Compromising Baltic salmon genetic diversity : conservation genetic risks associated with compensatory releases of salmon in the Baltic Sea

• Many aspects need to be considered when evaluating the consequences of halting compensatory releases of salmon in the Baltic area. The present report focuses strictly on genetic concerns associated with large scale salmon releases.  A majority of the original wild Baltic salmon populations, i.e. populations of Atlantic salmon (Salmo salar) in the Baltic Sea, has gone extinct. Historically, 84 rivers flowing into the Baltic Sea have harbored Atlantic salmon, but currently only 10 of these maintain self sustaining wild natural populations in safe numbers (CCB 2012). Large scale releases of salmon are carried out in the Baltic region to increase productivity of separate populations and to compensate for natural reproduction that has been lost due to hydroelectric power plants that are blocking previous migratory routes.  Already in the 1980s observations of pronounced genetic differentiation between populations inhabiting different rivers, coupled with indications that salmon hatchery stocks are genetically divergent from the wild populations they were meant to represent, have warranted conservation genetics researchers to warn against potential negative effects of large scale releases (Ståhl 1981, 1983, 1987). In brief, current large scale releases can cause the following four types of genetic risks for native populations: 1) loss of genetic variation, 2) loss of adaptations, 3) change of population composition, and 4) change of population structure (Laikre et al. 2010). These adverse genetic impacts have been recognized and documented for salmonid fishes for decades (Ryman 1981; Ryman & Utter 1987; Hindar et al.1991; Waples 1999; Naish et al. 2008; Nielsen & Hansen 2008). In 2011 these potential risks of large scale releases gained attention when the European Commission put forward a proposal of phasing out all compensatory releases of salmon in the Baltic area (European Commission 2011).  We have compiled and reviewed information regarding compensatory releases of salmon in Swedish rivers including spatio-temporal genetic variability patterns of wild and hatchery salmon populations in the Baltic region. We review and synthesize scientific information from both peer-review and “gray” literature, and have used available genetic data from both published and unpublished studies to address the following main questions:  What is currently known regarding the spatio-temporal genetic variability patterns of Atlantic salmon in the Baltic Sea? How has the loss of salmon populations affected the overall capacity for Baltic salmon to maintain genetic variation? What are the effects of releases on genetic variation between and within wild salmon populations? How much of the overall genetic variability of Baltic salmon exists exclusively in hatcheries or is maintained only through breedingrelease operations?Based on current genetic knowledge, what recommendations can be provided with respect to the proposal from the European Commission to halt compensatory releases of salmon in the Baltic?  A total of 37 scientific studies on Baltic salmon genetic diversity have been identified. "Gray" genetic literature on Swedish salmon populations comprises seven additional reports. Together they cover genetic information from populations representing 35 Baltic river systems (Rivers Umeälven and Vindelälven counted separately) and c. 23 000 genotyped individuals. The main conclusions from these studies are that the Baltic salmon is genetically divergent from other Atlantic salmon populations and that there is a high degree of genetic structuring between populations in different rivers within the Baltic area. Further, there is a hierarchical grouping of populations in the Baltic, and three larger genetic groups, corresponding to populations in the north, east and south Baltic Sea have been found.   In Sweden, compensatory releases of salmon are performed in eight rivers flowing into the Baltic Sea and a total of more than 1.8 million salmon smolt are released annually in Sweden. Despite the hydropower companies’ policy to use local strains, fish of non-local origin is sometimes released. Further, in some hatcheries relatively few spawners are used, which may lead to an increased loss of genetic variation. Information regarding the number of released salmon, number of females and males used in rearing, and strains used for stocking is not easily accessible, and therefore assessment of genetic effects of large scale releases is not straightforward  Our analyses of published and unpublished genetic data indicate that a large part of the original genetic variation in Baltic salmon has already been lost due to extinction of individual populations and reduction in population sizes. There is a clear pattern of isolation-by-distance among wild populations, whereas no such pattern is found among hatchery stocks. Further, hatchery stocks typically exhibit lower genetic variation and are less divergent from each other than wild populations. However, hatchery stocks can harbor unique genetic variation and may thus be important to conserve.   The genetic effects of releases have not been monitored in the Baltic, but one scientific study indicates strong genetic homogenization of wild populations. Many of the changes of Baltic salmon gene pools occurred prior to the time when molecular genetic studies were possible. Thus, we are not likely to ever clarify exactly the changes that have occurred. Studies of salmonid releases in other parts of the world have in several cases documented altered genetic composition and reduced variability and viability. The extent of this threat needs further investigation. Until such data is available large scale releases should be stopped in line with the precautionary principle, provided that essential actions are implemented to protect remaining wild stocks from e.g. overharvest. Likewise, as many previous spawning areas as possible need to be restored, to safeguard the continued existence of Baltic salmon.

• Laxen i Östersjön består av både vild och odlad lax. Den odlade laxen sätts ut för att kompensera för de hinder som finns i laxens vandringsvägar i våra älvar. Frågan är vad den odlade laxen har för betydelse för den vilda laxens genetik.En stor andel av den vilda laxen i Östersjön har försvunnit. Bara 10 av de totalt 84 älvar som historiskt sett haft bestånd av vild lax, kan idag stoltsera med stabila bestånd. Den här rapporten beskriver vilka genetiska risker det finns med storskalig utsättning av odlad lax i Östersjön. Författarna ger också ett antal rekommendationer om åtgärder som bedöms vara nödvändiga för att minimera riskerna för förändring i de vilda laxarnas genetiska uppsättning.

Ämnesord

LANTBRUKSVETENSKAPER  -- Lantbruksvetenskap, skogsbruk och fiske -- Fisk- och akvakulturforskning (hsv//swe)

AGRICULTURAL SCIENCES  -- Agriculture, Forestry and Fisheries -- Fish and Aquacultural Science (hsv//eng)

NATURVETENSKAP  -- Biologi -- Genetik (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Genetics (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Environmental Sciences (hsv//eng)

NATURVETENSKAP  -- Biologi -- Annan biologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Other Biological Topics (hsv//eng)

Nyckelord

Atlantic salmion

salmo salar

extinction

genetic differentiation

laxfiskeri

utdöende

genetisk differentiering

National

Nationellt finansierad miljöövervakning

Integrerad kustfiskövervakning

Coastal fish stocks

Ett rikt växt- och djurliv

A Rich Diversity of Plant and Animal Life

populationsgenetik

Publikations- och innehållstyp

vet (ämneskategori)

rap (ämneskategori)