Research Document 1998/84
The abundance of harp seals in the North Atlantic and recruitment of the North American stock of Atlantic salmon (Salmo salar)
By Peter G. Amiro
Abstract
The negative trend in marine survival of North American (NA) Atlantic salmon (Salmo salar) has persisted in spite of severely reduced commercial salmon fisheries. This document explores the potential of replacing YEAR, a simple metric vector that accounted for a significant proportion of the variation in recruitment of salmon, with annual population estimates of harp seals (Phoca groenlandica). Harp seals were investigated as a replacement for YEAR because of their increasing population and their estimated consumption of salmon in the Northern Gulf of St. Lawrence. In 1996, the consumption of salmon may have exceeded 3.0 x 106 salmon in the Northern Gulf of St. Lawrence. Harp seals are estimated to have consumed 82% of the total seal consumption of fish in 1996 and are coincident with NA salmon for extended periods. The population of harp seals, interaction with the marine habitat area of salmon and an index of annual NA smolt production accounted for 0.94 of the variation in recruitment of NA multi-sea-winter salmon for the period 1973 to 1995. Similar models accounted for 0.86 of the recruitment of NA one-sea-winter salmon for the period 1979 to 1996. Models using seal abundance rather than YEAR provide an ability to forecast trends and more importantly, because the variable is not a simple vector, turning points in the annual recruitment of salmon. Recruits of the 1977 smolt year were outliers to the models and did not follow a pattern of increased size of one-sea-winter salmon. Factors other than predation are suggested for the reduced marine survival of the 1977 smolt class.
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