Temporal Genetic Variance and Propagule-Driven Genetic Structure Characterize Naturalized Rainbow Trout (Oncorhynchus mykiss) from a Patagonian Lake Impacted by Trout Farming

Abstract

Knowledge about the genetic underpinnings of invasions—a theme addressed by invasion genetics as a discipline—is still scarce amid well documented ecological impacts of non-native species on ecosystems of Patagonia in South America. One of the most invasive species in Patagonia’s freshwater systems and elsewhere is rainbow trout (Oncorhynchus mykiss). This species was introduced to Chile during the early twentieth century for stocking and promoting recreational fishing; during the late twentieth century was reintroduced for farming purposes and is now naturalized. We used population- and individual-based inference from single nucleotide polymorphisms (SNPs) to illuminate three objectives related to the establishment and naturalization of Rainbow Trout in Lake Llanquihue. This lake has been intensively used for trout farming during the last three decades. Our results emanate from samples collected from five inlet streams over two seasons, winter and spring. First, we found that significant intra- population (temporal) genetic variance was greater than inter-population (spatial) genetic variance, downplaying the importance of spatial divergence during the process of naturalization. Allele frequency differences between cohorts, consistent with variation in fish length between spring and winter collections, might explain temporal genetic differences. Second, individual-based Bayesian clustering suggested that genetic structure within Lake Llanquihue was largely driven by putative farm propagules found at one single stream during spring, but not in winter. This suggests that farm broodstock might migrate upstream to breed during spring at that particular stream. It is unclear whether interbreeding has occurred between “pure” naturalized and farm trout in this and other streams. Third, estimates of the annual number of breeders (N b) were below 73 in half of the collections, suggestive of genetically small and recently founded populations that might experience substantial genetic drift. Our results reinforce the notion that naturalized trout originated recently from a small yet genetically diverse source and that farm propagules might have played a significant role in the invasion of Rainbow Trout within a single lake with intensive trout farming. Our results also argue for proficient mitigation measures that include management of escapes and strategies to minimize unintentional releases from farm facilities.

Contrasting genetic metrics and patterns among naturalized rainbow trout (Oncorhynchus mykiss) in two Patagonian lakes differentially impacted by trout aquaculture

“…Based on these results, we followed Benavente et al. () approach and kept all collections separate and considered them independent for subsequent tests involving contrasts between lakes.…”

“…Samples taken during 2013 at Lake Llanquihue were from the Benavente et al. () study after excluding one inlet stream likely populated by escaped trout (i.e. Pescado (PES), Blanco (BLA), Tepu (TEP), and Blanco Arenales (BAR)).…”

“…Hierarchical groups were variance components between year or season collections within streams (intrapopulation; F SC ) and between streams (interpopulation; F CT ) following Benavente et al. ().…”

“…Recently, Benavente et al. () successfully used single nucleotide polymorphisms (SNPs) developed from a panel of North American populations to estimate intra‐ and interpopulation divergence among rainbow trout in Lake Llanquihue. Naturalized rainbow trout populations harbored high genetic diversity, providing an important molecular resource for invasion genetics studies; putative escaped trout were also identified (Benavente et al., ).…”

“…() successfully used single nucleotide polymorphisms (SNPs) developed from a panel of North American populations to estimate intra‐ and interpopulation divergence among rainbow trout in Lake Llanquihue. Naturalized rainbow trout populations harbored high genetic diversity, providing an important molecular resource for invasion genetics studies; putative escaped trout were also identified (Benavente et al., ). Highly variable SNPs may allow fine‐scale resolution of population differences where other markers have failed to resolve them (Ackerman, Habicht, & Seeb, ; Aykanat et al., ; Hecht, Campbell, Holecek, & Narum, ; Zarraonaindia et al., ).…”