Exposure of goldsinny, rock cook and corkwing wrasse to low temperature and low salinity: Survival, blood physiology and seasonal variation

Wrasse used as cleaner fish with farmed Atlantic salmon Salmo salar can be subjected to large and rapid temperature and salinity fluctuations in late autumn and early winter, when summer-warmed surface water is affected by early snowmelt episodes. Because of their containment in sea cages, wrasse which are essentially acclimated to summer temperatures may be rapidly exposed to winter conditions. Short-term tolerance of low temperature and low salinity by three species of wrasse, goldsinny Ctenolabrus rupestris rock cook Centrolabrus exoletus corkwing Crenilabrus melops caught during the summer, and winter-caught corkwing, was investigated. A 3-day period at 30 or 32 parts per thousand salinity and temperature 8, 6 or 4 degrees C (for summer-caught fish; 4 degrees C only for winter-caught) was followed by a decline in salinity to 24, 16 or 8 parts per thousand over c. 36 h, followed by a further 24 h at these salinities held constant, at each of the three temperatures. Controls in 30 or 32 parts per thousand were maintained at 8, 6 or 4 degrees C. Mortality of summer-caught corkwing and rock cook was high at 4 degrees C, whereas the influence of salinity on mortality was small. Mortality of goldsinny was low or zero in all treatments. Surviving corkwing and rock cook after 3 days at 4 degrees C and 32 parts per thousand salinity had elevated plasma osmolality: in summer-caught corkwing, plasma [Cl-] and [Na+] were high, whereas in rock cook only [Na+] was high. Haematocrit was low in summer-caught corkwing, high in rock cook. In survivors of all three species at the end of the experiment, values of all these parameters were comparable with those of fish at the beginning of the experiment, except that survivors at low salinity (8, 16 parts per thousand) had low plasma osmolality, at all temperatures, and in surviving rock cook in these treatments haematocrit was high and plasma [Cl-] was low. Winter-caught corkwing had higher osmolality, [Na+] and [Cl-] than summer-caught corkwing; there was no difference in haematocrit. Survival of winter-caught corkwing exposed to four salinities at 4 degrees C was much higher than that of summer-caught corkwing under the same conditions. Little change in blood physiology was recorded for winter-caught corkwing, with only fish subjected to 8 parts per thousand and 4 degrees C showing signs of osmoregulatory stress. The interspecific and seasonal differences in survival and blood physiology at low temperature and low salinity are discussed in relation to wrasse survival over winter, both in the field and in salmon farms. (C) 1996 The Fisheries Society oi the British Isles.