Apparent Stock Recruitment Relationships in Mixed Stock Fisheries

Abstract

When several stocks of differing productivities are fished together and combined for stock recruitment analysis, the estimated productivity and size of the stock depends strongly on the previous exploitation history. As a mixed stock is harvested harder, it appears smaller in total size but more productive per individual. I analysed the mechanism behind this change. Passive feedback management policies perform well on mixed stocks, when starting from unexploited conditions. When starting from an overexploited condition, passive feedback management will fail to allow the less productive stocks to recover and will maintain overexploitation. This is also true in the presence of straying between stocks. Since few salmon fisheries operate on single stocks, stock recruitment analyses will usually underestimate the optimum escapement and overestimate the optimum harvest rate when mixed stocks are treated as a single stock. These conclusions will be true for any mixed stock fishery with different productivities of the stocks.