Ecological studies on the freshwater fishes of the Alligator Rivers Region, Northern Territory: Autecology
Supervising Scientist Report 145
Bishop KA, Allen SA, Pollard DA & Cook MG
Supervising Scientist, 2001
ISBN 0 642 24348 4
- SSR145 - Ecological studies on the freshwater fishes of the Alligator Rivers Region, Northern Territory: Autecology (PDF - 6,383 KB)
- Contents (PDF - 69 KB)
- Preliminary pages (PDF - 474 KB)
- Chapters 1 - 2 (PDF - 232 KB)
- Chapter 3 Part 1 (PDF - 1385 KB)
- Chapter 3 Part 2 (PDF - 1646 KB)
- Chapter 3 Part 3 (PDF - 1791 KB)
- Chapter 3 Part 4 (PDF - 1750 KB)
- Chapters 4 - 5 (PDF - 775 KB)
- References (PDF - 224 KB)
- Appendices (PDF - 237 KB)
The essential objective of the study was to identify the locations and timings of activities critical in the life cycles of each of the fish species considered. By cross-relating such knowledge to information on the location and timing of potential mining-induced physical and chemical (abiotic) impacts, the life-cycle components of species most at risk can be identified - an important task in impact risk assessment.
Primary biological/ecological information was obtained and analysed for each of the 37 freshwater fish taxa found in the Alligator Rivers Region in relation to the following generic species-account structure: size composition, environmental associations, reproduction and feeding habits. Each of these components revealed complementary information on the locations and timing of activities critical in the life cycles of the fish taxa considered.
Size composition. Smaller juveniles of the largest number fish species were found in lowland backflow billabongs and floodplain billabongs. These two habitats therefore appear to be particularly important as nursery areas. Larger species tended to have recruitment limited to a single season, primarily either the Early-wet or Mid-wet seasons. Smaller fish species tended to have continuous recruitments with a few peaks occuring during the year. The primary recruitment peaks of such species typically occurred in the Mid-wet season.
Environmental associations. Useful information indicative of the environmental associations of 32 fish species was obtained. Three main groups of species were distinguished by characteristic combinations of associated environmental factors. Escarpment-associated species are potentially the most vulnerable component of the fish fauna to mining induced abiotic impacts. They were usually associated with low temperature, high DO concentration, low conductivity, low turbidity and coarse substrate. These species therefore display a general 'clean water' association. Accordingly, they are at risk when they make incursions to the lowlands near mining areas in the Wet season, or are trapped therein throughout the Dry season. They are therefore likely to be the most useful candidates for monitoring and biological testing.
Reproduction. Six breeding strategies were exhibited by the fishes studied. Species which lay demersal eggs are most prone to impacts associated with siltation, and/or the release of toxic materials from sediments when anoxic conditions develop at depth within waterbodies. Accordingly, such species are therefore likely to be the most useful candidates for monitoring and biological testing. The most important spawning habitat was the lowland backflow billabongs, where 19 species showed evidence of spawning. Most fish species (25) bred around the onset of the Wet season. This is the time when initial flooding hugely increases the area and diversity of aquatic habitats available as well as initiating major increases in plankton and other foods.
Feeding habits. Nine feeding guilds were identified. If biomagnification effects are the focus of future investigations then the peak carnivores are the most suitable candidates for monitoring. Similarly, if exposure to disturbed contaminated sediments is a key factor, then the herbivore/detritivores, omnivores and/or benthic carnivores are the most suitable candidates. Downstream of the Ranger Uranium Mine (RUM) the highest feeding activity was recorded in lowland backflow billabongs followed by floodplain billabongs. Upstream of RUM the highest feeding activity was recorded in escarpment main channel waterbodies followed by lowland billabongs. The season of greatest feeding activity was, as other researchers in tropical seasonal rivers have found, the Wet season. Feeding activity thus increased most dramatically between the Late-dry season and the Early-wet season. By the Mid-wet season feeding activity had peaked, and then decreased slightly by the Late-wet-Early-dry season. An examination of variations in body condition indicated that most species obtained their best condition from the Mid-wet to the Mid-dry season, with a peak in the Late-wet-Early-dry season.
A key finding from the study is the crucial importance of lowland backflow billabongs to the ecology of the majority of the freshwater fish fauna.
The detailed information presented in this report constitutes a major contribution to the autecological knowledge of the freshwater fish fauna of the Alligator Rivers Region. Because many of the species have a wide distribution, the information will be valuable to researchers across Australia, especially those working in the Timor Sea, Gulf of Carpentaria and north-east coast drainage divisions, and within Papua New Guinea and Irian Jaya. Insights gained into the processes 'driving' the ecology of this tropical riverine fish fauna also have world-wide application. Importantly, the information arising from the present study will have considerable application when assessing the nature and magnitude of impacts arising from a range of freshwater-associated developments, and particularly those in the mining arena.