Spatial and temporal population structure of sea trout at the Island of Gotland, Sweden, delineated from mitochondrial DNA

Abstract

In a study of the genetic relationships among 879 anadromous brown trout Salmo trutta from 13 streams at the Island of Gotland, Sweden, using RFLP analysis of a mitochondrial DNA segment (NADH dehydrogenase-1 gene), six haplotypes were detected. Significant genetic divergence was observed among streams as well as between cohorts within streams. Approximately 8–9% of the total variation was due to differences between populations, and 4–5% was explained by differences between cohorts within populations. The female effective population size (Nef) was assessed from temporal haplotype frequency differences between consecutive cohorts; the estimated average Nef over all populations was just below 30, suggesting that these populations were effectively quite small. With such small effective sizes the populations are expected to loose genetic variability quickly, but the observed levels do not appear particularly low. This indicates that female migration between streams occurs. The observed level of differentiation does not support the presumption that a particular pre-smolt migratory behaviour observed in Gotland streams, with large portions of fry leaving for the sea soon after hatching, results in a reduced homing ability. From a conservation management perspective the Gotland brown trout streams should be regarded as a population system where the vitality and survival of brown trout in one stream is dependent on the opportunity of contact and exchange of individuals from other streams.