Modeling the effects of temperature on the survival and growth of North Sea cod ( Gadus morhua) through the first year of life

Temperature and body size are widely agreed to be the primary factors influencing vital rates (e.g., growth, mortality) in marine fishes. We created a biophysical individual-based model which included the effects of body size and temperature on development, growth and mortality rates of eggs, larvae and juveniles of Atlantic cod (Gadus morhua L.) in the North Sea. Temperature-dependent mortality rates in our model were based on the consumption rate of predators of cod early-life stages. The model predicted 35%, 53% and 12% of the total mortality to occur during the egg, larval and juvenile stages, respectively. A comparison of modeled and observed body size suggested that the growth of survivors through their first year of life is high and close to the growth rates in ad libitum feeding laboratory experiments. Furthermore, our model indicates that experiencing warmer temperatures during early life only benefits young cod (or theoretically any organism) if a high ratio exists between the temperature coefficients for the rate of growth and the rate of mortality. During the egg stage of cod, any benefit of developing more rapidly at warmer temperatures is largely counteracted by temperature-dependent increases in predation pressure. In contrast, juvenile (age-0) cod experiences a higher cumulative mortality at warmer temperatures in the North Sea. Thus, our study adds a new aspect to the ‘growth–survival’ hypothesis: faster growth is not always profitable for early-life stages particularly if it is caused by warmer temperatures.