Considerations for using spawner reference levels for managing single- and mixed-stock fisheries of Atlantic salmon

The probability of achieving the spawning requirement objective of Atlantic salmon (Salmo salar L.) is defined by the stochastic properties of small numbers and biological characteristics of the stock. The uncertainty in achieving the spawning escapement objective is greater for small stocks than for large ones, such that measures of annual performance are more variable for small stocks. Summing individual river spawner requirements into a regional requirement reduces the probability of meeting the objectives simultaneously in all rivers. Variations in productivity among stocks, when not accounted for, can result in under-escapement in areas of lower productivity. The impact of mixed-stock fisheries can be most important for small stocks, and especially if these are of low relative productivity. Increasing the regional spawner requirement in an attempt to compensate for lower productivity may alleviate the problem somewhat, but it is not a guaranteed solution to the challenge of protecting all stocks, specifically those of low productivity in mixed-stock fishery situations. Each mixed-stock fishery situation can be evaluated on a case-by-case basis using Monte Carlo techniques.