Use by fishes of temporary pools created by high seasonal tides, which occur each year from October until April, on the saltpans and littorals adjacent to the Norman River estuary in northern Queensland was investigated. Fish entering the pools were sampled with one-way tidal traps placed in inlet gutters, and fish in the pools were poisoned using rotenone. Recruitment to the pools by fish occurred soon after the initial tidal inundation. Juvenile fish of 37 species, including the commercially important food fish L. calcarifer, used the pools. The first barramundi were found 8 weeks after initial inundation and for the remainder of the study they continually migrated into these habitats. The total length of the smallest L. calcarifer sampled was 9.5 mm. Salinities in pools containing fish ranged from 94 × 10-3 to less than 1 × 10-3 and temperatures reached 36�C.

Larval barramundi in the size range 2.8-5 2 mm were collected from plankton in two estuaries in north-eastern Queensland from 31 October 1979 until 13 February 1980. After leaving the plankton, barramundi moved into nearby brackish and freshwater swamps. These areas acted as nursery grounds, offering both protection from predators, and abundant prey in the form of insect larvae, other fish and crustaceans. These habitats exhibit a wide range of salinities (fresh water-44 x lo3 mg 1-I) and surface water temperatures (23-36°C). Juvenile barramundi commenced migration from these swamps into permanent tidal creeks around April where they remained for up to 9 months before dispersal into the estuary, up rivers or along coastal foreshores. The diet of the barramundi in these tidal creeks was exclusively fish and crustaceans. Juvenile barramundi were resident in tidal creeks that had been subjected to substantial human interference through habitat alteration. Destruction of nursery swamps may pose a serious threat to local barramundi stocks near centres of human population on the eastern Queensland coast.

Lutjanus argentimaculatus, tagged and released in coastal rivers and estuaries, were found to have made inter- and intra-riverine, coastal and offshore movements. A small proportion of the recaptures made offshore movements to reef habitats of up to 315 km and these recaptures were fish that were at liberty, on average, more than twice as long as those fish that had made intra-riverine movements. Most juvenile fish <400-mm length to caudal fork (LCF) resident in rivers were recaptured less than a kilometre from where they were released. The proportion of fish making sizeable movements increased with increasing recapture size, with about of 20% of larger fish (400–500-mm LCF) making offshore, inter-riverine or coastal movements. Larger fish were primarily caught offshore, whereas smaller fish <~338-mm LCF were exclusively caught in estuarine and freshwater habitats. Recruitment of juveniles into estuarine and lower freshwater riverine habitats occurred from about February. There was temporal variability of recruitment of mangrove jack into some river systems and their relative abundance within the river system was inversely proportional to the distance from the sea. Overfishing of juveniles when they are concentrated in inshore areas could have adverse implications for mangrove jack stocks.

Abstract. The freshwater piscifauna of the watercourses of the Barron and Mitchell systems, two northern Australian catchments with adjacent headwaters, was surveyed. Fifty-eight species of fish from 26 families, including several marine vagrants, were sampled from the freshwater reaches of the Barron River, compared to 28 species from 15 families from the Mitchell River. Species diversity and richness was higher in the coastal section of the Barron catchment owing to the influence of marine vagrants and species that were estuarine dependent for part of their life cycle. Detrended correspondence analysis of fish abundance data showed that sites in the coastal Barron catchment and the Mitchell catchment were widely separated in ordination space. Historically, barriers to fish movement determined species composition in both catchments, but there have been major changes in species composition as a result of extensive translocations of native fish into the Barron catchment over the last 50 years. Five species of exotic fish, including the cichlids Tilapia mariae and Oreochromis mossambicus, were established in the Barron system and one species in the Mitchell catchment. Cherax quadricarinatus, a crustacean endemic to western drainages, has been translocated into the Barron catchment and has since become widely established. This has implications for the interbasin transfer of irrigation water between the Barron and the Mitchell catchments potentially providing a mechanism for fish, including the exotic species O. mossambicus, to spread across northern Australia. M F 0 2 0 4 6 F i s h a s s e m b l a g e s t r u c t u r e i n c o n n e c t e d n o r t h e r n A u s t r a l i a n s t r e a m s D . J . R u s s e l l e t a l .

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