Siblingship tests connect two seemingly independent farmed Atlantic salmon escape events

Abstract

Aquaculture escapees represent a threat to the genetic integrity of native populations, may spread infectious agents and display ecological interactions with wild fish. DNA-based identification methods are well established for tracing Atlantic salmon escapees back to their farms of origin. However, traditional genetic assignment approaches are not always able to single out the farm of origin in cases where several potential farm sources rear fish from the same genetic line, and display strongly overlapping allele frequencies. We investigated whether an alternative statistical approach, which involves ad hoc identification of sibling relationships, circumvents the challenge of overlapping allele frequencies. We analysed the following samples collected in 2013: (1) 221 farmed escapees captured in several rivers in the Ryfylke region of Norway, (2) 139 farmed escapees captured some 150 km away in an upstream fish migration trap in the River Etne, and (3) 779 farmed salmon sampled from 17 cages on 10 farms in Ryfylke. Siblingship tests increased the precision of identification of escapees back to their farm of origin over genetic assignment and population statistic approaches. Together with other non-genetic data, siblingship tests were also able to connect 2 seemingly independent escape events, demonstrating that some of the salmon escaping from 1 or 2 farms in Ryfylke took approximately 1 mo to migrate 150 km northwards before entering the River Etne. Finally, we demonstrated that the genetic background of the escapees captured in the River Etne during the course of an entire season was represented by the 3 major breeding programs in Norway.

Modeling fitness changes in wild Atlantic salmon populations faced by spawning intrusion of domesticated escapees

“…This population has been demonstrated to be admixed with farmed escapees at ~20% (Glover et al., ; Karlsson et al., ). Recently, an upstream trapping system that permits removal of nearly all escapees trying to enter the river was installed (Madhun et al., ; Quintela et al., ), thus offering the native population protection from further admixture and the ability to recover in a region where there are still persistently high numbers of farmed escapees. In the future, natural strayers from this recovering population may buffer domestication‐driven introgression in close‐by rivers.…”

Timing is everything: Fishing‐season placement may represent the most important angling‐induced evolutionary pressure on Atlantic salmon populations

“…The holding area is checked daily, and biological measurements (weight and length) are taken from all salmon and sea trout entering the trap. Based upon external morphological characteristics (including body size, fin erosion, coloration), wild salmon and sea trout are thereafter released to continue their upstream migration, while farmed salmon escapees are killed and thereafter sampled to verify their escape status using scale and DNA analysis (Lund & Hansen, ; Madhun et al., ; Quintela et al., ). Fish were identified as either farmed or wild; hybrids were not identified.…”

“…DNA extraction and amplification of the 18 microsatellite markers were performed as described in Quintela et al. (), with the addition of genetic sex markers to one of the multiplexes: Exon 2 and Exon 4 (modified from Eisbrenner et al., ). An additional set of 13 microsatellite markers (total of 31 markers for all fish) were amplified in two multiplexes (MP1: Ssa405, Ssa412 (Cairney, Taggart, & Hoyheim, ), Ssa98 (O'Reilly, Hamilton, McConnell, & Wright, ), SsOSL25 (Slettan, Olsaker, & Lie, ), SSsp2215 (Paterson, Piertney, Knox, Gilbey, & Verspoor, ), EST107, EST68 (Vasemagi, Nilsson, & Primmer, ) and MP2: EST28, EST19 (Vasemagi et al., ).…”

“…In this study, we sampled adult wild salmon returning to the River Etne in 2013 (distinguished from escaped farmed salmon based on morphological characteristics or using DNA and scale samples as detailed below (Madhun et al., ; Quintela et al., )), in addition to fish captured upstream of the trap by angling in the same year. Using DNA analysis to individually identify fish captured by angling upstream of the trap, with the data from sampling all fish on the trap, we were able to investigate previously unanswered questions, such as how quickly fish are captured upon entry to the river, the degree to which the angling catch reflects the populations characteristics (i.e., is angling random or selective?…”

Half a century of genetic interaction between farmed and wild Atlantic salmon: Status of knowledge and unanswered questions

“…This method is based on the fact that farmed salmon are fed a diet including a high concentration of terrestrial lipids that are high in medium chain polyunsaturated fatty acids (PUFAs) such as 18:2n‐6 (Olsen, Taranger, Svasand, & Skilbrei, ) and that its concentration decreases with time after escape (Skilbrei, Normann et al., ). Studies using this and other approaches have shown that one half or more of escapees entering freshwater have escaped from farms in the same year that they entered freshwater (Madhun et al., ; Quintela et al., ; Skilbrei, Normann et al., ).…”