Wetland connectivity: understanding the dispersal of organisms that occur in Victoria’s wetlands draft
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Freshwater fishIn Victoria, 32 native fish species frequently occupy wetlands, and many of those prefer still or slow-flowing water (Appendix 3). Almost half of the species are listed as threatened or vulnerable (Appendix 3). Several species are diadromous and migrate between freshwater and marine habitats to complete their life cycles. A number of introduced species also utilise wetland habitats, including Common Carp (Cyprinus carpio), Goldfish (Carassius auratus), Weather Loach (Misgurnus angullicaudatus), Mosquitofish (Gambusia holbrooki), Redfin Perch (Perca fluviatilis) and Tench (Inca tinca). Brown Trout (Salmo trutta) and Rainbow Trout (Oncorhynchus mykiss) are introduced species that mainly occur in lakes and farm dams. Many introduced fish are detrimental to native fish populations because of predation, competition for resources, habitat modification and the introduction of diseases and parasites (Crowl et al. 1992, Rowe et al. 2008). Understanding the processes that govern fish movement between freshwater habitats will inform the management of native species and identify mechanisms for controlling the spread of introduced species.
A variety of fish species utilise wetland habitats as nursery grounds, to access food resources and spawn, and as refuges from predation or competition (Poisal and Crivelli 1997). The type of wetland habitat varies from permanent to temporary and fresh to saline, and includes billabongs, anabranches, lakes, marshes and floodplains (Humphries et al. 1999). Exploratory movements in waterways are often undertaken by fish in search of alternative habitat, but the pattern, scale and constraints of fish movements among wetlands is not well studied. Movement abilities differ among fish species and will influence community assemblages and species distributions in the landscape. For example, the propensity for dispersal varies among fish species with some species being sedentary and others dispersive. Large bodied fish are generally able to disperse over greater distances than small bodied fish, but as water depth must be at least 1.5 x body depth for fish to swim, shallow water can present a barrier to movement in large fish (Lucus and Baras 2001). Conversely, small fish can disperse in shallow water but may avoid deep water as the risk of predation is higher (Harvey and Stewart 1991). Fish movements in a network of seasonal wetlands and a lake in Florida, USA were studied for over a year by Hohausova et al. (2010). The study found dispersal routes varied with the size and species of fish, and both hydrology and human disturbances influenced movement patterns. The depth of water connecting habitats was an important factor influencing dispersal, with shallow water favouring the dispersal of smaller bodied fish (e.g. Mosquitofish) which dispersed farther than large bodied fish. Dispersal distances of fish varied from 0.7 to 4 km. In temporary wetlands, where fish populations undergo frequent local extinctions associated with drying, the ability of species to recolonise these systems is a key determinant of community assemblage structure (Snodgrass et al. 1996). The role of connectivity in structuring fish assemblages in 24 temporary wetlands in Florida was examined by Baber et al. (2002). This study found that 71% of the study wetlands were colonised by fish. The most common species was the Mosquitofish, which inhabited all wetlands where fish were recorded. Fish were absent from wetlands that were:
Wetlands with diverse fish assemblages were connected to permanent water bodies and also positioned in a mosaic of wetlands. Deeper wetlands with a long hydroperiod also supported more diverse fish communities. Other studies similarly report that fish diversity in temporary wetlands is enhanced by connectivity to other waterbodies and the number of wetlands in the landscape (Snodgrass et al. 1996, Taylor 1997). Although hydrological connectivity was found to increase species diversity, Mosquitofish were also prevalent in hydrologically connected systems. In Australia, where the Mosquitofish is invasive and has been implicated in the decline of native fish, hydrologic connectivity in regions where this species (or other introduced species) occur may have negative effects on fish diversity (Rowe et al. 2008). River–floodplain movement — The ability of fish to migrate to floodplain wetlands is influenced by a variety of factors, including hydroperiod, distance, elevation gradient and the presence of water control structures such as canals, levees, and dams (Hohausova et al. 2010). In Australia, 50% of floodplain wetlands on developed rivers may be isolated from their source rivers (Kingsford 2000), presenting limitations on fish movement and local persistence. In some cases, structures such as channels, culverts and weirs can artificially connect wetlands, providing either permanent or periodic access depending on the type and method of operation (Beesley et al. 2011). Although many species of fish occupy both streams and wetland habitats, the significance of river–floodplain connections in maintaining fish populations is still not well understood in Australia. The flood pulse concept (FPC) proposed by Junk et al. (1989) posits that optimal conditions for fish recruitment occurs when floodplain inundation coincides with warmer temperatures. As some fish species spawn in the warmer months, floods that coincide with warmer temperatures allow larvae and juveniles to access and exploit plentiful resources available on productive floodplains. The FPC is well supported empirically in tropical regions of the world, where flood pulses regularly coincide with warmer temperatures (Junk et al. 1989; Humphries et al. 1999). In Australia, however, temporal variability in the flood pulse can uncouple floodplain inundation and warmer temperatures, making the significance of river–floodplain linkages to fish recruitment less clear (Humphries et al. 1999; King et al. 2003). For example, in the unregulated Ovens River in Victoria, native fish failed to recruit in inundated floodplains, probably because temperature thresholds for spawning were not synchronised with floods in this region; but Common Carp, which are capable of spawning at lower temperatures, successfully recruited (King et al. 2003). Beesley et al. (2011) reported that the colonising potential, recruitment success and fish abundance in floodplain wetlands in the Murray–Darling Basin was influenced by the source (river or irrigation channel) and mode of water delivery to floodplain (e.g. pipes, pumps, regulated and unregulated channels). Fish responses were poor when water was sourced from irrigation channels and pumped into floodplain wetlands.
The Pacific Blue-eye provides one example of connectivity over a large spatial scale. It is a widespread, salt-tolerant and putatively vagile species that inhabits coastal drainages across much of eastern Australia. It occupies a range of rivers and wetland habitats, from rainforest streams to estuaries. Genetic studies revealed that northern and southern populations along the east coast of Australia are genetically divergent, probably because of a dry corridor in mid-coastal Queensland (Wong et al 2004). At the southern end of the species range, populations were genetically more similar among drainages, suggesting that floods may be important in maintaining connectivity among populations. |