Salmo fario, S. lacustris; von Behr trout, Loch Leven trout, German brown trout

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

Realized:

Brown Trout has been implicated in reducing native fish populations (especially other salmonids) through predation, displacement, and food competition (Taylor et al. 1984). Brown Trout and Rainbow Trout (Oncorhynchus mykiss) were deemed at least partially responsible for the extirpation of Arctic Grayling (Thymallus arcticus) half a century ago in Michigan, its only known location in the Great Lakes basin (Crawford 2001, p. 143). Many studies looking at the effects of Brown Trout on Brook Trout (Salvelinus fontinalis) have been conducted, including Nyman (1970), Fausch and White (1981), Waters (1983), Fausch and White (1986), and DeWald and Wilzbach (1992). Taylor et al. (1984) listed a number of papers citing the effects of Brown Trout on native fishes. Fausch and White (1981) found that adult Brown Trout displaced adult native Brook Trout from the best habitats in a Michigan stream and from the northeast in general. Over a 15-year monitoring program, Waters (1999) observed a large-scale replacement of Brook Trout by Brown Trout following the introduction of Brown Trout in Valley Creek, MN. Brown Trout production increased to 95% of trout biomass in the stream by the end of the study (Waters 1999). Conversely, juvenile Brook Trout were dominant over juvenile Brown Trout of the same size in an artificial stream (Fausch and White 1986). The competitive advantage of the two species may change with size, age, temperature, stream size, or environmental adaptations of different populations (Fausch and White 1986). Brook Trout are also more susceptible to angling and predation than Brown Trout. Surges of resources (eggs and nutrients) due to salmon entering Great Lakes streams may provide greater benefit to the more competitive Brown Trout, and further increases in Brown Trout abundance may be at the expense of Brook Trout (Hermann et al. 2020). In Lake Ontario, Brown Trout has a high trophic overlap with other salmonids (Chinook, Coho, Lake and Rainbow Trout) (Yuille et al. 2015). In a laboratory experiment with artificial streams, the presence of Brown Trout reduced the survival and fitness related traits of Atlantic Salmon (Salmo salar) (Houde et al. 2015b), and ultimately may hinder the reintroduction of Atlantic Salmon in Lake Ontario and other native ranges (Hagelin and Bergman 2021). Also, the authors suggest that Brown Trout is a better competitor for food than Atlantic Salmon (Houde et al. 2017), except only for when Brown Trout abundance is low (Laroque et al. 2021).

Brown Trout is one of the few foreign species able to hybridize with natives, although the occurrence is considered rare (Brown 1966; Taylor et al. 1984). A study by Grant et al. (2002) in Valley Creek, MN, confirmed that hybridization between male Brown Trout and female Brown Trout occurs in the wild, resulting in a hybrid fish known as Tiger Trout. It also indicated that the interference of Brown Trout in conspecific Brown Trout reproduction could contribute to declines in Brown Trout populations (Grant et al. 2002).

Introduction of the bacteria Aeromanas salmonicida was likely a result of Brown Trout stocking and has led to cases of furunculosis in both native and non-native salmonids, including Brook Trout, Arctic Grayling (Thymallus arcticus), and Lake Whitefish (Coregonus clupeaformis) (Crawford et al. 2001; see GLANSIS fact sheet on A. salmonicida). Of salmonids in the Great Lakes, cultured Brown Trout had the highest rate of infection from bacterial coldwater disease (BCWD), caused by Flavobacterium psychrophilum (Van Vilet et al. 2015).

Potential:

Effects of Brown Trout on Great Lakes species can also be surmised by Brown Trout biology in general and the effects of Brown Trout on other species outside of the Great Lakes basin.
Brown Trout is a host for cestode parasites Diphyllobothrium spp. (Henrikson et al. 2016) and for the myxozoan endoparasite Tetracapsuloides bryosalmonae that causes proliferative kidney disease (PKD) in salmonids (Bruneaeux et al. 2017). Intensity of PKD infections also becomes more intense at water temperatures >19? compared to 16? (Waldner et al. 2021).
Natural hybridization between Brown Trout and Atlantic Salmon, a Lake Ontario native, has been frequently documented in Europe (Hartley 1996; Matthews et al. 2000; Álvarez and Garcia-Vasquez 2011).  Survival is reportedly highest among male trout x female salmon hybrids, which may have similar levels of survival to pure salmon while emerging earlier as fry (Álvarez and Garcia-Vasquez 2011).

The introduction of Brown Trout to various waters has resulted in the decline of some other native trout species. In California, competition and predation from Brown Trout may have contributed to the decline of Dolly Varden (S. malma) in the McCloud River (Moyle 1976), and of Golden Trout (Oncorhynchus aguabonita) in the Kern River (Krueger and May 1991; Courtenay and Williams 1992). Brown Trout has commonly replaced Cutthroat Trout (O. clarki) in large rivers (Behnke 1992). McAffee (1966) specifically reports that Lahontan Cutthroat Trout (O. c. henshawi) has been replaced by Brown Trout. Introduced Brown Trout, and other trout species, were likely responsible for the near-extinction of Lahontan Cutthroat Trout in Lake Tahoe in the 1940s (McAffee 1966). Yellowstone Cutthroat Trout in a Montana stream had significantly lower growth rates and had a considerable lack of recruitment where Brown Trout are also present (Al-Chokhachy and Sepulveda 2019). The removal of introduced Brown Trout led to recovery of Bonneville Cutthroat Trout in a small tributary in the intermountain west (Budy et al. 2021). Notably, hatchery-raised Brown Trout have a reduced predation threat to fishes relative to naturalized Brown Trout (Ward et al. 2018).

Brown Trout is also a significant threat to other non-trout species. For example, Brown Trout may have depleted the Modoc Sucker (Catostomus microps), an endangered species, in Rush Creek, Modoc County (Moyle and Marciochi 1975). Brown Trout predation threatens Humpback Chub in Colorado River (Ward et al. 2015), but the threat can be partially mitigated by increasing water turbidity to reduce predation success (Ward et al. 2016). In a human altered river in Utah, the introduction of Brown Trout resulted in its dominance and suppression of native fish populations (Belk et al. 2016). Because of their predatory nature, Brown Trout were introduced into Flaming Gorge Reservoir to reduce populations of the Utah Chub (Gila atraria) (Teuscher and Luecke 1996).  Competition with and predation by nonnative species (i.e., Catostomus sp., Creek Chub (Semotilus atromaculatus), Redside Shiner (Richardsonius balteatus), Burbot (Lota lota), Brown Trout (Salmo trutta), and Lake Trout (Salvelinus namaycush)) limit populations of the rare Bluehead Sucker (Catostomus discobolus) (Wyoming Game and Fish Department 2010). Brown Trout occupy similar habitat types as, and predate upon, Roundtail Chub (Gila robusta) (a species of conservation concern) in Wyoming lakes (Laske et al. 2012). Nonnative predators, including Brown Trout, have been shown to reduce the abundance and diversity of native prey species in several Pacific Northwest rivers (Hughes and Herlihy 2012).

In Chile, Brown Trout appeared to have adverse impacts on Aplochiton spp., a native galaxiid, marked by a complete absence of Aplochiton spp. in all streams containing Brown Trout (Young et al. 2010). Similar findings have been documented with other native Chilean stream fishes (Penaluna et al. 2009). A review by Townsend (1996) documented many impacts of Brown Trout introductions that have been studied in New Zealand, including predation of native galaxiids and their exclusion from stream habitat, potential reduction in insect and other invertebrate populations that may lead to reduced grazing, increased algal biomass and other trophic effects, and facilitation of the evolution of anti-predator behavior of some invertebrates. It is also possible that Brown Trout introduction played some role in the extinction of native New Zealand Grayling (Prototroctes oxyrhynchus) in the 1920s, although evidence from this time period is unavailable (Townsend 1996).

Brown Trout may have an impact on the food web due to its propensity for invertebrates. Its introduction into boreal lakes displaced macroinvertebrates from the pelagic zone and led to a trophic cascade effect that increased species richness of Cladocera (Milardi et al. 2016).

There is little or no evidence to support that Salmo trutta has significant socio-economic impacts in the Great Lakes.

Salmo trutta has a high beneficial effect in the Great Lakes.

Realized:

As of 2009, Brown Trout continues to be stocked as a sport fish to bolster recreational fisheries in all five Great Lakes and Lake St. Clair (USFWS/GLFC 2010; NYDEC 2011). While Brown Trout was reportedly less popular as a sport fish than Brook Trout (Bence and Smith 1999), it has since grown in popularity and contributed substantially to the recreational harvest in most of the Lakes (Bence and Smith 1999; Fisheries and Oceans Canada 2008).

Potential:

Brown Trout have the potential to control unwanted species. For example, because of their predatory nature, Brown Trout was introduced into Flaming Gorge Reservoir to reduce populations of the Utah Chub (Gila atraria) (Teuscher and Luecke 1996). Brown Trout also consumes high amounts of the invasive Round Goby in Lake Ontario, but its impact as a biocontrol agent was not directly assessed (Mumby et al. 2018).

In the United States and its territories, the importation or transportation of species in the family Salmonidae is prohibited unless otherwise stated (18 U.S.C. 42 (Lacey Act)). In Canada, the use or possession of fish as live bait in any province other than from which it was taken is prohibited (SOR/93-55). It is unlawful to transport, ship or convey live trout, salmon or char into Illinois unless a salmonid import permit has been issued to the source hatchery (Ill. Admin. Code Title 17 Statute 870.50). This species is considered nonnative in Minnesota but is not subject to regulation under invasive species statutes, however, it may have relevant fishing and hunting regulations (Statute 84D.07). It is illegal to bring any live fish into Ontario for use as bait (SOR/2007-237). In Quebec, both the use of this species as bait and the sale of dead fish of this species or its (CQLR c C-61.1, r7, SOR/90-214). It is a restricted species in Wisconsin, where there is a ban on the transport, transfer and introduction of this species, but possession is allowed (Chapter NR 40, Wis. Adm. Code).

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control

Biological

There are no known biological control methods for this species.

Physical

Barriers can be constructed and/or natural barriers augmented to prevent upstream migration and aid management and eradication efforts, though little research exists on effective barriers for Salmo trutta. Lintermans and Raadik (2003) noted 3 key aspects of successful barriers in relation to a Rainbow Trout control program: a 1.5 m or greater vertical drop; direction of water flow towards the middle in higher flows with no slower overland flow passing down the banks; and no deep pool below the barrier from which fish could jump.

The USACE Great Lakes and Mississippi River Interbasin Study notes the potential effectiveness of sensory deterrent systems in providing barriers to fish migration or eliciting fish movements, including underwater strobe lights, acoustic air bubble barriers, continuous and pulsed wave acoustic deterrents, and electric barriers (GLMRIS 2012). Most large-scale strobe systems consist of four individual lights that flash at a rate of 450 flashes/min., with an approximate intensity of 2634 lumens/flash (GLMRIS 2012).

The U.S. National Park Service uses physical removal through electrofishing to manage rainbow trout and Brown Trout populations that threaten native Brown Trout in Shenandoah National Park, Virginia (National Park Service 2011).

Chemical

Of the four chemical piscicides registered for use in the United States, antimycin A and rotenone are considered general piscicides (GLMRIS 2012).

In 1995, a total of 20 km of stream length (7 different streams) was treated in Victoria, Australia with a total of 60 L rotenone used, neutralized with 1100 kg of potassium permanganate, and with rotenone volumes ranging from 0.3 to 0.5 L per 100 m of stream, in order to eradicate Rainbow Trout (Oncorhynchus mykiss) and Brown Trout and protect native galaxiids (Lintermans and Raadik 2003). Areas in which trout and galaxiids overlapped were not treated with piscicides; they were instead intensively electrofished to selectively remove trout (Lintermans and Raadik 2003). Rotenone has also been used to eradicate Brown Trout in two streams of Kaiwharawhara catchment in Wellington, New Zealand using a concentration of 200 µg/L rotenone over initial treatment times of 4 and 5.5 hours for the smaller and larger stream, respectively (Pham et al. 2013). Ling (2003) noted that Brown Trout has an LC50 of 5.5 µg/L at 17°C for 1h exposure to rotenone.

Boogaard et al. (2003) found that Brown Trout are among the least sensitive fishes to the lampricide TFM and a TFM/1% niclosamide mixture in laboratory exposures, with a 12h LC50 of 8.90 mg/L for TFM and 12h LC50s of 5.01 and 5.68 mg/L for TFM/1% niclosamide. Sensitivity was slightly decreased in field tests (Boogaard et al. 2003).

Increasing CO2 concentrations, either by bubbling pressurized gas directly into water or by the addition of sodium bicarbonate (NaHCO3) has been used to sedate fish with minimal residual toxicity, and is a potential method of harvesting fish for removal, though maintaining adequate CO2 concentrations may be difficult in large/natural water bodies (Clearwater et al. 2008). Salmonids are considered to be among the most sensitive fishes to low dissolved oxygen levels, with a DO concentration of 1-3 mg/L sufficient to cause mortality or loss of equilibrium (Clearwater et al. 2008). However, CO2 is approved only for use as an anesthetic for cold, cool, and warm water fishes the US, not for use as euthanasia (Clearwater et al. 2008). Exposure to NaHCO3 concentration of 142-642 mg/L for 5 min. is sufficient to anesthetize most fish (Clearwater et al. 2008).

Low pH is known to affect fish behavior. Ikuta et al. (2003) documented the effects of low pH on S. trutta, noting that trout would not swim upstream into areas of pH lower than 5.5. Acute exposure to low pH levels can directly kill fish by discharge of sodium and chloride ions from body fluid, and sub-lethal levels can affect reproduction (Ikuta et al. 2003). In the case of Brown Trout, weak acidic conditions of <pH 6.4 were enough to depress prespawning digging behavior. Salmonids in general show significant avoidance of acidic environments (Ikuta et al. 2003).

It should be noted that chemical treatment will often lead to non-target kills, and so all options for management of a species should be adequately studied before a decision is made to use piscicides or other chemicals. Potential effects on non-target plants and organisms, including macroinvertebrates and other fishes, should always be deliberately evaluated and analyzed. The effects of combinations of management chemicals and other toxicants, whether intentional or unintentional, should be understood prior to chemical treatment. Boogaard et al. (2003) found that the lampricides 3-trifluoromethyl-4-nitrophenol (TFM) and 2’,5-dichloro-4’-nitrosalicylanilide (niclosamide) demonstrate additive toxicity when combined. In another study on cumulative toxicity, combinations of niclosamide and TFM with contaminants common in the Great Lakes (pesticides, heavy metals, industrial organics, phosphorus, and sediments) were found to be mostly additive in toxicity to Rainbow Trout, and one combination of TFM, Delnav, and malathion was synergistic, with toxicity magnified 7.9 times (Marking and Bills 1985). This highlights the need for managers to conduct on-site toxicity testing and to give serious consideration to determining the total toxic burden to which organisms may be exposed when using chemical treatments (Marking and Bills 1985). Other non-selective alterations of water quality, such as reducing dissolved oxygen levels or altering pH, could also have a deleterious impact on native fish, invertebrates, and other fauna or flora, and their potential harmful effects should therefore be evaluated thoroughly.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.