to lower levels than would be appropriate for fisheries targeting on discrete stocks to avoid risks of overharvesting some stocks.