THE CRITICAL THERMAL LIMITS FOR THE BULLHEAD, COTTUS-GOBIO, FROM 3 POPULATIONS IN NORTH-WEST ENGLAND

1. The objective was to determine the thermal limits for feeding and survival in the bullhead, Cottus gobio, using juveniles (total length 20-30 mm, live weight 0.5-1.5 g) from one population and adults (50-70 mm, 3.5-5.5 g) from three populations. 2. Fish were acclimated to constant temperatures (3, 7, 10, 15, 20, 25 or 27 degrees C) and the temperature was then changed at a rate of 1 degrees C/30 min to determine the critical limits for feeding, survival over 7 days (incipient lethal temperature), or survival for 10 min or less (ultimate lethal temperature). The rate of 1 degrees C/30 min was the optimum value from preliminary experiments, using nine rates from 0.5 degrees C/48 h to 18 degrees C h(-1). As values for adults were not significantly different between populations, they were pooled to provide arithmetic means (with 95% CL) for the thermal limits at each acclimation temperature. 3. Feeding limits increased with acclimation temperature to upper and lower mean values (+/- 95% CL) of 26.5 +/- 0.16 degrees C and 4.2 +/- 0.20 degrees C for adults, 26.6 +/- 0.59 degrees C and 5.0 +/- 0.55 degrees C for juveniles. Incipient lethal levels defined a tolerance zone within which fish survive indefinitely; upper limits increased with acclimation temperature to a plateau of 27.6 +/- 0.22 degrees C for adults and 27.5 +/- 0.47 degrees C for juveniles, lower limits increased from near 0 degrees C to 2.5 +/- 0.31 degrees C for adults and 2.7 +/- 0.47 degrees C for juveniles. Ultimate lethal levels increased with acclimation temperature to a plateau of 32.5 +/- 0.24 OC for adults and 32.6 +/- 0.46 degrees C for juveniles, whilst the lower limits increased from near 0 to 0.9 +/- 0.29 degrees C. Upper feeding, incipient and ultimate lethal values were significantly lower for juveniles than those for adults at acclimation temperatures < 20, < 20 and < 15 degrees C, respectively. 4. The thermal tolerance of bullheads was slightly lower than that of stone leach, similar to that of juvenile Atlantic salmon and higher than that of brown trout; the thermal limits for feeding were much wider than those for salmon or trout.