Open access publications in the SLU publication database

A previous pilot study of adult salmon sampled 2011 in River Mörrumsån, southern Sweden, yielded surprising and alarming results. A significant proportion (c. 20%) of the putative salmon turned out to be hybrids with sea trout, despite that experienced personnel had handled the fish. Additionally, a high proportion (10-20%) of the pure salmon (confirmed with DNA) carried genotypes expected to be very rare among salmon from the river, indicating that those individuals were "strays" from other rivers. However, the above results were based on a low number of fish collected late within a single year. In order to allow general conclusions, a more comprehensive genetic survey was needed. Purposes of the present study were thus to genetically analyze a considerably larger material in order to: · obtain a better picture of the percentage hybrids occurring among adult "salmon" collected in different years and months, · to estimate the total hybrid proportion among all adult salmonids, · to identify and estimate the proportion of salmon strays, and · to study temporal genetic change in the R Mörrumsån salmon population since the 1960s, including signs of genetic impact from strayed salmon. The proportion of salmon-trout hybrids among putative salmon in the extended material from 1961, 1985, and 2004-2012 was lower (c. 6%) than in the pilot study from 2011 (c. 20%). The reason seems to be a considerably lower proportion of hybrids during the salmon spawning run peak (May-June) than in the subsequent fishing months (July-October). One reason for this seasonal variation may be that the hybrids, like the sea trout, exhibit a later spawning run time than the salmon in R. Mörrumsån. From catch statistics and fish counting data combined with DNA-results, we estimated the proportion of species hybrids among all ascending salmonids (salmon, trout, and hybrids combined) to have increased to >10% in recent years. Such a high proportion may result in a significant production loss due to "wasted reproductive effort" when effectively sterile hybrid males spawn with salmon and trout females. A more informed risk analysis is warranted, however, based on better knowledge of breeding the behavior and reproductive success of hybrids under natural or semi-natural conditions. The estimated proportion of salmon strays (2-3%) was clearly lower than in the previous pilot study (10-20%). A clear majority of the putative strays from different sampling years was caught late during the season (September-October), which may explain the high proportion of strays among the equally late caught salmon in 2011. At the same time, small but significant genetic changes in combination with evidence of increased genetic variability over time indicated that "foreign genes" have been introduced into the salmon population since the 1960s, although it remains largely unclear when and how this gene flow has occurred.