Fishes in groundwater dependent pools of the Fortescue and Yule Rivers; Pilbara, Western Australia

摘要

In order to provide baseline information on the fish in groundwater‐fed ecosystems in two Pilbara rivers, the fish fauna of Bilanoo Pool and Mungajee Pool on the Fortescue River, and Li Lin Pool and Jelliabidina Pool on the Yule River were sampled for fish using a variety of methods that included: gill netting, seine netting and, angling and visual surveys. The majority of the fish (i.e. 95.64%) were captured with seine nets with the remainder captured in gill nets (4.28%) or via hand line (0.08%). Population demographics of each species at the different sites were determined and comparisons made within and between systems. A total of 6650 individual fish from 17 species were captured during this study. This includes 13 species from the Fortescue River sites and 10 species from the Yule River sites; with seven species common to both systems. The sampling and analysis of previous research revealed that the Fortescue River is the most diverse, in terms of freshwater fishes, of the Pilbara (Indian Ocean) Drainage Division.ududA total of six native freshwater species had previously been recorded from the freshwaters of the Yule River, including Melanotaenia australis, Nematalosa erebi, Amniataba percoides, Leiopotherapon unicolor, Neosilurus hyrtlii and Anguilla bicolor (Morgan et al. 2003, Morgan & Gill 2004). During this study we recorded four of the above species in the target pools. A further eight marine/estuarine species are known from the river, including H. compressa (which is often referred to as a freshwater species) (Morgan et al. 2003, Morgan & Gill 2004). During this study we recorded seven marine/estuarine species in the Yule River pools, including two species that were previously unrecorded from the river, M. cephalus and S. multifasciata.ududWithin the Fortescue River pools sampled during this study, we recorded nine freshwater species, each of which has previously been recorded from the river (Morgan et al. 2003, Morgan & Gill 2004, Beesley 2006). Although during this study we sampled only two sites, we recorded all of the known freshwater fish species from this system. However, there are potentially two further undescribed species that were recorded during this study, i.e. Neosilurus sp. and Terapontid sp. (see Table 2). A further four marine/estuarine species were recorded from the freshwaters of river, which represents all but two species previously recorded from it, i.e. S. multifasciata and Hypseleotris compressa, both of which were previously known from only one site reported in Morgan et al. (2003). Thus, the pools sampled within the Fortescue River during this study collectively provide habitat for all known fishes in the entire catchment. On comparing with all rivers in the Pilbara Drainage Division, it is important to note that the Fortescue River has the highest diversity of freshwater fish species of all the rivers of the region.ududUtilising data collected both during this study and in those of Morgan et al. (2003) and Morgan & Gill (2004), it was revealed that the diversity of fishes in both the Yule and Fortescue Rivers is considerably greater in the lower sections of these rivers compared to the middle and upper reaches. While this can partly be attributed to the presence of species that are of marine/estuarine origin in the lower reaches, these downstream most riverine sections are ecologically important as they are the only sections of these rivers that house all of the freshwater species of each catchment. Within each system, there is an overall significant difference between riverine reach with the upper sections providing habitat for few species compared to the middle and lower reaches.ududWithin the Yule River system, the lower section acts as a nursery for at least eight species of marine/estuarine origin, compared to the middle section which houses only one of these species. The mean number of the fish species in the lower Fortescue River is higher but not significantly greater than the middle section of the river. The upper section of the river is significantly lower in terms of species diversity.ududPool connectivity to the estuary and also pool stability (i.e. depth) and habitat complexity are important in maintaining the diversity of fishes in at least the Fortescue system (see Beesley 2006). However, other mechanisms may also influence the diversity of fishes, particularly within the upper Fortescue River, where waterfalls would limit the dispersal throughout the system. The upper pools on the Fortescue River, namely, Hamersley Gorge and Fern Pool are also important refuges for the restricted and endemic Fortescue Grunter (L. aheneus).ududBeesley (2006) demonstrated a significant linear relationship between pool persistence and fish species diversity and also habitat heterogeneity and species diversity within the lower and middle reaches of the Fortescue River. Most notably, permanent pools afford fishes permanent habitat, and it is likely that the intrusion of groundwater is crucial in maintaining the assemblages in the middle and lower sections of the Yule and Fortescue Rivers. Within Jelliabidina Pool in the Yule River, it is important to note that there was little difference between the assemblage of fishes captured in spring 2001 (Morgan et al. 2003) compared to spring 2008 (this study). For example, four freshwater species and one marine species were recorded on each sampling event, with a further marine/estuarine species recorded during 2008.ududThe maintenance of higher water levels, during flooding or through links to groundwater are likely to be important in the survivorship and therefore rates of recruitment of off‐spring, and this is particularly important in desert fish communities where rainfall is sporadic. Higher water levels generally provide larval and juvenile fishes with lower levels of predation and permanent waters are often associated with increased vegetation growth and thus increased egg laying and larval habitats. It is known that while some species breed year round (see Allen et al. 2005, Beesley 2006), others have less protracted breeding periods that often coincide with high water levels during flood events (see Beesley 2006, Morgan & Gill 2006).ududThere is also likely to be strong relationships between stream order and species occurrences, with lower diversities typical of tributaries compared to main channel waters (see Morgan & Gill 2004). For example, tributaries are more likely to contain L. unicolor, M. australis and A. percoides over larger bodied species such as N. erebi, N. graeffei and the marine/estuarine species.ududThe study suggests that groundwater intrusion is maintaining pool stability and is critical in sustaining the fish assemblages in these systems.