Elucidating the decline of North American Atlantic salmon with a time-dependent matrix model

Abstract

North American Atlantic salmon populations have declined significantly since the 1990s, especially the elder life stages. The total average number of returned two-sea-winter (2SW) in the period 1997 to 2007 dropped by 57% compared to that in the period 1972 to 1982. Evidence is emerging that the decline is largely due to poor marine survival. Since the marine phase of the salmon life cycle lasts several years and spans a large geographic range, it remains unclear at which specific life stage the most significant decline occurs. To address this question, I developed a new method to assess the status and fluctuations of the North American Atlantic salmon population. By applying an optimization algorithm to fit an age- and stage-structured matrix model to available observations for the period from 1972 to 2011, I assess stage-specific mortality rates over time. The model is able to closely replicate the observations and provides insights into the temporal variation of one-sea-winter (1SW) and two-sea-winter (2SW) salmon returns. Results suggest that changes in the relative proportion of the 1SW and 2SW returns resulted from a 28% decrease of survival during the second year at sea since 1992. By combining model outputs, and homewater and distant fishery catch data, I quantified the relative influence of bottom-up (i.e., environmental changes) and top-down effects (i.e., fishing pressure). It shows the environmental impact has a generally negative effect on Atlantic salmon survival, with a 41% decrease near West Greenland during the second year at sea and a 17% decrease near Canadian homewater during migration to their spawning grounds since 1992. In addition to the importance of external environmental change impacting the population dynamics of North American Atlantic salmon, I show that the moratorium on commercial fishing is likely insufficient for recovery of Atlantic salmon to previous abundance levels. However, the moratorium is crucial, at least in the short term, to maintain the relatively low yet stable abundance of Atlantic salmon.