Fish spawning and recruitment in a large floodplain river in the wet-dry tropics of northern Australia : are low flows important?

摘要

The rivers of the wet-dry tropics in northern Australia support highly productive and diverse ecosystems, with a wide variety and abundance of freshwater fish. However, there are significant and imminent anthropogenic threats to these near-pristine ecosystems and the implications for freshwater fishes are difficult to predict, as there is little information known about basic life history, particularly reproduction and early life stages of these fishes.This study investigated the temporal and spatial variability of spawning and recruitment of freshwater fish during the dry season, low-flow period in the Daly River, Northern Territory. Sampling of fish early life stages occurred at 6 weekly intervals (May- October 2013) at the Daly River Crossing, to investigate temporal patterns. On two occasions (early and late dry season 2013) sampling also occurred at Claravale and Oolloo Crossings, when spatial and habitat patterns were explored. A diverse range of sampling gears was used across a range of slackwater habitats and the mid-channel environment, during both the day and night.The dry season low-flow period was found to be important for freshwater fish spawning (indicated by presence of larvae) and recruitment (indicated by presence of juveniles). Twenty species were recorded spawning during the 2013 dry season, from a range of different life history strategies and taxonomic groups. Spawning occurred throughout the dry season, although the transition times (coming from the wet to the dry season, and dry to wet season) seemed to be key spawning periods for the majority of species collected. Temporal variability was largely due to a peak in larval abundance towards the end of the dry season, attributable to the increased abundance of larvae of opportunistic species (i.e. Craterocephalus stramineus and Craterocephalus stercusmuscarum). Larval and juvenile fish assemblages also varied among river reaches; with larval abundance lower and juvenile species richness higher at the downstream site (Daly River Crossing), compared to the other two sites. Variability may be due to broad differences in habitat characteristics (e.g. submerged vegetation) and position within the catchment. Within-reach spatial variability was also evident, as the majority of fish in early life stages used low flow slackwater habitats along the bank edges, rather than the mid-channel. This is consistent with the low flow recruitment hypothesis (Humphries et al. 1999). The recorded environmental variables within slackwaters only explained 31.9 % and 27.6% of the variance in the larval and juvenile assemblages respectively, with dissolved oxygen and turbidity being responsible for most of the variation.This study has made a key contribution to understanding the significance of dry season low flows for spawning and recruitment of freshwater fishes in wet-dry tropical rivers. However, longer term and more spatially replicated sampling is required to confidently describe the importance of the period, and the spawning period and cues of freshwater fishes in these rivers.