Oncorhynchus mykiss has a high environmental impact in the Great Lakes.
Realized:
Rainbow trout and brown trout (Salmo trutta) were deemed at least partially responsible for the extirpation of Arctic grayling (Thymallus arcticus) in Michigan, its only known location in the Great Lakes Basin (Crawford 2001). Fausch (1988), Clark and Rose (1997), and numerous papers cited in both discussed several factors affecting competitive interactions between rainbow and brook trout (Salvelinus fontinalis), although the overall impact of this competition on brook trout is not well known (Crawford 2001). Reportedly, rainbow trout also drive nongame fishes such as suckers and northern pikeminnow (Ptychocheilus oregonensis) from feeding territories (Li, personal communication to P. Moyle in Moyle 1976a). Rainbow trout hybridize with other rarer trout species, thereby affecting their genetic integrity (Page and Burr 1991, Rinne and Minckley 1985) (see Potential Impacts for examples outside of the Great Lakes). Rainbow trout may also affect invertebrate populations. Feltmate and Williams (1989) found that the introduction of rainbow trout to an enclosure within a Great Lakes tributary in Ontario caused a 35% decline in stonefly abundance relative to areas without rainbow trout. Stonefly populations were adversely affected by both predation and disturbance, which led to emigration (Feltmate and Williams 1989).
Rainbow trout is an important aquaculture species produced in both land-based and water-based hatcheries. Although there are hundreds of land-based hatcheries and fewer than 15 freshwater cage farms in Canada (many of which are in Lake Huron), cage farms produce the majority of fish and rainbow trout are the numerically dominant species (Rooney and Podemski 2010). High volumes of aquaculture production have had adverse impacts in freshwater lakes. In a study of cage production in a freshwater lake in Ontario, Canada, Rooney and Podemski (2010) documented increases in ammonia and metal concentrations, such that Cu and Zn exceed sediment quality guidelines. Cage farms also release solid wastes, nitrogen, and phosphorus, which can cause increased algal blooms and decreased oxygen availability (Hamblin and Gale 2002). Such trends have already been documented in northern Lake Huron and nearby Lake Wolsey (Hamblin and Gale 2002). Additionally, stocking of hatchery rainbow trout in rivers has led to the introduction of whirling disease into open waters of approximately 20 states including, most recently, the Madison River and its tributaries in Montana (B. Nehring and R. White, personal communication). Both non-native and native salmonids are susceptible to the disease (Yoder 1972). In the Madison River, the disease has reduced the rainbow trout population by 90% (White, personal communication).
Crawford et al. (2001) pointed out that salmonids have the potential to alter the energy and nutrient cycles of the Great Lakes system through increased energy transfer between open water and streams/tributaries. This energy transfer includes the addition of nitrogen and phosphorus to tributaries through decaying salmonine carcasses, as well as the addition of salmon eggs and dead fish as a food source in streams (Ivan et al. 2011, Parmenter and Lamarra 1991, Rand et al. 1992). Rand et al. (1992) found that phosphorus released from salmon carcasses was responsible for >50% of the total phosphorus discharged in some Lake Ontario streams during parts of the spring. The presence of live salmonids may have an even greater effect on nutrients in streams through the excretion of ammonium and soluable reactive phosphorus and their mechanical disturbance of the stream bottom during spawning runs (Ivan et al. 2011, Tiegs et al. 2009).
Potential:
Rainbow trout have the potential to consume native fishes and compete with native salmonids (Page and Laird 1993). Introduced rainbow trout eat endangered humpback chub (Gila cypha) in the Little Colorado River and may exert a major negative effect on the population there (Marsh and Douglas 1997). Rainbow trout have been found to negatively affect Little Colorado spinedace (Lepidomeda vittata) through predation and by affecting spinedace behavior. The trout occupied undercut banks that the spinedace normally used for refuge. As a result, spinedace were displaced from preferred microhabitats and pushed into open water, making them vulnerable to predation (Blinn et al. 1993). Introduced predatory fishes, including the rainbow trout, are likely at least partially responsible for the decline of the Chiricahua leopard frog (Rana chiricahuensis) in southeastern Arizona (Rosen et al. 1995).
In California, rainbow trout have hybridized with Lahontan cutthroat trout (O. clarki henshawi), golden trout (O. aguabonita), and redband trout (O. mykiss subsp.) to the point that all three are included in the threatened trout management program of the California Department of Fish and Game (Behnke 1992, McAffee 1966b, Moyle 1976b). In the Lahontan drainage and various Rocky Mountain rivers, hybridization with rainbow trout has been a major factor in the decline of native cutthroat trout (O. clarkii henshawi) (McAffee 1966a). In fact, rainbow trout have replaced Lahontan cutthroat trout in many areas where the cutthroat is native and rainbow trout have been introduced (McAffee 1966b). For instance, introduced rainbow trout were likely partially responsible for the near-extinction of Lahontan cutthroat in Lake Tahoe in the 1940s (McAffee 1966b). Rainbow trout have been shown to hybridize with Westslope cutthroat trout (O. clarki lewisi) throughout the Flathead River system in Montana (Muhlfeld et al. 2009). Oncorhynchus clarkii lewisi hybridization with O. mykiss, and the resulting backcrossing to pure parent populations, has also resulted in strong introgression toward both populations in the Upper Oldman River, Alberta, Canada (Rasmussen et al. 2010). In Nevada, this species is also held responsible for the virtual extinction of Alvord cutthroat O. mykiss subsp. (Behnke 1992). In Arizona, the species has hybridized with native Gila trout (O. gilae) and Arizona trout (O. apache) (Minckley 1973, U.S. Fish and Wildlife Service 1979). The latter species is also known as the Apache trout and is considered a subspecies of O. gilae (i.e. O. g. apache).
There is little or no evidence to support that Oncorhynchus mykiss has significant socio-economic impacts in the Great Lakes.
Oncorhynchus mykiss has a high beneficial effect in the Great Lakes.
Realized:
Rainbow trout has been stocked as a recreational species in the Great Lakes since the 1800s and are currently stocked in all five lakes and Lake St. Clair (FWS/GLFC 2010, NYDEC 2011). The U.S. Fish and Wildlife Service (U.S. FWS 2006) estimated that nationally, every dollar spent on hatchery programs for rainbow trout returns over $36 of net economic value. One survey estimated the rainbow trout recreational fishery to be worth up to $12-14 million annually in Lake Erie, compared to a stocking cost of $600,000 (Kelch et al. 2006). Additionally, the global production of aquacultured rainbow trout has grown continuously, annually producing over 700,000 tons as of 2010 (FAO 2011).
Potential:
Eggs spawned by steelhead have been found to comprise an important part of the native brown trout diet in Great Lakes tributaries, but the effects of this consumption have yet to be understood (Ivan et al. 2011).