Behavioural and metabolic responses to the cold in winter-dormant fishes: reductions in activity as the key strategy underlying energy savings

Abstract

Metabolic rate depression (MRD) is a reversible downregulation of standard (resting) metabolic rate (SMR) that facilitates survival in energy-limited environments. Previous studies suggesting interspecific variation in the capacity for MRD among winter-dormant fishes may be confounded by unaccounted variation in activity, which affects the accuracy of SMR estimates. When winter activity reductions were controlled for, a recent study on cunner (Tautogolabrus adspersus) found no MRD. I investigated whether inactivity is the central strategy underlying the metabolic phenotype of winter-dormant fishes. I characterized winter-dormant behaviour in four temperate fish species, finding that activity reductions are ubiquitous albeit varying in magnitude. I then investigated the relationship between activity and metabolic rate using video tracking and intermittent respirometry in two species (mummichog, Fundulus heteroclitus; pumpkinseed sunfish, Lepomis gibbosus) during acute cooling and after winter temperature acclimation. Low winter metabolic rates resulted from reduced activity combined with passive physicochemical effects of cooling, not MRD. Inactivity is the key energy saving strategy of winter-dormant fishes.