The biology of the south-western Australian catfish Tandanus bostocki Whitely (Plotosidae).

Some aspects of the biology of the south-western Australian freshwater catfish Tandanus bostocki are described on the basis of monthly gill and seine net samples collected in Wungong Reservoir, September 1990 to August 1991. Several ageing techniques were tested using two different anatomical structures, otoliths and dorsal spines, to establish the most suitable means of ageing the species. Thin sectioning of the dorsal spine using a resin embedding technique usually employed in otolith sectioning, was found to be the most time effective and accurate method, also producing the clearest growth rings. Circuli formation was very irregular and was reflected in highly variable mean values of the monthly marginal increments. This was attributed to the oligotrophic status of the reservoir. Due to the atypical behaviour of the growth ring formation in T. bostocki ageing, and consequently growth, of the species was not possible in the time constrains of this thesis. Increasing GSI values and the appearance of ripe oocytes in the ovary showed that spawning of the species occurs in November and December and may occur as early as October. Rainfall was low and temperatures were in excess of I9°C during this period. Increasing day length appeared important to the inducement of spawning while an annually varying environmental factor, such as temperature, is likely to be important in the timing of breeding. Spawning was asynchronous, individuals of the same size having ovaries at different stages of development during the breeding period. Histological sections of ovaries and oocyte size-frequency distributions provided evidence of an asynchronous ovarian development. The diet of T. bostocki was typical of the diet described for other freshwater plotosid species. The diets of small and large fish were different through the emphasis on different prey sizes. Insects larvae, particularly the dipteran family Chironomidae, and small crustaceans were the dominant food types of small cobbler. In contrast, the marron Cherax tenuimanus was the dominant food of large fish. Seasonal changes were most marked in small cobbler where the dominant food category changed with the season