Effects of temperature on larval fish swimming performance: the importance of physics to physiology

Temperature influences both the physiology offish larvae and the physics of the flow conditions under which they swim. For small larvae in low Reynolds number (Re) hydrodynamic environments dominated by frictional drag, temperature‐induced changes in the physics of water flow have the greatest effect on swimming performance. For larger larvae, in higher Re environments, temperature‐induced changes in physiology become more important as larvae swim faster and changes in swimming patterns and mechanics occur. Physiological rates at different temperatures have been quantified using Q₁₀s with the assumption that temperature only affected physiological variables. Consequently, Q₁₀s that did not consider temperature‐induced changes in viscosity overestimated the effect of temperature on physiology by 58% and 56% in cold‐water herring and cod larvae respectively. In contrast, in warm‐water Danube bleak larvae, Q₁₀s overestimated temperature‐induced effects on physiology by only 5–7%. This may be because in warm water, temperature‐induced changes affect viscosity to a smaller degree than in cold water. Temperature also affects muscle contractility and efficiency and at high swimming velocities, efficiency decreases more rapidly in cold‐exposed than in warm‐exposed muscle fibres. Further experiments are needed to determine whether temperature acts differently on swimming metabolism in different thermal environments. While hydrodynamic factors appear to be very important to larval fish swimming performance in cold water, they appear to lose importance in warm water where temperature effects on physiology dominate. This may suggest that major differences exist among locomotory capacities of larval fish that inhabit cold, temperate waters compared to those that live in warm tropical waters. It is possible that fish larvae may have developed strategies that affect dispersal and recruitment in different aquatic habitats in order to cope not only with temperature‐induced physiological challenges, but physical challenges as well.