Can changes in length-at-age and maturation timing in Scotian Shelf haddock (Melanogrammus aeglefinus) be explained by fishing?

Factors affecting size-at-age in fish populations include temperature and fishing where the latter can represent a strong selective force on size-at-age variation through changes in population growth and maturation. Over the past three decades, Scotian Shelf haddock (Melanogrammus aeglefinus) exhibited declines in maturation timing and mature fish length-at-age. Here, we examine these declines with respect to temperature, stock biomass, and fishing. We employ the thermal integral (growing degree-day, GDD, degrees C.day) to examine the variation in length-at-age (length-at-day, LaD, cm) and maturity (age-at-50%-maturity) that is attributable to temperature. Unexplained variation in LaD and age-at-50%-maturity remains and is characterized by declines in the LaD-at-GDD regression parameters and the thermal constant for maturity with increasing year-class. We find no significant correlation between the temperature-independent declines in LaD and stock biomass. The combination of high fishing mortality (favouring early maturation) and sustained harvesting of large fish (fast-growing, late-maturing individuals) offers the simplest explanation for the systematic decline in inferred growth and age-at-maturity for Scotian Shelf haddock. These results are consistent with other exploited populations and recent laboratory experiments quantifying the effects of fishing on size-at-age and age-at-maturity.