Search

Walleye pollock (Theragra chalcogramma) is the single most abundant fish species in the North Pacific Ocean. Concentrations of this species, which occur in the eastern Bering Sea and western Gulf of Alaska, support the largest single-species commercial fishery in the world. Exploitation of these stocks began in the early 1960s primarily by foreign national fleets. Currently the fishery is in a transitional phase. Over a relatively short time a combination of events dramatically reshaped the fishery from a multispecies foreign bottom-trawl fishery to a joint-venture fishery and finally to an expanding domestic presence. In the process, patterns and methods of fishing have changed as have the fishery parameters estimated from assessment models. In the past, fishery assessments were based on limited non-directed bottom-trawl survey c.p.u.e., fishery c.p.u.e., and length-frequency analysis. Beginning in 1979, the use of hydroacoustic surveys allowed the assessment of the midwater portion of the stock which normally was not available to the bottom-trawl surveys or the commercial fishery. The hydroacoustic survey provided data which made it possible to tune sequential population models. Also described are current assessment meth­ods which allow direct integration of hydroacoustic survey information into separable fishery assessment models. With an integrated approach, estimated abundance levels are more consistent with survey estimates. The hydroacoustic surveys in coming years will be especially important to the estimation of new tuning parameters. This paper reviews past catch, management, and assessment results. Also pre­sented are results from current assessments. In particular we emphasize how survey results, especially results from hydroacoustic surveys, can be integrated with age-structured fishery assessment models to examine the validity of assumptions inherent in each method.

The use of scientific survey data for assessing abundance of commercially important fish stocks has become a managerial necessity in many regions. Errors and bias in estimates from such surveys may thus have great economic impact. In the Northeast Atlantic abundance of cod has been estimated from bottom trawl surveys and acoustic surveys since 1981. In the Svalbard area, such survey abundance indices have differed considerably with the discrepancy varying, from year to year. At low stock abundance and when the population is comprised of young fish, cod are distributed close to bottom and are thus optimally located for a bottom trawl survey, but are poorly recorded by the acoustic technique. In contrast, when abundant year-classes attain an age of 2-3 years and older, they are more pelagically distributed and clearly available for acoustic observation. Dependency on only one of the survey techniques could seriously underestimate the stock. A combination of estimates from the two methods is at present hindered by the lack of an absolute estimate of abundance, and by uncertainty in the actual volume sampled volume due to movement of fish during survey sampling. The vertical profile of the acoustic recordings and size composition and density of the population are factors which will affect fish availability to the two survey techniques differently. Such variation in availability seems to affect gadoid surveys in general. With a limited amount of additional effort changes in availability-selectivity can be monitored during the survey, and observed changes can be incorporated into analyses to compensate for potential bias due to variation in these parameters.

Pacific cod have been exploited in Alaska for over 100 years. Originally, the fishery was exploited in coastal waters by U. S. fishermen. This domestic fishery declined following World War I, ceasing by the early 1950's . In the mid-1950's Japanese and Soviet distant water fleets began operations in the northeastern Pacific Ocean. Pacific cod was primarily an incidental species in the harvests of these fleets. In recent years a U. S. cod fishery has redeveloped and is rapidly expanding. Coincidental to the renewed interest in a directed U. S. cod fishery has been a strong increase in Pacific cod abundance. Most of the increase appears to have been due to the occurrence of an exceptionally strong 1977 year-class. Since cod is becoming a major commercial fishery in the northeastern Pacific Ocean it is important to develop long term estimates of yield and accompanying management strategies. One shortcoming of such efforts is the short time series of cod data i n Alaskan waters. Some consequences resulting from above-average cod recruitment in 1977 and the current high abundance level are examined.