Use of fatty acid profiles to monitor the escape history of farmed Atlantic salmon

Abstract

Farmed Atlantic salmon can escape from fish farms at various stages of their life, from juveniles to large mature fish. Escapees that enter rivers to spawn pose a threat to the genetic integrity of wild populations. Knowledge about the timing of these escapes can provide important information for wildlife management and the aquaculture industry, enabling them to prevent or mitigate the negative impacts of escapees. Farmed salmon food has a high content of terrestrial lipids; thus, we used fatty acid (FA) profiling to monitor the escape history of farmed salmon. Escaped salmon captured in rivers (n = 251) presented a wide range of FA profiles that we used to classify the fish as (1) early-escaped wild-like fish that were assumed to have escaped at smolt or early post-smolt stage (24%), (2) recently escaped fish with high levels of FAs typically found in commercial salmon food (61%) and (3) intermediate escapees whose FA profiles lay between those 2 groups (15%). To estimate the size at escape of the intermediate escapees, we performed a feeding experiment that monitored the development of FA profiles after a shift in diet from terrestrial to marine lipids. Most intermediate escapees appeared to have escaped when they were < 3 kg, and ranged from 3 to 11 kg when recaptured in rivers. We conclude that FA profiling is a promising tool to monitor escape histories, and that the proportion of post-smolt escapees in this study was high compared to official escape statistics which include very few reports of young fish escaping.

Fatty acid composition of sprat (Sprattus sprattus) and herring (Clupea harengus) in the Baltic Sea as potential prey for salmon (Salmo salar)

“…However, despite consuming the same prey, individual salmon can also differ in terms of their retention of lipids [57]. Nevertheless, the dietary FA composition should be reflected in the FA profiles of salmon [31][32][33]. Thus, the results of the present study are also important…”

“…We do not know whether this variation is dependent on the feeding preferences of individual salmon or is related to the feeding area. It would be possible to resolve this if we knew the FA composition of prey fish, because the dietary FA composition is reflected in the FA composition of salmon [31][32][33]. The proportions of FAs can be used in signature analysis to assess prey and predator relationships [34].…”

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

“…Reading fish scales provides an opportunity to identify the stage at which the salmon escaped from a farm (Thorstad et al., ). Also, recent developments in fatty acid profiling now make it possible to identify early (those salmon having been in the wild for some time, a year or more before entry to freshwater) as opposed to late (those having recently escaped, and certainly the same year in which they entered the river) escapees accurately (Skilbrei, Normann, Meier, & Olsen, ). 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., ).…”

Siblingship tests connect two seemingly independent farmed Atlantic salmon escape events

“…The level of this fatty acid is normally below 2.5% in sea-run and farmed salmon that has escaped early in life, and approximately 10 to 12% in fish feed and recently escaped farmed salmon (Jónsson et al 1997, Olsen & Skilbrei 2010. According to the method described by Skilbrei et al (2015b), the fish here were classified into 2 categories based on the level of the fatty acid 18:2(n-6): recently escaped (i.e. the same year as capture) farmed salmon with 18:2(n-6) > 7% and farmed salmon believed to have escaped at an early age (i.e.…”

“…2). Non-genetic supplementary methods, such as fatty acid (GrahlNielsen & Glover 2010) or pathogen profiling (Glover et al 2013b, Madhun et al 2015 have been successfully used as additional information to assist the identification of the specific farm of origin when the genetic background of several nearby farms is very similar, and also to monitor the escape history of the fish (Skilbrei et al 2015b). However, thus far, other statistical approaches using the available genetic data to identify farms of origin for escapees, e.g.…”