Abstract
The impacts and interactions between hatchery-bred fish and wild fish populations has been a topic of active investigation in recent decades. In some instances, the benefits of stocking can be overshadowed by negative effects such as genetic introgression with natural populations, loss of genetic diversity, and dilution of local adaptations. Methods that facilitate the identification of stocked fish enable us to estimate not only the effectiveness of stocking but also the level of natural reproduction and the degree of hybridization. The longest Baltic river, the Vistula, also has the second highest discharge. Historically, it hosted numerous populations of the anadromous form of brown trout (sea trout); however, dam construction has since interfered with and reduced spawning migration to a rate that is much lower than before. Reduced spawning has resulted in a population collapse and a negative flow-on effect on commercial catches. In response, Poland (along with many other Baltic countries) initiated an intensive stocking program which continues today and which sees the average annual release of 700,000 smolts. As a consequence, today’s main-river and inshore catches come from stock-enhanced populations. High-throughput single-nucleotide polymorphism (SNP) genotyping was performed on samples of sea trout from southern Baltic populations; results suggest that a significant portion of the sea trout catches in the Vistula mouth region have direct hatchery origin and indicate the presence of Pomeranian specimens. SNP loci identified as outliers indicate a potential selection pressure that may be related with effects of hatchery breeding and mixing with natural populations. The brown trout SNP array applied in this study showed high effectiveness not only for population differentiation, but more importantly, it emerged as a sensitive tool to provide evidence of detection selection.
Highlights
Mixed-stock fisheries may target multiple anadromous fish populations; one strategy for documenting stock composition is genetic stock analysis [1]
A 5.5 K Illumina single-nucleotide polymorphism (SNP) iSelect genotyping array developed for brown trout (S. trutta) was used to analyze the genetic relationship between sea trout specimens from the southern Baltic
The share of the hatchery origin specimens inhabiting the Vistula mouth area is high, which can be indicative of relatively good stocking effectiveness
Summary
Mixed-stock fisheries may target multiple anadromous fish populations; one strategy for documenting stock composition is genetic stock analysis [1]. This approach is routinely applied for determining migratory routes, timing, and catch share for several marine and anadromous fish [2], [3] and offers a way to perform stock-specific management and facilitates the establishment of targeted stock-specific regulatory measures tailored towards safeguarding vulnerable populations [4]. In Poland, evidence suggest that 30 rivers have sea trout populations, and all of them have mixed status according to the Baltic Marine Environment
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