Ontogenetic development in swimming ability of two species of cold-water marine fish larvae and the role of temperature

This study examines the effect of temperature on the development of active swimming ability in the larvae of two cold-water fish species, the pelagic spawner Gadus morhua (Atlantic cod) and the bottom spawner Myoxocephalus scorpius (shorthorn sculpin). The first chapter provides a brief overview of studies on swim speed in larval fishes and how they have been used in studies of dispersal. The second chapter examines the development of critical swimming speed in larvae of the two species reared at different temperatures. These swimming tunnel experiments show that temperature has a clear effect on critical swimming ability of sculpin larvae as they increase in size and age. By contrast, a temperature effect on development of swimming ability in cod larvae was only observed with thermal summation age. Sculpin larvae hatched at a bigger size and stronger swim speed than cod larvae, and also exhibited faster development rate of swimming speed relative to both size and age than cod. The third chapter examines the influence of temperature on development of sustained swimming ability in larvae of the same two species. For sculpin larvae, temperature only has a significant effect on development of sustained swimming ability as a function of absolute age, although sustained swimming speed increased with both age and size. For cod larvae, however, there was a significant temperature effect on development of sustained swimming ability only as a function of size. Cod larval sustained swim speeds lagged behind those of sculpin larvae as a function of age because of the smaller size of cod larvae at hatch. The fourth chapter presents larval transport simulations for Placentia Bay, Newfoundland, in which larvae were initially assumed to be entirely passive and thereby transported exclusively by mean currents. These results were compared to simulations that combined larval active swimming speeds with the influence of mean currents. These simulations, which were conducted for spring and summer temperatures, were then compared to field patterns of larval concentrations from multiple biological surveys in Placentia Bay. The simulations indicated that larvae of bottom-spawning sculpin could take advantage of currents and their strong swimming ability to influence dispersal and improve retention within the embayment, particularly in warmer water temperatures. Larvae of pelagic-spawning cod were almost entirely passive, irrespective of water temperature. Field patterns of cod and sculpin larvae were generally consistent with the simulations, particularly in the context of the active retention simulation for sculpin. This thesis demonstrates that active swimming by larval fish has the potential to significantly influence transport and dispersal, but this potential differs for species and environmental conditions.