Crocodylus johnstoni and C. porosus Coexisting in a Tidal River

Abstract

A 52.5-km section of the Adelaide River, N.T. (12°13'S., 131°13'E.). was spotlight-surveyed 20 times between June 1979 and September 1981. C, johnstoni (15.3 ± 9.2 sighted per survey) were less abundant than C. porosus (137.6 ± 36.5 sighted per survey), and were mainly in the upstream 20 km of the survey route (96% of C. johnstoni sightings); here considered a zone of syntopy within the survey route. C. johnstoni congregate in the main stream during the dry season and disperse from it during the wet season, which parallels similar seasonal movements to and from dry-season refuges in non-tidal areas lacking C. porosus. As the dry season progresses, C. johnstoni are located further and further upstream, and this movement (or loss ofanimals) appears unrelated to changes in salinity. Numbers of C.johnstoni within the zone of syntopy are negatively correlated with numbers of C. porosus (r*2 = 0.50, P=0.005). and competitive exclusion may be occurring. Independent of seasonal factors, numbers of C. johnstoni within the zone of syntopy declined with consecutive month (1979-81: r*2=0.47, P= O.004), whereas numbers ofthe more recently protected C, porosus increased (r2 = 0.48, P= 0,006). The location of the syntopic zone was unchanged.