Salinity preference and swimming activity of the gray snapper Lutjanus griseus

Nearshore, shallow-water habitats frequented by the gray snapper Lutjanus griseus are often characterized by substantial fluctuation in salinity levels which can represent a significant source of osmotic stress for associated organisms. We investigated the potential links between salinity preference, swimming activity and habitat selection in a newly developed salinity choice electronic shuttlebox via 48-hr trials. Eleven individual sub-adult and adult gray snapper ranging between 18-23 cm (overall mean of 19.8 ± 0.49 cm) were tested using this system. Results revealed that most fish selected intermediate salinities in the range of 9-23 ppt as their final preference. Within this range, six preferred salinities between 9-15 ppt while five preferred salinities between 18-23 ppt. Although there was a tendency for fish to select slightly lower salinities during the light phase (mean of 13.8 ± 1.8ppt for the light period; 16.9 ± 1.4 ppt for the dark) this difference was not statistically significant. Parabolic salinity-swimming speed relationships emerged with a peak swimming speeds at intermediate salinities (nearly isosmotic) and a decrease towards both salinity extremes, particularly at high salinities. Across all salinities swimming speeds were significantly (p<0.05) elevated by 0.043 body lengths per second during the dark period as compared to the light period. Overall, our findings reveal that gray snapper prefer slightly hyperosmotic salinities that may minimize the physiological costs of osmoregulation. We speculate that the reduced swimming speeds we observed at our salinity extremes directly reflect compensation for higher osmoregulatory costs.