Critical Thermal Minima of age-0 Australian bass, Macquaria novemaculeata, fingerlings: implications for stocking programmes

Fishes are often stocked outside natural distribution ranges with inadequate information on target streams, particularly thermal regimes. Australian bass, Macquaria novemaculeata (Steindachner), is a catadromous species that is regularly stocked into upland reaches of rivers and impoundments in south-eastern Australia. Critical Thermal Minima (CTMin) were determined for age-0 Australian bass fingerlings with a mean fork length of 64.4 +/- 0.4 mm and weighing 3.8 +/- 0.8 g. Four treatments were used, including three replicate aquaria for each treatment. Fingerlings were acclimated at either 8 or 15 degrees C at densities of 15 fish in 56-L glass aquaria. Water temperatures were then decreased at either 1 degrees C day-1 or 1 degrees C h-1 until loss of equilibrium (LOE), which occurred between 3 and 7 degrees C. Mean CTMin among treatments was 3.224.64 degrees C and was influenced by acclimation temperature and rate of temperature decline. Fingerlings acclimated at 8 degrees C subjected to a temperature decline of 1 degrees C h-1 experienced highest LOE temperature. Post-LOE mortality among treatments was highest at 100% in the 8 degrees C acclimation with a 1 degrees C day-1 temperature decline. Mortalities following LOE occurred within 5 days. The results suggest that stocking age-0 Australian bass is unlikely to be successful in areas where winter temperatures fall below 6 degrees C.