Research Document - 2000/147
Ecosystem effects on pre-recruit survival of cod in the southern Gulf of St. Lawrence.
By D.P. Swain and A.F. Sinclair, G.A. Chouinard, and K.F. Drinkwater
Abstract
The recruitment rate of cod in the southern Gulf of St. Lawrence was remarkably high from the mid 1970s to the early 1980s. This high recruitment rate contributed to the rapid recovery of this stock from low levels of abundance in the mid 1970s. We sought an explanation for this unusually high recruitment rate by examining relationships between apparent pre-recruit survival of southern Gulf cod and ecosystem factors that might be expected to influence early survival, i.e. climate variables and potential predators of cod eggs and larvae (pelagic fishes) or cod juveniles (seals). We tested effects of these factors by including them as covariates in the stock-recruitment relationship of southern Gulf cod. The stock-recruit relationship was strongly compensatory, with a higher recruitment rate at low spawning stock biomass. A negative effect of pelagic fish biomass on cod recruitment rate was also highly significant for this stock. Cod recruitment rate was unrelated to the indices of seal abundance. With one exception, no significant effects of the climate variables were apparent after accounting for autocorrelation. The exception was a tendency for recruitment rate to be higher at intermediate dates for the last appearance of ice on the Magdalen Shallows, though this effect was relatively weak compared to the effects of pelagic fish biomass and cod spawning stock biomass on recruitment rate. The unusually high recruitment rate of southern Gulf cod in the mid to late 1970s can be explained by the low levels of both cod spawning biomass and pelagic fish biomass during this period.
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