Orientation behaviour of Lepomis macrochirus Rafinesque to salinity fluctuations

The impact of the rate of salinity change has never been studied in primary freshwater fish occupying oligohaline marshes, where they are near their upper physiological limits. We conducted experiments on the bluegill sunfish, Lepomis macrochirus Rafinesque, to examine their orientation to salinity changes under winter temperature and photoperiod. We found significant difference in behavioural responses to rates of salinity change. The responses to the two flow rate changes indicate orthokinesis for ± 1.0‰ and −12.0‰ change h −1 treatments; however, a ‘closure’ type of response is apparent during +12.0‰ change h −1 treatment. These results indicate that bluegill sunfish respond behaviourally to salinity change, but reduce all overt activity when rate changes are unnatural and potentially lethal. It is apparent that behavioural and physiological responses are synergistic when the salinity exceeds a critical rate of change, which presumably is based upon the species' recent acclimatization history. Moreover, an orthokinetic response to the rate of salinity change may allow bluegill sunfish to move into and out of oligohaline marshes.