Fish fauna of the Vasse River and the colonisation by feral goldfish (Carassius auratus)
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During December 2003 and March 2004 a total of 7895 fish from nine species were captured during a survey of the Vasse River, including two sites in the Vasse River Diversion Drain. Of the fish captured, four were native freshwater fishes that are endemic to the south-west, three were native estuarine species and two were introduced fishes. Two estuarine species dominated the native catches with the western hardyhead (Leptatherina wallacei) being captured at the most downstream sites and the Swan River goby (Pseudogobius olorum) being captured throughout the river aside from the upper-most headwater site. The endemic western pygmy perch (Edelia vittata) was also widely distributed in the river, however, it was captured in relatively low numbers aside from immediately downstream of the Diversion Drain at a site with more complex in-stream and riparian habitat. The western minnow (Galaxias occidentalis) and nightfish (Bostockia porosa) were only captured in relatively low numbers at two sites each in the lower Vasse River. This study also captured the rare mud minnow (Galaxiella munda) at the headwater site, which is the first record of this species in the Vasse River.
The Vasse River is generally heavily modified and two introduced feral species, the mosquitofish (Gambusia holbrooki) and goldfish (Carassius auratus) have become established and accounted for over 60% of all fish captures. These species were particularly dominant in more degraded reaches of the river, such as the Diversion Drain and the lower Vasse River. The goldfish were captured in the lower Vasse River, between the Diversion Drain and immediately below the Old Butter Factory slot-boards. This stretch of the river has essentially become a heavily silted, stagnant pool due to the restriction of flow from the Diversion Drain and the presence of slot-boards, which together prevent adequate flushing of the system and that is likely to have facilitated the establishment of the feral species, as well as exacerbating algal blooms.
It is suggested that the introduction of goldfish into the Vasse River is relatively recent and the dominance of juveniles born in October last year will result in a rapid increase in the population in the next few years. Growth rates of goldfish in the Vasse River far exceed those reported elsewhere with individuals attaining lengths of over 180 mm TL at the end of their first year, the age at which they also mature. Goldfish are known to be vectors for disease introduction, may prey on native fish and their eggs and larvae, reduce aquatic plant biomass and re-suspend nutrients further fuelling algal blooms. Furthermore, recent studies have demonstrated that significant growth of cyanobacteria is stimulated by the passage through goldfish intestines. Goldfish, which attained lengths of over 40 cm in the Vasse River, therefore have the potential to contribute to algal blooms.
It is recommended that flushing of the river could occur by removal of the slot-boards at the Old Butter Factory and diverting flows from the Diversion Drain back into the river, thus attempting to return environmental flows to a more natural regime. Riparian vegetation should also continue to occur in this stretch of the River with estuarine species able to tolerate the higher salinities expected to occur as the River returns to a more natural, estuarine system.
A goldfish eradication program should be implemented in the lower Vasse River. This program would also provide an excellent opportunity to gain further information on their biology and ecological impact in the system, particularly with regard to their role in algal blooms. An education program should also be implemented outlining the problems caused by releasing aquarium fish into wild aquatic systems.
|Murdoch Affiliation:||Centre for Fish and Fisheries Research|
|Series Name:||Murdoch University Report to the Department of Environment, Government of Western Australia|
|Publisher:||Murdoch University. Centre for Fish & Fisheries Research|
|Copyright:||2004 Murdoch University. Centre for Fish & Fisheries Research|
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