NOAA Technical Memorandum
NMFS-AFSC-29
Estimation of salmonid bycatch in the 1989 Japanese squid driftnet fishery
Abstract
Salmonid (Oncorhynchus spp.) bycatch data from the 1989 U.S., Canada, and Japan monitoring program for Japanese squid driftnet vessels were analyzed. Sampling effort was consistently less than expected by proportionate sampling between 170°E and 180° long., especially during the first part of the month when capture of salmonids would most likely occur. Salmonid incidence (the proportion of monitored operations encountering salmonids) was highest at the western and eastern boundaries of the fishery.
Fishing-effort data for the bycatch calculations were divided into three categories: a portion represented by observer data, an unrepresented portion that could be estimated by extrapolation of sampled data, and an unrepresented portion that could not be estimated. The observers reported a catch of 79 salmonids in 57 monitored operations (4% of total fishing effort). For the estimated portion (81% of total fishing effort), a bycatch estimator based on an extension of Aitchison's delta distribution produced a bycatch estimate of 2,648 salmonids. Although the unestimated portion of Japanese fishing effort is small, salmonid bycatches there may have been significant; a Republic of Korea research vessel operating in an unsampled portion of the fishing area and using 37 tans of driftnet caught 13 salmonids. This represents over 16% of the total salmon observed in 1,427,225 tans through the monitoring program.
The problem of unestimated strata may be unique to migratory species like salmonids which are found only within a small portion of the total fishing area. In addition, the warm ocean conditions of 1989 probably displaced salmonids northward, restricting the amount of fishing area intersecting salmonid waters, and inducing some of the estimation problems associated with our data.
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