GROWTH-MODELS FOR 2 SPECIES OF FRESH-WATER TURTLE, CHELODINA-RUGOSA AND ELSEYA-DENTATA, FROM THE WET-DRY TROPICS OF NORTHERN AUSTRALIA

Chelodina rugosa is a fast-growing, carnivorous turtle that occupies s easonally ephemeral habitats, whereas Elseya dentata is relatively slo w-growing, largely herbivorous, and confined to permanent water. The r elationship between carapace length and growth rate for each species w as modeled using ''split-stick'' regression. The regression procedure estimated the carapace length at which the slope of the growth rate-bo dy size relationship changed from rapid juvenile growth to slow adult growth. This estimate of body size at maturity compared favorably with estimates of size at maturity based on independent criteria. The von Bertalanffy growth model provided the best fit to recapture data, and the age-size curve closely matched individual ages obtained by countin g plastral annuli. Both species are sexually dimorphic in body size at maturity and at maximum size, with females being the larger. On avera ge, males of C. rugosa mature at 3.9 yr and 165 mm in carapace length and reach a maximum size of 250 mm, whereas females mature at 6.5 yr a nd 210 mm and reach a maximum size of 300 mm. Males of E. dentata matu re at 8.6 yr and 220 mm and reach a maximum size of 280 mm, whereas fe males mature at 13.5 yr and 276 mm and reach a maximum size of 330 mm. The rapid growth and early maturity of C. rugosa reflects its high pr otein diet and occupancy of highly productive ephemeral environments w here selection for rapid growth may be intense. The slower growth and late maturity of E. dentata, despite year-round warm temperatures, sug gests that its growth may be energetically limited by a largely herbiv orous, protein-poor diet.