Temperature and Density-Dependent Effects on the Size-at-Age of Haddock (Melanogrammus aeglefinus) on the Scotian Shelf

Declining trends in weight-at-age of adult haddock were observed in NAFO Div. 4VW on the eastern Scotian Shelf off Nova Scotia from survey data collected between 1970 and 1996. The rate of decline was relatively slow through the 1980s but increased rapidly near the mid-1980s.  Since then, the weights-at-age have remained low, approximately 30-40% of the pre-1980s observations. Correspondingly, temperatures in the eastern Shelf declined gradually during the early 1980s, decreased rapidly in the mid-1980s and were near their long-term minimum values in the 1990s. Regression analysis revealed that temperature accounted for 67% of the variation in the weight-at-age of seven year-old haddock. Higher growth was associated with warmer periods.  Predictions of weight-at-age 7 haddock during the late 1940s to 1970 were made from available temperature data.  The predicted weights showed very good agreement with published weights from the commercial fishery.  The relationship between weights-at-age 7 with spawning stock biomass (SSB) shows two distinct periods. Prior to 1985, there was a negative relationship with SSB suggesting density-dependent growth.  During the post-1985 period when extreme cold conditions prevailed, the relationship was positive.  Finally, the regression model was used to predict the weights-at-age for the years 1997 to 2000.  While temperatures warmed during this period and an increase in growth was predicted, this was not observed.  The continued low growth is suggested to be related to the earlier maturity that has been observed during the late 1990s.