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

Recruitment success in living marine resources is variable due to high survival variability of early life stages. Improving our understanding of how environmental and ecological factors mechanistically interact and influence larval fish growth and survival is necessary to better predict year-class strength and expected physiological and behavioural responses to climate warming. We use a state-dependent optimality model for the behaviour of larval Atlantic cod Gadus morhua to analyse trade-offs related to growth and survival. Temperature-dependent maximum growth rates and vertical profiles of temperature and stochastic prey availability are used as inputs within a mechanistic modelling framework that finds optimal behavioural strategies of vertical migration and foraging activity. The fitness criterion used is maximization of survival probability until the larvae reach a given body size (15 mm). Detailed descriptions of predation, physiology, growth and survival of larval cod emerge from simulations of the optimal strategies. The model shows that the effect of temperature on survival is complex. Increasing temperature may lead to faster growth and higher survival, but only when there is sufficient food. In poor food environments, higher temperatures make larvae more susceptible to predation as they take higher risks to satisfy their metabolic costs. Overall, these results suggest that larval Atlantic cod, especially those from warmer-water stocks, may experience reduced survival and recruitment in climate-change scenarios that predict both elevated temperatures and reduced food supply