Publication Abstract

Do farm escapes distort estimates of salmon pre fishery abundance and national conservation limits?

E.C.E. Potter and L.P.Hansen

The model used to estimate the pre-fishery abundance (PFA) of salmon in the NASCO-NEAC area and conservation limits for some national stocks is based upon national or regional catch data. Fish farm escapees caught in the Faroes fishery have been excluded from the analysis because they are not derived from natural recruitment and do not contribute to the spawning escapement. However, no account has been taken of the presence of farm escapees in homewater fisheries. The main effect is in Norway, where escapees have comprised up to 72% of salmon sampled in some coastal fisheries, up to 91% in some fjord fisheries, and up to 95 % in rod fisheries. We have developed an alternative formulation of the NEAC-PFA model, which more appropriately accounts for the farm escapees. This results in the estimated PFA of 1SW and MSW salmon being reduced by an average of 10 % and 18 %, respectively, over the past 10 years. The effects of farmed fish on the spawning escapement and hence the estimated national conservation limit depends upon the spawning success of farm escapees and viability of their eggs relative to wild fish. Assuming they have half the spawning success of wild fish, the effective spawning numbers (and hence egg deposition) for 1SW and MSW salmon are reduced by 2 % and 4 %, respectively. Thus taking account of the farm escapees in this way results in the national conservation limit for Norway, derived using the lagged egg-deposition model, being reduced by only about 4%.

Reference:

E.C.E. Potter and L.P. Hansen, 2001. Do farm escapes distort estimates of salmon pre fishery abundance and national conservation limits? ICES CM 2001/M:05. 10pp.