Research Document - 1999/179
Assessment of by-catch in the 1997 and 1998 shrimp trawl fisheries in British Columbia, with emphasis on eulachons
By D.E. Hay, R. Harbo, J. Boutillier, E. Wylie, L. Convey, and P.B. McCarter
Abstract
A coast-wide observer program to determine the composition of catches in shrimp trawls in British Columbia started in 1997. The objective was to sample catches approximately according to the fishing effort, season, area and type of gear. This report presents the results for all bycatch species for both years, but concentrates on the bycatch of an anadromous smelt (Osmeridae), the eulachon (Thaleichthys pacificus). There is a potential conservation concern for eulachons in many areas of the BC coast. There are relatively few eulachon populations, perhaps 15 in BC and only 30-40 throughout the entire range, from California to Alaska. Nearly all populations in the south have declined sharply in recent years. Therefore one objective of this paper was to estimate eulachon bycatch from different areas of the coast and from the two major fishing gears (otter trawl and beam trawl). A second objective was to comment on the bycatch of other species. We present two basic methods for estimating eulachon bycatch. One is based on the relative proportion of eulachons to shrimp in the catches. This method is applicable mainly for eulachons and was used for in-season monitoring of eulachon catches. In this paper the authors show that catch ratio estimates based on the means of individual tows, called MI-ratio estimates ('mean in-season catch ratio' estimates) appear to overestimate bycatch. Catch ratio estimates based on the ratios of the cumulative totals eulachon and shrimp catches, PI-ratio or 'pooled in-season catch ratio estimates, are summed over an aggregate of time or space (i.e. a Shrimp Management Area). It is concluded that PI-ratio estimates are more reliable and useful for in-season monitoring of bycatch. A 'post-season' method for estimating bycatch based on catch rates, called the 'MP-rate' method (mean post-season catch rate) is used. This method follows closely the methods used to estimate bycatch in other fisheries and other species. This is called a 'post-season' estimate because catch rate data (kg of catch/h) are not available until the end of the fishing season. The authors conclude that the MP-rate is the most reliable estimate of bycatch.
The catch rate of eulachons varies greatly among different areas of the BC coast and also between 1997 and 1998. In general, bycatch rates were lower in 1998. In part, the reason for the lower 1998 estimates may have been a lower encounter rate, but there also is evidence that the fleet took evasive action to lower eulachon bycatch. The bycatch estimate methods developed for eulachons also are applicable for estimation of other species. In this regard, the authors observe that the catch composition of 'non-target species' varies widely with different areas. Several observations are made about areas of potential concern, especially with respect to high bycatch rates of pleuronectids (flatfishes). The paper concludes with some perspectives on bycatch in the BC shrimp fishery.
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