T. F. Cross

Fitness reduction and potential extinction of wild populations of Atlantic salmon, Salmo salar, as a result of interactions with escaped farm salmon.

The high level of escapes from Atlantic salmon farms, up to two million fishes per year in the North Atlantic, has raised concern about the potential impact on wild populations. We report on a two-generation experiment examining the estimated lifetime successes, relative to wild natives, of farm, F1 and F2 hybrids and BC1 backcrosses to wild and farm salmon. Offspring of farm and ‘hybrids’ (i.e. all F1, F2 and BC1 groups) showed reduced survival compared with wild salmon but grew faster as juveniles and displaced wild parr, which as a group were significantly smaller. Where suitable habitat for these emigrant parr is absent, this competition would result in reduced wild smolt production. In the experimental conditions, where emigrants survived downstream, the relative estimated lifetime success ranged from 2% (farm) to 89% (BC1 wild) of that of wild salmon, indicating additive genetic variation for survival. Wild salmon primarily returned to fresh water after one sea winter (1SW) but farm and ‘hybrids’ produced proportionately more 2SW salmon. However, lower overall survival means that this would result in reduced recruitment despite increased 2SW fecundity. We thus demonstrate that interaction of farm with wild salmon results in lowered fitness, with repeated escapes causing cumulative fitness depression and potentially an extinction vortex in vulnerable populations.

Genetic impact of escaped farmed Atlantic salmon (Salmo salar L.) on native populations: use of DNA profiling to assess freshwater performance of wild, farmed, and hybrid progeny in a natural river environment

McGinnity, P., Stone, C., Taggart, J. B., Cooke, D., Cotter, D., Hynes, R., McCamley, C., Cross, T., and Ferguson, A. 1997. Genetic impact of escaped farmed Atlantic salmon (Sulmo salar L.) on native populations: use of DNA profiling to assess freshwater performance of wild, farmed, and hybrid progeny in a natural river environment. - ICES Journal of Marine Science, 54: 998-1008. Since Atlantic salmon (Salmo salur L.) used for farming are usually genetically different from local wild populations, breeding of escaped farmed salmon potentially results in genetic changes in wild populations. To determine the likelihood and impact of such genetic change, an experiment was undertaken, in a natural spawning tributary of the Burrishoole system in western Ireland, to compare the performance of wild, farmed, and hybrid Atlantic salmon progeny. Juveniles were assigned to family and group parentage by DNA profiling based on composite genotypes at seven minisatellite loci. Survival of the progeny of farmed salmon to the smolt stage was significantly lower than that of wild salmon, with increased mortality being greatest in the period from the eyed egg to the first summer. However, progeny of farmed salmon grew fastest and competitively displaced the smaller native fish downstream. The offspring of farmed salmon showed a reduced incidence of male parr maturity compared with native fish. The latter also showed a greater tendency to migrate as autumn pre-smolts. Growth and performance of hybrids were generally either intermediate or not significantly different from the wild fish. The demonstration that farmed and hybrid progeny can survive in the wild to the smolt stage, taken together with unpublished data that show that these smolts can survive at sea and home to their river of origin, indicates that escaped farmed salmon can produce long-term genetic changes in natural populations. These changes affect both single-locus and high-heritability quantitative traits, e.g. growth, sea age of maturity. While some of these changes may be advantageous from an angling management perspective, they are likely, in specific circumstances, to reduce population fitness and productivity. Full assessment of these changes will require details of marine survival, homing and reproductive performance of the adults together with information on the F, generation. 0 1997 International Council for the Exploration of the Sea

Genetic effects of hatchery rearing in Atlantic salmon

Abstract Six polymorphic enzyme loci were examined electrophoretically in a sample of wild Atalntic salmon smolts from the Burrishoole river in western Ireland and in samples of artificially-reared fry hatched in 1981 and parr hatched in 1979. These hatchery reared fish were the progeny of five generations of artificially reared sea ranched salmon which had originally come from the Burrishoole river. Selection for growth and disease resistance was practised and between ten and 30 females and similar numbers of males were used as parents in each generation. Gene frequencies differed significantly at a number of loci between the wild and the artificially reared samples. Erosion of genetic variability, as measured by mean heterozygosity and mean number of alleles over the six loci, was evident in both hatchery reared samples. It is argued that the observed genetic changes are caused by founder effects and genetic drift rather than selection by some aspects of the artificial rearing regime. The importance of using adequate numbers of parents in hatchery rearing is stressed, since it is shown that differences between wild and reared populations are as great as between natural populations from Irish rivers.

Impact of naturally spawning captive-bred Atlantic salmon on wild populations: depressed recruitment and increased risk of climate-mediated extinction

The assessment report of the 4th International Panel on Climate Change confirms that global warming is strongly affecting biological systems and that 20–30% of species risk extinction from projected future increases in temperature. It is essential that any measures taken to conserve individual species and their constituent populations against climate-mediated declines are appropriate. The release of captive bred animals to augment wild populations is a widespread management strategy for many species but has proven controversial. Using a regression model based on a 37-year study of wild and sea ranched Atlantic salmon (Salmo salar) spawning together in the wild, we show that the escape of captive bred animals into the wild can substantially depress recruitment and more specifically disrupt the capacity of natural populations to adapt to higher winter water temperatures associated with climate variability. We speculate the mechanisms underlying this seasonal response and suggest that an explanation based on bio-energetic processes with physiological responses synchronized by photoperiod is plausible. Furthermore, we predict, by running the model forward using projected future climate scenarios, that these cultured fish substantially increase the risk of extinction for the studied population within 20 generations. In contrast, we show that positive outcomes to climate change are possible if captive bred animals are prevented from breeding in the wild. Rather than imposing an additional genetic load on wild populations by releasing maladapted captive bred animals, we propose that conservation efforts should focus on optimizing conditions for adaptation to occur by reducing exploitation and protecting critical habitats. Our findings are likely to hold true for most poikilothermic species where captive breeding programmes are used in population management.

Management of salmonid fisheries in the British Isles: towards a practical approach based on population genetics

The evidence for structuring of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) into distinct reproductive populations and for genetic differentiation and local adaptation is compelling. The effect of genetic variation among populations is demonstrably a factor determining the economic value of salmonid fisheries in the British Isles. Genetic considerations are, therefore, a matter of self-interest for fisheries managers and a shared interest with those advocating more general approaches to the conservation of diversity and variation. The local population is the basic unit of production and, therefore, the preferred unit of management. However, salmonid populations are numerous and many are small. These factors limit practical possibilities for management at the population level. We suggest that this difficulty can be addressed by combining populations in fisheries-biased management units that comprise interchangeable, nested groupings of populations that are both genetically and biologically meaningful. This population-based approach addresses the necessity of managing the fisheries in ways that are consistent with the conservation of adaptive potential in relation to the dynamic aspects of populations, their capacity to respond to changing environmental conditions, and the likelihood that salmonids will remain a worthwhile resource for the future.

Natural selection acts on Atlantic salmon major histocompatibility (MH) variability in the wild

Pathogen-driven balancing selection is thought to maintain polymorphism in major histocompatibility (MH) genes. However, there have been few empirical demonstrations of selection acting on MH loci in natural populations. To determine whether natural selection on MH genes has fitness consequences for wild Atlantic salmon in natural conditions, we compared observed genotype frequencies of Atlantic salmon (Salmo salar) surviving in a river six months after their introduction as eggs with frequencies expected from parental crosses. We found significant differences between expected and observed genotype frequencies at the MH class II alpha locus, but not at a MH class I-linked microsatellite or at seven non-MH-linked microsatellite loci. We therefore conclude that selection at the MH class II alpha locus was a result of disease-mediated natural selection, rather than any demographic event. We also show that survival was associated with additive allelic effects at the MH class II alpha locus. Our results have implications for both the conservation of wild salmon stocks and the management of disease in hatchery fish. We conclude that natural or hatchery populations have the best chance of dealing with episodic and variable disease challenges if MH genetic variation is preserved both within and among populations.

Allozyme variation in populations of Atlantic salmon located throughout Europe: diversity that could be compromised by introductions of reared fish

A comprehensive understanding of the population structure of Atlantic salmon (Salmo salar L.) throughout the species range would help to determine the impact that cultured fish could have on wild populations. To help achieve this aim, Atlantic salmon samples were obtained from 14 locations throughout Europe (including Iceland) and screened for variation at 32 allozyme loci. A sample was also obtained from Canada to serve as an out-group. Seventeen allozyme loci were found to be variable in one or more of the populations studied and three, s AAT-4 * , IDDH-2 * , and m MEP-2 * were variable across the range. This is the widest-ranging study to include ESTD-2 * , FBALD-3 * , and TPI-3 * , which combined contributed 28% to the total genetic diversity detected. Genotype frequencies complied with Hardy-Weinberg expected proportions. Over all loci, highly significant heterogeneity was observed between samples. Alternate alleles segregating at ESTD-2 * were found to be largely exclusive to Europe or North America. A neighbour-joining dendrogram was constructed to visualize relationships between populations and was consistent with previous findings that revealed Baltic and European clusters, with the Canadian population being the most genetically distinct. A significant association was observed between geographic and genetic distance, which suggests the potential for local adaptation, thus highlighting the need for conservation of wild populations.

Demographics and landscape features determine intrariver population structure in Atlantic salmon (Salmo salar L.): the case of the River Moy in Ireland

Contemporary genetic structure of Atlantic salmon (Salmo salar L.) in the River Moy in Ireland is shown here to be strongly related to landscape features and population demographics, with populations being defined largely by their degree of physical isolation and their size. Samples of juvenile salmon were collected from the 17 major spawning areas on the river Moy and from one spawning area in each of five smaller nearby rivers. No temporal allele frequency differences were observed within locations for 12 microsatellite loci, whereas nearly all spatial samples differed significantly, suggesting that each was a separate population. Bayesian clustering and landscape genetic analyses suggest that these populations can be combined hierarchically into five genetically informative larger groupings. Lakes were found to be the single most important determinant of the observed population structure. Spawning area size was also an important factor. The salmon population of the closest nearby river resembled genetically the largest Moy population grouping. In addition, we showed that anthropogenic influences on spawning habitats, in this case arterial drainage, can affect relationships between populations. Our results show that Atlantic salmon biodiversity can be largely defined by geography, and thus, knowledge of landscape features (for example, as characterized within Geographical Information Systems) has the potential to predict population structure in other rivers without an intensive genetic survey, or at least to help direct sampling. This approach of combining genetics and geography, for sampling and in subsequent statistical analyses, has wider application to the investigation of population structure in other freshwater/anadromous fish species and possibly in marine fish and other organisms.

Brooding in the intertidal barnacles Chthamalus stellatus (Poli) and Chthalamus montagui Southward in south-western Ireland

At Lough Hyne Marine Nature Reserve, western Cork, Ireland, the intertidal barnacles Chthamalus stellatus (Poli) and Chthamalus montagui Southward were shown to be capable of breeding in their first year after settlement. In C. montagui, zero, one or two broods and in C. stellatus, zero, one, two or in a single case three broods were produced in the first summer. Whether or not barnacles brood in their first year is probably due to a combination of the time since settlement and size attained. In their second year after settlement, both species are multiple brooders, producing two broods during the summer season.

Genetic characterisation of Atlantic salmon (Salmo salar) lines farmed in Ireland

Abstract Five reared lines of Atlantic salmon, Salmo salar L., constituted over 90% of the smolt production of the Irish farming industry in 1990. They were assayed for the six polymorphic enzyme loci, which contribute most of the reported electrophoretically-detectable variation in the enzymes of S. salar . The lines, four of Norwegian and one of chiefly Scottish origin, were sampled as parr in late 1989 and early 1990. Mean heterozygosity ( H ) and mean number of alleles per locus ( N a ) in the most popular farmed line were comparable to the values observed in wild Irish populations, but the other lines had lower values. It seems likely that genetic variability in the latter lines has been decreased by management practices, though it should be noted that wild donor populations were not investigated. The five reared lines analysed differed significantly from each other in genetic composition despite the fact that four lines originated from the same Norwegian source. The probability that many sub-groups of the major farmed lines differ from each other genetically is suggested and the implications for genetic investigations of escape incidents is discussed.