Effects of species, culture history, size and residency on relative competitive ability of salmonids

The relative competitive ability of juvenile farm and wild salmonids was investigated to provide insight into the potential effects of introduction of cultured salmon on wild Pacific salmonid ( Oncorhynchus ) species. Aquarium experiments involving equal contests ( i.e. size matched, simultaneously introduced individuals) indicated that two wild coho salmon Oncorhynchus kisutch populations were competitively equal to a farm coho salmon population. In equal contests between farm Atlantic salmon Salmo salar (Mowi strain) and these wild coho salmon populations or coastal cutthroat trout Oncorhynchus clarki clarki , Atlantic salmon were subordinate in all cases. When Atlantic salmon were given a residence advantage, however, they were competitively equal to both wild coho salmon populations, but remained subordinate to coastal cutthroat trout. When Atlantic salmon were given a 10–30% length advantage, they were competitively equal to one wild coho salmon population but remained subordinate to the other. In equal contests in semi-natural stream channels, both wild coho and farm Atlantic salmon grew significantly more in the presence of the other species than when alone. It appears that coho salmon obtain additional food ration by out competing Atlantic salmon, whereas Atlantic salmon were stimulated to feed more in the presence of coho salmon competitors. These results suggest that wild coho salmon and cutthroat trout should out compete farm Atlantic salmon of a similar size in nature. As the relative competitive ability of Atlantic salmon improves when they have a size and residence advantage, should feral populations become established, they may exist on a more equal competitive footing owing to the long freshwater residence of Atlantic salmon.     

Phylogenetic Evidence of Long Distance Dispersal and Transmission of Piscine Reovirus (PRV) between Farmed and Wild Atlantic Salmon

The extent and effect of disease interaction and pathogen exchange between wild and farmed fish populations is an ongoing debate and an area of research that is difficult to explore. The objective of this study was to investigate pathogen transmission between farmed and wild Atlantic salmon (Salmo salar L.) populations in Norway by means of molecular epidemiology. Piscine reovirus (PRV) was selected as the model organism as it is widely distributed in both farmed and wild Atlantic salmon in Norway, and because infection not necessarily will lead to mortality through development of disease. A matrix comprised of PRV protein coding sequences S1, S2 and S4 from wild, hatchery-reared and farmed Atlantic salmon in addition to one sea-trout (Salmo trutta L.) was examined. Phylogenetic analyses based on maximum likelihood and Bayesian inference indicate long distance transport of PRV and exchange of virus between populations. The results are discussed in the context of Atlantic salmon ecology and the structure of the Norwegian salmon industry. We conclude that the lack of a geographical pattern in the phylogenetic trees is caused by extensive exchange of PRV. In addition, the detailed topography of the trees indicates long distance transportation of PRV. Through its size, structure and infection status, the Atlantic salmon farming industry has the capacity to play a central role in both long distance transportation and transmission of pathogens. Despite extensive migration, wild salmon probably play a minor role as they are fewer in numbers, appear at lower densities and are less likely to be infected. An open question is the relationship between the PRV sequences found in marine fish and those originating from salmon.     

The survival of semi‐wild, wild and hatchery‐reared Atlantic salmon smolts of the Simojoki River in the Baltic Sea

The recapture rate and survival of hatchery‐reared Atlantic salmon Salmo salar stocked as 1 year‐old parr (semi‐wild) with that of hatchery‐reared Atlantic salmon stocked as 2 year‐old smolts and wild smolts of Atlantic salmon in the northern Baltic Sea were compared. This was done through tagging experiments carried out in 1986–1988 and 1992. The recapture rate of the semi‐wild groups varied from 1·0 to 13·1%, being similar in 3 tagging years and lower in 1 year than that of the wild groups (1·7–17·0%). The recapture rate of the semi‐wild groups was similar (in 2 years) or higher (in 2 years) than that of the hatchery‐reared groups stocked as smolts (1·3–6·3%). The survival of semi‐wild smolts during the sea migration was as high as that of wild Atlantic salmon of an equal size and two to three times higher than hatchery‐reared Atlantic salmon stocked as smolts. The survival rate was positively associated with smolt size. The suitability of hatchery‐reared parr and smolts in the management of reduced Atlantic salmon stocks is compared.     

Evidence for a carrier state of infectious hematopoietic necrosis virus in chinook salmon Oncorhynchus tshawytscha.

In British Columbia, Canada, infectious hematopoietic necrosis virus (IHNV) is prevalent in wild sockeye salmon Oncorhynchus nerka and has caused disease in seawater net-pen reared Atlantic salmon Salmo salar. In this study, chinook salmon Oncorhynchus tshawytscha experimentally exposed to an isolate of IHNV found in British Columbia became carriers of the virus. When Atlantic salmon were cohabited with these virus-exposed chinook salmon, IHNV was isolated from the Atlantic salmon. Identification of chinook salmon populations that have been exposed to IHNV may be difficult, as virus isolation was successful only in fish that were concurrently infected with either Renibacterium salmoninarum or Piscirickettisia salmonis. Also, IHNV-specific antibodies were detected in only 2 of the 70 fish experimentally exposed to the virus. Two samples collected from chinook salmon exposed to IHNV while at a salt water net-pen site had a seroprevalence of 19 and 22%; however, the inconsistencies between our laboratory and field data suggest that further research is required before we can rely on serological analysis for identifying potential carrier populations. Because of the difficulty in determining the exposure status of populations of chinook salmon, especially if there is no concurrent disease, it may be prudent not to cohabit Atlantic salmon with chinook salmon on a farm if there is any possibility that the latter have been exposed to the virus.     

First detection of piscine reovirus (PRV) in marine fish species

Heart and skeletal muscle inflammation (HSMI) is a disease that affects farmed Atlantic salmon Salmo salar L. several months after the fish have been transferred to seawater. Recently, a new virus called piscine reovirus (PRV) was identified in Atlantic salmon from an outbreak of HSMI and in experimentally challenged fish. PRV is associated with the development of HSMI, and has until now only been detected in Atlantic salmon. This study investigates whether the virus is also present in wild fish populations that may serve as vectors for the virus. The virus was found in few of the analyzed samples so there is probably a more complex relationship that involves several carriers and virus -reservoirs.     

Molecular Genetic Analysis of Stomach Contents Reveals Wild Atlantic Cod Feeding on Piscine Reovirus (PRV) Infected Atlantic Salmon Originating from a Commercial Fish Farm

In March 2012, fishermen operating in a fjord in Northern Norway reported catching Atlantic cod, a native fish forming an economically important marine fishery in this region, with unusual prey in their stomachs. It was speculated that these could be Atlantic salmon, which is not typical prey for cod at this time of the year in the coastal zone. These observations were therefore reported to the Norwegian Directorate of Fisheries as a suspected interaction between a local fish farm and this commercial fishery. Statistical analyses of genetic data from 17 microsatellite markers genotyped on 36 partially-degraded prey, samples of salmon from a local fish farm, and samples from the nearest wild population permitted the following conclusions: 1. The prey were Atlantic salmon, 2. These salmon did not originate from the local wild population, and 3. The local farm was the most probable source of these prey. Additional tests demonstrated that 21 of the 36 prey were infected with piscine reovirus. While the potential link between piscine reovirus and the disease heart and skeletal muscle inflammation is still under scientific debate, this disease had caused mortality of large numbers of salmon in the farm in the month prior to the fishermen''s observations. These analyses provide new insights into interactions between domesticated and wild fish.     

Serum agglutinating titres against Renibacterium salmoninarum the causative agent of bacterial kidney disease, in rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L.

Serum agglutinating titres against Renibacterium salmoninarum were determined from clinically infected, sub-clinically infected and non-infected rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L. Titres ≥16 occurred in clinically infected rainbow trout, but declined after a few months when disease signs were absent. Non-infected rainbow trout had titres ≤ 16. No correlation between the agglutinating titre and the level of infection was found in farmed Atlantic salmon. Variable titres were found in wild salmon broodstock although there was no evidence of BKD during 4 years of testing.     

Does increased abundance of sea lice influence survival of wild Atlantic salmon post‐smolt?

A synthesis of results from two projects was assessed to analyse possible influence of sea lice Lepeophtheirus salmonis on marine Atlantic salmon Salmo salar survival. During the years 1992–2004, trawling for wild migrating post-smolts was performed in Trondheimsfjord, a fjord in which no Atlantic salmon aquaculture activity is permitted. Prevalence and intensity of sea lice infections on migrating wild post-smolts differed between years. A correlation analysis between 1 sea-winter (SW) Atlantic salmon catch statistics from the River Orkla (a Trondheimsfjord river) and sea lice infections on the migrating smolts in the Trondheimsfjord was not significant. Up to 2% reduction in adult returns due to sea-lice infection was expected. In addition, experimental releases from 1996 to 1998 with individually tagged groups of hatchery-reared Atlantic salmon smolts given protection against sea-lice infection was performed. Higher recaptures of adult Atlantic salmon from 1998 treated smolts compared to the control group may correspond to high abundance of sea lice found on the wild smolt, and may indicate influence on post-smolt mortality. These studies indicate that post-smolt mortality in Trondheimsfjord is marginally influenced by sea lice infection; however, the methods for assessing wild smolt mortality might be insufficient. Higher infections of sea lice farther out in the fjord may indicate more loss in Atlantic salmon returns in some years.     

Why do some fish do it younger than others? learning from experiments

Atlantic salmon in Maine were once abundant but have become depleted, and are listed as endangered under the federal Endangered Species Act. Historically, salmon numbers in Maine may have been as high as 100 000 adults, but habitat loss, pollution and overfishing have contributed to the decline of the species. In 2000, only 110 adults returned to spawn in Maine rivers. Maine produces c. 15 000 metric tons/year of aquacultured Atlantic salmon from a total of nearly 600 coastal net pens. Escapees from these pens may interact with the wild salmon. The dynamics of salmon populations under such conditions are poorly understood. In order to illuminate the role aquaculture may play in such a system, we have developed a model for simulating population trajectories for both wild salmon and competing populations derived from aquaculture escapes. The model simulates a small population of wild salmon based in a stream/estuary system, into which an aquaculture facility is losing fish to escapes. Biological parameters in the model were estimated as much as possible from data in the USFWS report on Maine salmon. We used the model to investigate the consequences of a variety of ecological interactions between the wild and cultured fish including competitive, genetic and disease effects. Initial results indicate that many of these effects allow the aquaculture‐derived population to supplant the wild fish, but that wild populations may still persist under some conditions.     

Ecological model of interactions between escaped and wild Atlantic salmon Salmo salar

Atlantic salmon in Maine were once abundant but have become depleted, and are listed as endangered under the federal Endangered Species Act. Historically, salmon numbers in Maine may have been as high as 100 000 adults, but habitat loss, pollution and overfishing have contributed to the decline of the species. In 2000, only 110 adults returned to spawn in Maine rivers. Maine produces c. 15 000 metric tons/year of aquacultured Atlantic salmon from a total of nearly 600 coastal net pens. Escapees from these pens may interact with the wild salmon. The dynamics of salmon populations under such conditions are poorly understood. In order to illuminate the role aquaculture may play in such a system, we have developed a model for simulating population trajectories for both wild salmon and competing populations derived from aquaculture escapes. The model simulates a small population of wild salmon based in a stream/estuary system, into which an aquaculture facility is losing fish to escapes. Biological parameters in the model were estimated as much as possible from data in the USFWS report on Maine salmon. We used the model to investigate the consequences of a variety of ecological interactions between the wild and cultured fish including competitive, genetic and disease effects. Initial results indicate that many of these effects allow the aquaculture‐derived population to supplant the wild fish, but that wild populations may still persist under some conditions.     

A global assessment of salmon aquaculture impacts on wild salmonids

Since the late 1980s, wild salmon catch and abundance have declined dramatically in the North Atlantic and in much of the northeastern Pacific south of Alaska. In these areas, there has been a concomitant increase in the production of farmed salmon. Previous studies have shown negative impacts on wild salmonids, but these results have been difficult to translate into predictions of change in wild population survival and abundance. We compared marine survival of salmonids in areas with salmon farming to adjacent areas without farms in Scotland, Ireland, Atlantic Canada, and Pacific Canada to estimate changes in marine survival concurrent with the growth of salmon aquaculture. Through a meta-analysis of existing data, we show a reduction in survival or abundance of Atlantic salmon; sea trout; and pink, chum, and coho salmon in association with increased production of farmed salmon. In many cases, these reductions in survival or abundance are greater than 50%. Meta-analytic estimates of the mean effect are significant and negative, suggesting that salmon farming has reduced survival of wild salmon and trout in many populations and countries.     

Effect of density on competition between wild and hatchery‐reared Atlantic salmon for shelter in winter

The effect of varying the density of hatchery‐reared Atlantic salmon Salmo salar on the ability of single wild fish to occupy a shelter is assessed. Although there was strong density‐dependence on sheltering overall, the ability of wild Atlantic salmon parr to occupy a shelter was not affected by the presence of hatchery‐reared fish even when outnumbered by four to one. These findings illustrate a competitive asymmetry for shelter in favour of the wild fish at the densities tested.     

Changes in polyamines during embryonic development of Atlantic salmon, Salmo salar.

1. Polyamine composition of eggs and alevins from cultured and wild Atlantic salmon, Salmo salar, was compared during early development. 2. Eggs collected from wild stock had higher amounts of bound and free putrescine, spermidine and spermine than those collected from cultured stock. 3. Cadaverine was present in bound form in cultured and wild stocks before and after fertilization, respectively. However, free cadaverine was detected in both stocks throughout development. 4. Atlantic salmon embryos and alevins can synthesize polyamines which could be associated with their growth and development.     

Alternative mating strategies in Atlantic salmon and brown trout

By screening variable number of tandem repeat (VNTR) loci, multiple paternity within clutches has been found in wild populations of southern European Atlantic salmon (Salmo salar) and brown trout (Salmo trutta). For Atlantic salmon, we determined the relative contribution of alternative male phenotypes to the next generation. Individual males that are morphologically juvenile yet sexually mature fertilized a large proportion of eggs, and they thereby contributed to an increase of genetic variability in wild populations via (1) balancing the sex ratio, (2) increasing outbreeding, and (3) enlarging the effective population size, in part a consequence of (1) and (2). In addition, these precocious males ensured that interspecific spawns involving Atlantic salmon females and brown trout males (a fairly common occurrence in southern Europe where the two species are sympatric) resulted mostly in Atlantic salmon progeny. For brown trout, preliminary genetic results indicated that multiple paternity, when present, was not due to alternative mating strategies by males, but rather to successive fertilizations by adult suitors.     

Genetic interactions between marine finfish species in European aquaculture and wild conspecifics

The principal species of marine aquaculture in Europe are Atlantic salmon (Salmo salar), sea bass (Dicentrarchus labrax) and sea bream (Sparus auratus). For Atlantic salmon and sea bass, a substantial part of total genetic variation is partitioned at the geographical population level. In the case of sea bream, gene flow across the Azores/Mediterranean scale appears to be extensive and population structuring is not detected. For Atlantic salmon and sea bass, natural population structure is at risk from genetic interaction with escaped aquaculture conspecifics. The locally adaptive features of populations are at risk from interbreeding with non‐local aquaculture fish. Wild populations, generally, are at risk from interactions with aquaculture fish that have been subject to artificial selection or domestication. Atlantic salmon is the main European aquaculture species and its population genetics and ecology have been well‐studied. A general case regarding genetic interactions can be based on the information available for salmon and extended to cover other species, in the appropriate context. A generalized flow chart for interactions is presented. Salmon escape from aquaculture at all life stages, and some survive to breed among wild salmon. Reproductive fitness in the escaped fish is lower than in native, wild fish because of behavioural deficiencies at spawning. However, as the number of salmon in aquaculture greatly exceeds the number of wild fish, even small fractional rates of escape may result in the local presence of large numbers, and high frequencies, of escaped fish. At present, policy and legislation in relation to minimizing genetic interactions between wild and aquaculture fish is best developed for Atlantic salmon, through the recommendations of the Oslo Agreement developed by the North Atlantic Salmon Conservation Organization and subsequent agreements on their implementation. In future, the potential use of genetically modified fish in aquaculture will make additional policy development necessary. Improved containment is recommended as the key to minimizing the numbers and therefore the effects of escaped fish. Emergency recovery procedures are recommended as a back‐up measure in the case of containment failure. Reproductive sterility is recommended as a future key to eliminating the genetic potential of escaped fish. The maintenance of robust populations of wild fish is recommended as a key to minimizing the effects of escaped fish on wild populations.     

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.     

Using Agent-Based Modelling to Predict the Role of Wild Refugia in the Evolution of Resistance of Sea Lice to Chemotherapeutants

A major challenge for Atlantic salmon farming in the northern hemisphere is infestation by the sea louse parasite Lepeophtheirus salmonis. The most frequent method of controlling these sea louse infestations is through the use of chemical treatments. However, most major salmon farming areas have observed resistance to common chemotherapeutants. In terrestrial environments, many strategies employed to manage the evolution of resistance involve the use of refugia, where a portion of the population is left untreated to maintain susceptibility. While refugia have not been deliberately used in Atlantic salmon farming, wild salmon populations that migrate close to salmon farms may act as natural refugia. In this paper we describe an agent-based model that explores the influence of different sizes of wild salmon populations on resistance evolution in sea lice on a salmon farm. Using the model, we demonstrate that wild salmon populations can act as refugia that limit the evolution of resistance in the sea louse populations. Additionally, we demonstrate that an increase in the size of the population of wild salmon results in an increased effect in slowing the evolution of resistance. We explore the effect of a population fitness cost associated with resistance, finding that in some cases it substantially reduces the speed of evolution to chemical treatments.     

Expression of olfactory receptors in different life stages and life histories of wild Atlantic salmon (Salmo salar)

It has been hypothesized that salmonids use olfactory cues to return to their natal rivers and streams. However, the key components of the molecular pathway involved in imprinting and homing are still unknown. If odorants are involved in salmon homing migration, then olfactory receptors should play a critical role in the dissipation of information from the environment to the fish. Therefore, we examined the expression profiles of a suite of genes encoding olfactory receptors and other olfactory-related genes in the olfactory rosettes of different life stages in two anadromous and one non-anadromous wild Atlantic salmon populations from Newfoundland, Canada. We identified seven differentially expressed OlfC genes in juvenile anadromous salmon compared to returning adults in both populations of anadromous Atlantic salmon. The salmon from the Campbellton River had an additional 10 genes that were differentially expressed in juveniles compared to returning adults. There was no statistically significant difference in gene expression of any of the genes in the non-anadromous population (P < 0.01). The function of the OlfC gene products is not clear, but they are predicted to be amino acid receptors. Other studies have suggested that salmon use amino acids for imprinting and homing. This study, the first to examine the expression of olfactory-related genes in wild North American Atlantic salmon, has identified seven OlfC genes that may be involved in the imprinting and homeward migration of anadromous Atlantic salmon.