Research Document 1997/88
Decision rules for overwintering herring fisheries
By R. Claytor
A computer simulation model demonstrates the risk of over-exploitation to small stocks in overwintering herring fisheries that take place where mixtures of large and small stocks occur. This model leads to the general principles that (1) if mixing of stocks is random, then (a)exploitation on average will be equal among all stocks, (b) exploitation rates will be more variable on the smaller stocks, but (2) if fishing occurs where small stocks are concentrated; exploitation rates much higher than expected will occur on the small stocks.
These principles were used, in combination with fishery and research survey results to develop decision rules for the 4Vn overwinter herring fishery. The first rule determines that the overwinter catch is not to exceed recent average landings, because these levels have not been detrimental to local spawning components. The second determines that the starting date should be November 1, because results of acoustic and bottom trawl surveys indicate that the migration of 4T herring is well established by this date. The third, is that no more than 10% of the catch by number can be below 24.5 cm fork length, as this limit has been effective in keeping the landings of immature herring to less than 10% and a higher percentage of the immature herring are likely to be of local origin than adults. The fourth rule concerns fishing area, which was not able to be defined using the data analyzed to date and was identified as an item to be determined among science, management, and industry.
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