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While growth variation in fishes is the result of a number of different factors (temperature, food consumption, genetic composition, etc.), temperature has been identified as one controlling factor on fish sizeat-age. Thus, variation in size-at-age due to variation in temperature should be explained before the other factors are explored. We argue that the relevant metric for explaining temperature-dependent size-at-age variation in most fishes is physiologically relevant temperature; i.e., the thermal integral parameterized as the growing degree-day (GDD, oC × d) metric. Accordingly, we employ GDD to examine the change in size-at-age (length-at-day, LaD) among eastern Scotian Shelf (Northwest Atlantic Fisheries Organization statistical divisions 4VW) haddock (Melanogrammus aeglefinus ) from 1970 through 2003. Significant varia tion in length-at-day (LaD) among year classes (decline in slopes and intercepts of LaD-at-GDD relations) remains after LaD variation due to GDD (temperature) is explained. The unexplained variation is postulated to be a result of temperature-independent factors such as condition or size-selective fishing or both.