Salmon Watersheds Program

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Atlantic salmon mortality at sea: Developing an evidence-based “Likely Suspects” Framework

The Atlantic salmon (Salmon salar) has suffered from significant declines in survival at sea since the late 1990s, manifest in persistently poor return rates of various sea ages for a wide range of wild stocks across the north Atlantic range. While observed survival trends differ among individual stocks, there are also common trends across groups of stocks; from which it can be inferred that local and wider scale factors may be implicated in the salmon’s decline.

In this regard, the EU-funded SALSEA research collaboration led to a step change in the state of knowledge of European Atlantic salmon at sea. SALSEA established the concept of an annual “conveyer belt” of northward migrating smolts. Along this route, the number of smolts declines due to cumulative effects of natural mortality as well as any fishing mortality. In particular, SALSEA identified the existence of “choke points” in the ocean, where there is coalescence of migrating smolts in relatively restricted geographical areas and where there is believed to be potential for variations in oceanic conditions to alter the destinations of migrating smolts. Additional mortality factors likely operate during the overwinter feeding phase and on the return migration to home waters and entry to rivers.

While SALSEA identified potential sources of mortality during the initial smolt migration to the feeding grounds, it did not aim to quantify or fully understand these. It is evident however, that since these factors can vary in time and space, such variation may be expected to account for some, if not most of, the variability in return rates observed among salmon stocks and between years. However, although much of the research on Atlantic salmon marine survival in the past 15 years has focused on factors at sea, it is acknowledged that conditions experienced by juvenile salmon before and during the critical phase of migration from freshwater to the sea may also impact survival during the subsequent marine phase.

The scale and complexity of survival problems facing Atlantic salmon pose unique challenges for conducting scientific research both in freshwater and the marine environment, and we must consider how it can best be coordinated and targeted. This is particularly the case with “at sea” research programs since these are very expensive and typically require international collaboration across several countries.