In the absence of direct counting systems, the abundance of Atlantic salmon stocks in Scotland is generally estimated indirectly using reported rod fishery data. The widespread adoption of catch and release in Scottish rod fisheries provides a challenge for such assessment techniques, however, as a proportion of fish released from the rod fishery may be re-caught and hence inflate the catch statistics by appearing in the reported data more than once.
This report describes a modelling approach which has been developed to correct the reported catch of Scottish rod fisheries by accounting for recaptures associated with these data. The number of recaptures is modelled using estimates of exploitation rate, retention rate in the rod fishery and mortality following release. Model output at a range of notional exploitation rates was compared with reported catch over the period 1994 (when catch and release was first reported within the fishery) to 2013.
It is concluded that catch and release inflates the reported catch data and that this effect increases through the time series as the take-up of catch and release within the rod fishery increased. However, regardless of this effect, the general trends in the reported catch data over the time series were similar to those generated by the model. This was true even at high notional exploitation rates which emphasize the differences between reported catch and model output. It is important that the potential effects of catch and release on rod catch trends is kept under review and that efforts are made to collect data that increase the reliability of our abundance indicators in the future. An important development in this regard is the construction of a network of well validated counters that will not only provide direct estimates of stock abundance in relation to particular river systems, but will also improve our knowledge of exploitation rates across a wide range of Scottish salmon stocks.