Effects of temperature and food on larval cod survival: a model for behaviour in vertical gradients

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Recruitment success of many living marine resources can vary considerably between years and the early life stages are a key determining factor. Understanding how environmental factors mechanistically influence growth and survival of fish larvae will allow better predictions for recruitment and are also central for predicting expected responses to climate warming. A statedependent optimality model for the behaviour of larval Atlantic cod Gadus morhua was used to analyse trade-offs related to temperature and food abundance and their importance for the growth and survival of larval cod. Temperature-dependent maximum growth rates and vertical gradients of temperature and stochastic prey availability were used as inputs within a mechanistic model that finds optimal state-dependent behaviour for vertical migration and foraging and gives detailed descriptions of predation, physiology, growth and survival of larval cod. Warmer water raises the metabolic demand for food and increased foraging involves an increased risk of predation; the total effect on survival from temperature is not obvious. The model emphasises that food availability is more important for survival and recruitment success than for growth. It also predicts that increasing temperature leads to faster growth and higher survival, but only if there is sufficient availability and quality of food