Smolt Survival and Adult Utilization of Little Codroy River, Newfoundland, Atlantic Salmon

Abstract

Between 1954 and 1961, seaward-migrating Atlantic salmon smolts were marked by the removal of their dorsal and adipose fins and released from the counting fence in the estuary of the Little Codroy River, Newfoundland, to determine their survival to, and utilization as, adults. Not all smolts were marked: in 1957 and 1958, about half, and in 1959, none were marked to assess the effects of marking on survival; in 1962, smolts were counted without being handled in a specially designed trap to assess the effects of handling on survival. A total of 61,394 smolts were finclipped; 523 marked adults (0.82% of the smolts marked) returned to the fence, and 876 marked adults (1.47%) were estimated to have been taken by the commercial fishery in Newfoundland and Labrador. In addition, there was an unknown but probably large number of grilse landed that did not get into commercial channels and so were not sampled. Five age-groups of smolts migrated, but two of these were scarce and only three were found among the adult survivors. These were age-2 smolts (1.79% of the smolts marked), age-3 smolts (2.43%), and age-4 smolts (1.64%). Three sea-age groups were represented among the recorded survivors: grilse (0.96% of the smolts marked), 2 sea-year salmon (1.21%), and 3 sea-year salmon (0.07%). About 62% of the recorded survivors were utilized by the commercial fishery, 4% by anglers on the Little Codroy, and the remaining 34% escaped and may have spawned. The commercial fishery is estimated to have taken 48% of the grilse (plus some unrecorded grilse), 75% of the 2 sea-year salmon, and 43% of the 3 sea-year salmon. The handicap created by the loss of the dorsal and adipose fins and defined by the ratio of the instantaneous mortality rates of marked and unmarked fish was estimated to be 1.10. Handling smolts was estimated to cause an immediate instantaneous mortality of 0.23. The overall average survival rate, conservatively adjusted, becomes 3.64%, that is, 2235 adult salmon would have survived from the migration of 61,394 smolts if they had not been handled and marked. However, if an estimate of unrecorded grilse be added, and a greater adjustment made for marking mortality, the survival rate becomes 9.42%. In addition to handling and marking, smolt survival also appears to have been affected by the size, age, and number of smolts migrating, as well as by the mark itself. Returns of Little Codroy smolts to the fence were lower than returns of salmon with missing adipose and ventral fins reported from several other streams.