A long time ago in a galaxy (not so) far, far away … aliens infiltrated the ranks of many native fish populations and risked their very survival. Fish may not be an alien concept to us, however, for Australia’s native flora and fauna, alien species pose a real threat.

Native fish declines are a big concern in Australia, with approximately 47% of native fish listed on threatened species lists at both the federal and State levels. A number of impacts have caused these native fish declines. Habitat degradation and destruction, for example, are well-known causes, and easily recognisable. But the impacts of introducing alien species, and often, repeatedly stocking them, on native fish species are much less known, and much less obvious or recognisable. This is despite alien species affecting 82% of all threatened species listed under the Commonwealth Environmental Protection and Biodiversity Conservation Act 1999 (EPBC Act) (our national environmental law).

Brown trout. Source: Arthur Rylah Institute.

In the Murray Darling Basin (MDB), alien fish species are extremely prominent, with 12 species currently known. Alien trout are a popular species for sport fishing, providing recreation to the angling community. A recent survey by the Australian Bureau of Agricultural and Resource Economics and Sciences (ABARES), found that “one in five Australian adults participate in recreational fishing every year, improving wellbeing and contributing 100,000 jobs and $11 billion to the Australian economy.” This survey provides a snapshot of the current state of recreational fishing in Australia, highlighting the significance of fishing for human sport and recreation.

As for trout specifically, a search on social media is quick to reveal the significant number of rec fishers interested in catching the two types of alien trout found within the MBD, the rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta).

Brown trout (top) and Rainbow trout (bottom). Credit: NSW DPI

Originally from the western coastal drainages of North America, rainbow trout are predominately found in south-eastern parts of the MDB, preferring cool habitats including montane/upland creeks, streams, rivers and reservoirs. However, a small number of populations reside in streams within SA, and within the upper Condamine-Balonne in southern QLD. Vic and NSW fisheries agencies release approximately 2.5 million rainbow trout annually, with private hatcheries also making releases.

Native to Europe and western Asia, brown trout prefer cool upland streams, rivers and reservoirs, having a similar distribution in the MDB to rainbow trout, yet are not present in QLD. Brown trout is a target species for rec fishers across south-eastern Australia, with NSW and VIC fisheries combined releasing approximately one million each year. Brown trout is ‘less catchable’ than rainbow trout, making their release numbers significantly lower.

Alien trout species were introduced to the Murray Darling Basin in the late 1800s because of human activities such as introductory releases and stockings. In NSW, the first introductions of alien trout species to the MDB took place in the Upper Goodradigbee River in 1888, where they were noted for rapidly eliminating the native mountain galaxias species previously present. (We now know this was at least two species, the “standard” mountain galaxias and stocky galaxias.)

Screen grab from Mountain galaxias - Jamieson Victoria (Galaxias olidus). Credit: Henry Albert

Whilst trout are stocked in many places each year to be caught for recreation, many populations also sustain themselves by strong breeding and recruitment in the wild. Both brown trout and rainbow trout are listed in the top 100 of the world’s worst invasive alien species list by the International Union for Conservation of Nature (IUCN). A common issue with each of these trout species can be attributed to diet. Trout feed on small aquatic animals, including freshwater insect larvae, wind-blown terrestrial insects, crustaceans and small fish. Both species are however noted for preying more on fish as they get larger. These species’ preference for small animals poses a severe problem for small and juvenile native fish species and other native fauna including frogs, crayfish and macroinvertebrates. Beyond predation, food and habitat competition are also impacts of introduced trout.

The awareness of the plight of galaxiids against alien trout is growing. Trout predation is confining many galaxiid populations to tiny sections of headwater streams made inaccessible to trout by natural barriers such as waterfalls and cascades. The enormous reduction in abundance and distribution of stocky galaxias, mountain galaxias and the threatened barred galaxias is directly attributed to alien trout. Nine out of fifteen recently described galaxiids have been listed as critically endangered and all are threatened by trout. It is important to note that trout predation has without doubt eradicated some galaxias species before they could be discovered or described. Trout are also known to threaten Macquarie perch, trout cod, blackfish species, southern pygmy perch, spiny crayfish, and the spotted and Booroolong tree frogs.

International Union for Conservation of Nature categories. Credit: IUCN Red List.

A 2023 study by Simon Kaminskas investigated the relationship between alien fish stockings and declining numbers, and extinctions, of large-bodied native fish species in the relatively pristine Goodradigbee River system. Using historical records combined with modern fish sampling data, scientific literature and field observations, the study found a multitude of evidence that the declines and extinctions of native fish were due to the impacts of alien fish.

Within the case study specifically, large-bodied native fish species comprising Murray cod, Macquarie perch and silver perch, as well as the intermediately sized two-spined blackfish and the small-bodied mountain galaxias species and carp gudgeon species, were found to be nearly extinct or wholly extinct in the Lower Goodradigbee River. Macquarie perch have not been sampled since ~2002, even though the Lower Goodradigbee was once noted for the abundance of this iconic species. Silver perch became extinct in the 1980s, despite also once being common in the Lower Goodradigbee and other regional rivers. The long-term domination of alien trout and constant trout stockings in this system, as well as the more recent invasion of alien carp, are directly linked to this dismal abundance of native fish species. The study suggests a cessation of ongoing trout stockings in favour of replenishing stocks of Murray cod and other native fish species.

3 two-spined blackfish have been predated between 2 trout, with the biggest blackfish being 17.5 cm long. Source: Kaminskas (2023).
Rainbow trout predation on two-spined blackfish. Source: Kaminskas (2023).

The events in the Lower Goodradigbee River are a strong warning that alien fish impacts, including through stockings, can be solely responsible for extinctions of large-bodied native fish in montane, upland and slope stream habitats (rather than habitat degradation or river regulation). Additionally, the study calls for a major re-evaluation of the impacts of alien fish and alien fish stocking in Australia, accompanied by thorough and dedicated research.

The stocking of brown trout is now restricted in some streams and dams for conservation reasons where threatened species are known to be present. However, rainbow trout stockings are continuing in some threatened species habitats, such as the Upper Murrumbidgee where endangered Macquarie perch populations hang on. A broader community discussion on the appropriateness of such trout stockings is needed, recognising for instance that while we don’t need the Upper Murrumbidgee for trout stocking, the Macquarie perch do need the Upper Murrumbidgee for survival.

Clearly, more specific and tailored research is needed within Australian aquatic ecosystems to best inform the delicate balance between conservation and recreation. Generating greater awareness of this thorny issue and sparking constructive community discussions on how to achieve this balance, will be a great start. The question remains though as to whether this is enough to support the plight of our native fish in the future.

Featured image: Brown trout.

Credit: Adobestock 

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