Crocodylus johnstoni in the McKinlay River Area N. T, III.* Growth, Movement and the Population Age Structure

Abstract

Regression constants for predicting C. johnstoni body measurements from each other are presented. In the McKinlay River area, C. johnstoni grow primarily in the wet season, dry-season growth appearing negligible. Males grow faster and attain larger sizes than females, and those downstream in the study area grow faster than those upstream. Neither sex nor location (upstream v. downstream) accounts for a significant proportion of the variation in growth of C. johnstoni of <210 mm HL (head length). A single exponential curve is a poor fit to the size-age relationship, which is more adequately modelled by two exponentials. Separate mean age-size curves for C. johnstoni <200 and >200 mm HL are derived, and methods for predicting (and correcting) the age of individuals are tested. Accuracy of prediction is increased by using C. johnstoni recaught at least once (n = 240), and the age structure of this segment of the population is derived. The size range of animals recaught is not significantly different from that of those originally marked, so this age structure is considered a good estimate of the total population age structure. Between 3 and 4% of the number of eggs laid annually are represented as 2-year-olds, indicating a high mortality before this age. In general, mortality appears greater downstream than upstream, and may be independent of crocodile density; in the few years before and after the introduction of protection (1964) recruitment downstream was still minimal, even though numbers of crocodiles had been much reduced. Females mature when between 9 and 14 years of age, with a mean age of about 12 years (240-254 mm HL). For the small sample of males examined, half of those 15-19 years of age (n = 4) and all of those 20 years or older (n = 7) were mature. A 13-year-old male had two sperm cells in its smear, and maturity may be attained then, although 16-17 appears a more usual age (17 y; 287-294 mm HL). Of all C. johnstoni recaught after 1 year, 83.4% were within 1 km of the pool in which they had been marked and released. Of those which had moved, more were upstream than downstream of their capture site, and there was no difference between the proportions of males and females moving. Younger animals moved more than older ones, and those from upstream in the study area moved more than those from downstream.