Stocking impact and migration pattern in an anadromous brown trout (Salmo trutta) complex: where have all the stocked spawning sea trout gone?

Abstract

We examined polymorphism at seven microsatellite loci among sea trout (Salmo trutta) (n = 846) collected from three areas in the Limfjord (Denmark). We then assessed their potential population source by comparing, using a mixed stock analysis (MSA) Bayesian framework, their genetic composition to that of brown trout collected from 32 tributaries pooled into nine geographical regions (n = 3801) and two hatcheries (n = 222) used for stocking. For each of the three regional sea trout groups (western, central and eastern Limfjord, n = 91, n = 426, n = 329, respectively), MSA was conducted with (i) all individuals in the group, (ii) with the subset of spawning sea trout only and (iii) with the subset of foraging, nonspawning individuals only, a subset that consisted primarily of sea trout caught during their first year at sea. For all three regional sea trout groups, a higher proportion of individuals (regardless of whether they were foraging or spawning) appear to have originated from the rivers that drain locally, than from the rivers that drain in other parts of the Limfjord. This suggests (1) that sea trout, at least during their first year at sea, undertake limited migrations within the Limfjord system and (2) that sea trout on their spawning run were caught close to their natal rivers. The proportion of sea trout of hatchery origin varied widely among all three Limfjord areas and broadly reflected regional stocking histories, with high proportions of sea trout of domestic origin in the east (39.3%), where stocking with domestic trout was practised intensely at the time of sampling, and in the west (57.2%), where a programme of coastal stocking of post smolts took place over several years in the early 1990s. In contrast, in the central Limfjord, where stocking with domestic trout was largely abandoned in the early 1990s, the proportion of sea trout of domestic origin was only 8.5%. Interestingly, for all three regional sea trout groups, virtually no sea trout of hatchery origin were found among the spawning individuals, which were on average larger than the nonspawning sea trout. These results suggest that stocked domestic brown trout that become anadromous experience high mortality at sea and are therefore largely absent among the larger, spawning individuals. We conclude that sea trout of domestic origin exhibit much reduced ability to reproduce and are unlikely to contribute significantly to the local gene pool largely because of a relatively high mortality at sea before the onset of maturity.

Genetic mixed-stock analysis of Atlantic herring populations in a mixed feeding area

“…Such effects are, however, expected to contribute little to allele frequency variation in marine organisms with extensive overlap between generations and the presumed very large effective population sizes. Our analysis comprised populations exhibiting lower differentiation than MSA studies in most other marine species (Wirgin et al 1997, Ruzzante et al 2000, Beacham et al 2008, Wennevik et al 2008) and approximately an order of magnitude lower than MSA studies in salmonids (Ruzzante et al 2004, Beacham et al 2005, Koljonen et al 2005, Smith et al 2005, Gauthier-Ouellet et al 2009). Using BAYES with data for 8 microsatellite loci typed in 26 Atlantic salmon populations, Koljonen et al (2005) reported that deviation between true and estimated single population contributions averaged 3% in a composite self-assignment test (i.e.…”

“…MSA has been applied widely in anadromous salmonids (e.g. Ruzzante et al 2004, Beacham et al 2005, Koljonen et al 2005, Smith et al 2005, Gauthier-Ouellet et al 2009, Miller et al 2010), but in spite of the method's large potential for determining spatially and temporally explicit migratory behaviour, relatively few studies have yet been conducted in marine fishes (Waples & Naish 2009). One of the reasons for this paucity is that the generally modest levels of differentiation among marine populations (e.g.…”

Mixed stock analysis of juvenile loggerheads (Caretta caretta) in Indian River Lagoon, Florida: implications for conservation planning

“…Similar MCMC analyses have been used to estimate stock composition and rookery contribution in other marine turtles and fishes (Fernandez et al 2002;Fillatre et al 2003;Herwerden et al 2003;Luke et al 2004;Ruzzante et al 2004). We also test the hypothesis that large rookeries in close geographic proximity to juvenile aggregations contributed more individuals to those populations than would be expected, given their relative size and geographic proximity.…”

Genetic restoration of a stocked brown trout Salmo trutta population using microsatellite DNA analysis of historical and contemporary samples

“…Natural selection against domesticated fish and their descendants is generally assumed to be strong and may involve juvenile survival (McGinnity et al 2003), marine survival and return rate (McGinnity et al 2003;Ruzzante et al 2004) and spawning competition (Fleming et al 2000). However, in the case of the Skjern River selection against domesticated trout has not overcome the rate of 'immigration' because of stocking, as evidenced by the high and permanent admixture proportions of domesticated trout.…”