Estimate of the population structure of the estern water dragon, Physignathus lesueurii (Reptilia : Agamidae), along riverside habitat

Abstract

A population of eastern water dragons, Physignathus lesueurii, was investigated along 1.5km of the Gloucester River in central eastern New South Wales from November 1989 to December 1992. Dragons were caught in all months from September to April, but not in June. In all, 373 dragons were marked and 69 of these were recaptured on one or more occasions. Females are ovigarous in late October, November and December. Only about one third of females have palpable eggs during these months. Hatchlings enter the population in February and March at 4.5g, with a snout-vent length (SVL) of 53mm. In the next season they are 19.5g (SVL 80mm). The dragons grow rapidly for four years to reach adult size. Growth rates are fastest during the first year at 2.25mm SVL per month or 1.25 g per month. The Jolley-Seber population estimate for adult females was 103 +/-49 in the study site or 69 per km of river. Estimates for males and for the total population could not be calculated because individuals were not equally catchable (Leslie's test for equal catchability). However, theoretical considerations place the density of adult dragons at 138-215 per km of river. The largest male was 304mm SVL, 54mm longer than the next-largest water dragon reported. One male was heavier than 1000g. The largest female was 230mm SVL and the heaviest was 490g, or approximately half the mass of the heaviest male. Adult males and adult females have the same proportion of broken tails. Water dragons are fairly sedentary, with an average distance of 76m (range 0-785m) between captures.