Anadromous Brown Trout (Salmo trutta) Contribute Disproportionally to Recruitment: Insights From Genomics and Multi-Tissue Stable Isotope Chemistry

Brown trout (Salmo trutta L.) exhibit a variety of life-history strategies. The two physiological and phenotypic extremes are represented by freshwater residents that remain in freshwater for their entire life and anadromous sea trout that migrate to sea to grow. The cue to initiate anadromy appears to be a combination of environmental factors, genetics, and non-genetic parental effects. We currently lack a systematic understanding of the interplay between these two life-history strategies, in particular their relative importance in the recruitment of populations displaying facultative migration. We assessed the importance of anadromous females in the recruitment of S. trutta at sampling locations 16–44 river km upstream of the tidal limit in a productive chalk stream using a combination of tissue stable isotope ratios and SNP genotyping of newly emerged fry, with a primary goal of understanding the migratory strategy of their mother and hence the relative contribution of the two strategies to recruitment. A secondary goal was to test a new method of assessing maternal strategy using the δ¹³C of the core of the eye lens. By combining information from muscle tissue and eye lens core stable isotope ratios, and SNP genotyping, we were able to allocate 97.5% of fry to either anadromous or freshwater-resident mothers. In most cases, the δ¹³C of the core of the eye lens appeared to provide greater discrimination among maternal strategies than muscle tissue. Given the inert nature of this archival structure, lens core chemistry represents a promising approach to assess maternal strategy in any aged fish. Anadromous females contributed disproportionately to recruitment throughout the catchment, where the proportion of fry with anadromous mothers followed a non-linear decline with increasing distance from the tidal limit. Close to the tidal limit (16 river km upstream) 100% of fry had anadromous mothers, declining to 43% at the site furthest from the tidal limit (44 river km upstream). As well as a disproportionate contribution of anadromous fish to young-of-the-year, the number of full-sibling family groups involving anadromous mothers was higher than for freshwater resident mothers at all sites. Anadromous S. trutta mothers contribute disproportionately to recruitment even in the headwaters of a productive chalk stream. Whilst we do not know how the contribution of anadromous mothers influences the balance of the two life history strategies, recruitment appears highly dependent on unobstructed migration pathways to and from the marine environment.