Weight-at-age of cod varies greatly for different stocks throughout the North Atlantic. For example, the average weight of a 4-year-old cod ranges from about 0.6 kg at Labrador to 7.3 kg in the Celtic Sea. A simple exponential model in which mean catch weight-at-age is a function of the product of age × temperature (mean annual ambient temperature) accounts for 92% of the variance for 2–4-year-old cod from 17 stocks throughout the North Atlantic.
In(weight-at-age in kg)=0.06821 × (temperature × age)– 1.042.
A logistic model accounts for 91% of the variance over the age range 2–9 for 12 of the 17 stocks
Weight-at-age=10.28/(1 + exp(0.085(41.27 – temperature × age))).
Parameters were estimated by fitting both models to data from many stocks (i.e a between-stock comparison, using average temperature), but the same models and parameters also explain observed time trends in weight-at-age within stocks at Faroe and West Greenland and differences between areas around Iceland. Within these stocks weight-at-age can vary by a factor of two or more, with very important consequences for stock assessment (including forecasting) and for the evaluation of possible effects of climate change on cod stocks. The conclusion that most of the observed variability in growth is due to temperature needs to be tested thoroughly, and this will require detailed information on the ambient temperature experienced by different ages of cod in different years.