Research Document - 2003/101

Marine mortality of Atlantic salmon, Salmo salar L: methods and measures

By Potter, E.C.E., O'Maoileideigh, N. and Chaput, G.

Abstract

This publication contains the contributed papers to an international workshop on marine mortality of Atlantic salmon (Salmo salar L.) convened at the Department of Fisheries and Oceans, Gulf Fisheries Centre, Moncton, New Brunswick (Canada) from August 20-22, 2001. The Workshop was organised under an EU Concerted Action programme (SALMODEL) to facilitate a detailed review of the information on the survival of Atlantic salmon in the sea, methods to estimate natural mortality and the values currently used in modelling studies. The Workshop reviewed estimates of marine survival for salmon from a number of monitored rivers around the North East Atlantic and in North America, investigating the trends in marine mortality. While some common trends were evident, other contradictory indicators suggest that the migrations of salmon at sea and the factors contributing to mortality may be quite complex. The methods used to estimate mortality rates at sea are reviewed, and both the inverse-weight and the maturity schedule models indicate that M in the second year of sea life has probably been underestimated in the past. Finally the workshop considered factors operating in fresh water, estuarine (transitional) and marine environments that might influence survival at sea. Investigations have indicated some strong associations between growth at sea and survival. Growth can be mediated by several factors including temperature, food, competition (intra and inter-specific) and may influence subsequent vulnerability to predation. Associations between several of the factors and survival rates at sea may be indicative of possible causal mechanisms. Additionally, marine phase survival may be influenced by conditions experienced during the juveniles in freshwater and transitional phases of the life cycle or by factors which delay the entry of returning adults into their river of origin. Research to date has attempted to identify a dominant factor affecting mortality, but it is more likely that many factors are acting together and influencing survival. In the context of run-reconstruction modelling for the development of fisheries advice, it is concluded that mortality of salmon in the second year at sea is more likely to be in the order of 3% per month rather than the 1% per month previously used in ICES assessments.

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