Climate and fishing: Disentangling factors affecting growth in Scotian shelf haddock (Melanogrammus aeglefinus)

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Fish growth is affected by variations in a number of factors including climate (temperature) and size-selective fishing. The effects of these factors on resulting size-atage variation must be disentangled to allow for the development of successful management strategies. Of the factors impacting variation in growth, temperature is a controlling factor governing growth via reaction rates at the cellular (metabolic) level and the effects of temperature on size-at-age variation must be examined foremost. Disentangling effects of temperature on size-at-age requires a metric relevant to the integrated growth metric that is size-at-age. In previous work, we show this physiologically relevant metric is the growing degree-day (GDD, ºC d). Here we employ the GDD metric to quantify the influence of temperature variation on a 30-yr decline in length-at-age among mature, eastern Scotian Shelf (Northwest Atlantic Fisheries Organization statistical divisions 4VW) haddock (Melanogrammus aeglefinus). We present evidence that remaining, temperature-independent variation in length-at-age among year-classes is consistent with sustained size-selective fishing of large (i.e. fastgrowing and late-maturing) fish. We argue that size-selective fishing is the most parsimonious explanation for the systematic declines in size-at-age and age-at-maturity found in Scotian Shelf haddock.