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

Abstract

1. Brown trout (Salmo trutta L.) exhibit a variety of life-history strategies. The two physiological and phenotypic extremes arerepresented by freshwater residents that remain in freshwater for their entire life and anadromous sea trout that migrate tosea to grow. The cue to initiate anadromy appears to be a combination of environmental factors, genetics, and non-geneticparental effects. We currently lack a systematic understanding of the interplay between these two life-history strategies, inparticular their relative importance in the recruitment of populations displaying facultative migration. 2. We assessed the importance of anadromous females in the recruitment of S. trutta at sampling locations 16–44 river km up-stream of the tidal limit in a productive chalk stream using a combination of tissue stable isotope ratios and SNP genotypingof newly emerged fry, with a primary goal of understanding the migratory strategy of their mother and hence the relativecontribution of the two strategies to recruitment. A secondary goal was to test a new method of assessing maternal strategyusing the δ13 C of the core of the eye lens. 3. By combining information from muscle tissue and eye lens core stable isotope ratios, and SNP genotyping, we were able toallocate 97.5% of fry to either anadromous or freshwater-resident mothers. In most cases, the δ13 C of the core of the eye lensappeared to provide greater discrimination among maternal strategies than muscle tissue. Given the inert nature of this ar-chival structure, lens core chemistry represents a promising approach to assess maternal strategy in any aged fish. 4. Anadromous females contributed disproportionately to recruitment throughout the catchment, where the proportion of frywith 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 riverkm upstream). As well as a disproportionate contribution of anadromous fish to young-of-the-year, the number of full-siblingfamily groups involving anadromous mothers was higher than for freshwater resident mothers at all sites. 5. Anadromous S. trutta mothers contribute disproportionately to recruitment even in the headwaters of a productive chalkstream. Whilst we do not know how the contribution of anadromous mothers influences the balance of the two life historystrategies, recruitment appears highly dependent on unobstructed migration pathways to and from the marine environment.