Research Document - 2000/035

Further evaluation of juvenile Atlantic salmon (Salmo salar L.) abundance in the Experimental Ponds area relative to subsequent adult returns to Gander River and the empirical evidence of 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, in concert with rapidly increasing juvenile abundance, a pattern consistent with an interpretation of density-dependent mortality. Juvenile abundance was low in 1998 leading to a prediction of an increase in the marine survival index in 1999; this prediction is confirmed in the present study. We herein extend the relationship between the marine survival index and EPA juvenile abundance further back in time by using long-term records of adult returns to the Salmon Brook fishway as an index of whole river returns. This analysis indicates that the apparent negative density dependence is consistent over a span of slightly more than two decades. The EPA juvenile abundance increased from very low levels in 1998 to near record levels in 1999. If the density-dependent model is correct, marine survival this year should be low and year 2000 returns of small adult salmon to the Gander River should be only 13,782 which is far below the conservation requirement of 21,828. In contrast, small adult returns are predicted to increase to a record level of 49,073 if the high survival index calculated for 1999 is applied to year 2000 returns. Continued monitoring of the adult returns in 2000 should provide a strong test of the apparent historic relationship.

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