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 s urvival in the bullhead, Cottus gobio, using juveniles (total length 2 0-30 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 cons tant temperatures (3, 7, 10, 15, 20, 25 or 27 degrees C) and the tempe rature was then changed at a rate of 1 degrees C/30 min to determine t he critical limits for feeding, survival over 7 days (incipient lethal temperature), or survival for 10 min or less (ultimate lethal tempera ture). The rate of 1 degrees C/30 min was the optimum value from preli minary experiments, using nine rates from 0.5 degrees C/48 h to 18 deg rees C h(-1). As values for adults were not significantly different be tween populations, they were pooled to provide arithmetic means (with 95% CL) for the thermal limits at each acclimation temperature. 3. Fee ding 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 d egrees 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 with in which fish survive indefinitely; upper limits increased with acclim ation temperature to a plateau of 27.6 +/- 0.22 degrees C for adults a nd 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 acclim ation 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 f rom near 0 to 0.9 +/- 0.29 degrees C. Upper feeding, incipient and ult imate 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 w ere much wider than those for salmon or trout.