Winning the invasion roulette: escapes from fish farms increase admixture and facilitate establishment of non‐native rainbow trout

Abstract

Aquaculture is a major source of invasive aquatic species, despite the fact that cultured organisms often have low genetic diversity and tend to be maladapted to survive in the wild. Yet, to what extent aquaculture escapees become established by means of high propagule pressure and multiple origins is not clear. We analysed the genetic diversity of 15 established populations and four farmed stocks of non-native rainbow trout in Chile, a species first introduced for recreational fishing around 1900, but which has in recent decades escaped in large numbers from fish farms and become widespread. Aquaculture propagule pressure was a good predictor of the incidence of farm escapees, which represented 16% of all free-ranging rainbow trout and were present in 80% of the study rivers. Hybrids between farm escapes and established trout were present in all rivers at frequencies ranging between 7 and 69%, and population admixture was positively correlated with genetic diversity. We suggest that non-native salmonids introduced into the Southern Hemisphere could benefit from admixture because local adaptations may not have yet developed, and there may be initially little fitness loss resulting from outbreeding depression.

Native and exotic fishes in a Patagonian reservoir with rainbow trout cage culture: spatial and trophic resource use

“…This pattern could be favored by enhanced prey availability near the farms, as has been suggested for Canadian lakes (Podemski and Blanchfield, 2006). The comparison of body shape to assess nutritional status between farmed escapees and their free-living conspecifics is known to be affected by shape differences resulting from particular rearing environment and genetics (Swain et al, 1991;Jonsson and Jonsson, 2006;Consuegra et al, 2011). In this regard, our results showed a deeper body shape (higher body condition, SMI) for ERT compared to WRT.…”

“…Wild and farmed rainbow trout were identified according to their external characteristics (Temporetti et al, 2001;Fiske et al, 2005). However, since previous studies in Chilean Patagonia have found that ERTand WRTcan exhibit no obvious phenotypic differences (Consuegra et al, 2011), estimated abundances of ERT in the present study are probably underestimated (conversely, diet overlap between ERT and WRT may be overestimated). Stomachs were extracted in the laboratory and individually preserved in alcohol at À20°C until content analysis under a stereoscopic microscope.…”

“…Johnston and Wilson (2014), studying ERT in a Canadian lake, found a similar fashion of correct classification by visual assessment (85%, compared to genetic analysis). A lower overall precision was reported for rainbow trout in Chile (Consuegra et al, 2011). Therefore, it is reasonable to assume some degree of overestimation of wild individuals at the expense of farmed rainbow trout in our data.…”

“…Soto et al, 2001;Podemski and Blanchfield, 2006;Arismendi et al, 2009;Consuegra et al, 2011;Johnston and Wilson, 2014). Recent captures of mature ERT more than 30 km upstream in the main affluent of Alicurá reservoir (D. Nabaes Jodar, unpublished data), suggest that ERT is an expanding component of the regional fish community.…”

Escaped farmed salmon and trout in Chile: incidence, impacts, and the need for an ecosystem view

“…Coho, Chinook salmon, and rainbow trout have also been part of ranching programs in the past which eventually may also play a role in their establishment, especially in the case of Chinook salmon (Astorga et al 2008). It is also possible that salmonid escapees might increase the probability of establishing self-sustaining populations when those escapes are greater than from slow leakages or 'silent' escapes (Consuegra et al 2011). According to Consuegra et al (2011), invasion success may also de pend on pro pagule pressure.…”