Research Document - 1999/087

Juvenile Atlantic salmon (Salmo salar L.) Abundance in the Experimental Ponds Area relative to subsequent adult returns to Gander River as a index of marine survival: apparent evidence for density-dependant marine mortality.

By R. Knoechel, P.M. Ryan, and M.F. O'Connell

Abstract

A marine survival ratio index was calculated as the number of adult salmon returning to the Gander River divided by the total juvenile salmon populations in the Experimental Ponds Area (EPA) at the headwaters of the river in the previous spring. This survival index increased more than four-fold in the first four years (1992-95) following closure of the commercial fishery in 1992 but then dropped moderately in 1996 and then precipitously in 1997, suggesting a large decrease in marine survival despite the continued closure of the commercial fishery in insular Newfoundland. The decreases in the marine survival index occurred in the context of increasing juvenile abundance, a pattern consistent with an interpretation of density-dependent mortality. Juvenile abundance declined in 1997 and the marine survival index increased, as would be expected if mortality were density-dependent. The negative correlation between juvenile abundance and the marine survival index leads to a prediction of a dome-shaped relationship between juvenile abundance and subsequent adult returns to the Gander River. This relation suggests that the Gander River conservation requirement of 21,828 small adult spawners will be met when EPA juvenile abundance in the preceding year is in the range of 3000 - 5000 individuals. EPA juvenile abundance in 1998 was 2385 individuals which yields an estimated 1999 return of 19,740 small adults which would be 10% below the conservation requirement. The predicted return would increase to 25,643 small adults if the high marine survival observed during the early post-moratorium years (1992-95) was achieved.

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