Introgression affects Salmo trutta juvenile life-history traits generations after stocking with non-native strains

Abstract

Introgression of non-native conspecifics changes the genetic composition of wildpopulations, potentially leading to loss of local adaptations and fitness declines.However, long-term data from wild populations are still relatively few. Here, westudied the effects of introgression in a Danish brown trout (Salmo trutta, L.) pop-ulation, subjected to intensive stocking with domesticated hatchery fish of non-native origin. We used wild-caught genetically wild and admixed trout as well asfish from the partly domesticated hatchery strain used for stocking the river upuntil ~15 years prior to this study, to produce 22 families varying in hatchery/wildadmixture. Following a replicated common-garden experiment conducted in fishtanks from first feeding through 23 weeks at 7, 12, and 16°C, we observed a signifi-cant positive relationship between family admixture and fish size upon termination,an effect observed through all levels of admixture. Furthermore, the admixtureeffect was most distinct at the higher rearing temperatures. Although the hatcherystrain used for stocking had been in culture for ~7 generations, it had not beendeliberately selected for increased growth. These data thus demonstrate: (i) thatgrowth had increased in the hatchery strain even in the absence of deliberate di-rectional selection for this trait, (ii) that the increasing effect of admixture by tem-perature could represent inadvertent selection for performance in the hatcherystrain at higher temperatures, and most significantly, (iii) that despite undergoingup to five generations of natural selection in the admixed wild population, the ge-netically increased growth potential was still detectable and thus persistent. Ourfindings suggest that altered growth patterns and potentially their cascading ef-fects are of importance to the severity of hatchery/wild introgression, especiallyunder changing-climate scenarios and are of general significance to conservationpractitioners seeking to evaluate long-term effects of intra-specific hybridizationincluding under recovery.