Vertebral deformities in interspecific diploid and triploid salmonid hybrids

Vertebral deformities in salmonid interspecific hybrids, some of which were triploidised, were assessed across three separate year classes during the freshwater life stage. Initially, eggs from a farmed Atlantic salmon Salmo salar were crossed with the sperm from a S. salar, arctic char Salvelinus alpinus or brown trout Salmo trutta. For S. salar × S. trutta, half the eggs were triploidised. In a second- and third-year class, the eggs from a farmed S. salar were crossed with the sperm from either a S. salar or a S. trutta, and half of each group was triploidised. In the two initial-year classes, all hybrids were larger than the S. salar controls, and triploid S. salar × S. trutta were larger than diploid counterparts. In the third-year class, the S. salar × S. trutta were smaller than the S. salar, in contrast to the initial 2 year classes, although the triploid hybrids were still larger than the diploids. In the third-year class, a high degree of spontaneous triploidy was also observed in the putative diploid groups (between 16 and 39%). Vertebral deformities were consistently higher in pressure-shocked triploids than diploids, irrespective of hybridisation, but there was no consistent effect of hybridisation among experiments. Although this study was not able to explain the contrasting results for vertebral deformities between year classes, triploid S. salar × S. trutta can demonstrate impressive freshwater growth that could be of interest for future farming programmes.     

Farmed Atlantic salmon Salmo salar L. parr may reduce early survival of wild fish

The study examined the density‐mediated effects on growth, survival and dispersal of wild and farmed Atlantic salmon Salmo salar offspring in the period immediately following emergence, using a substitutive design. In small confined stream channels, wild parr coexisting with farmed parr had a significantly poorer survival, than wild parr alone. Density did not affect this relationship. In larger unconfined stream channels, wild parr coexisting with farmed parr entered a downstream trap in higher numbers than wild parr in allopatry. The results suggests that during the earliest life stages, farmed S. salar can outcompete wild S. salar, resulting in a reduced survival of wild S. salar.     

Plasticity in response to feed availability: Does feeding regime influence the relative growth performance of domesticated,wild and hybrid Atlantic salmon Salmo salar parr?

Growth of farmed, wild and F1 hybrid Atlantic salmon parr Salmo salar was investigated under three contrasting feeding regimes in order to understand how varying levels of food availability affects relative growth. Treatments consisted of standard hatchery feeding (ad libitum), access to feed for 4 h every day, and access to feed for 24 h on three alternate days weekly. Mortality was low in all treatments, and food availability had no effect on survival of all groups. The offspring of farmed S. salar significantly outgrew the wild S. salar, while hybrids displayed intermediate growth. Furthermore, the relative growth differences between the farmed and wild S. salar did not change across feeding treatments, indicating a similar plasticity in response to feed availability. Although undertaken in a hatchery setting, these results suggest that food availability may not be the sole driver behind the observed reduced growth differences found between farmed and wild fishes under natural conditions.     

Influence of feed ration size on somatic and muscle growth in landlocked dwarf and farmed Atlantic salmon Salmo salar

We examined the possible adaptation of the dwarf Bleke population of Atlantic salmon Salmo salar from Lake Byglandsfjord in southern Norway to limited food resources. The growth performance and muscle development in juvenile Bleke and farmed S. salar under satiated or restricted (50%) feeding were examined for 10 months, starting 3 weeks after first-feeding stage. Four-thousand fish were divided into four replicated groups and random samples of 16–40 fish per group were measured six times during the experiment. The two strains showed no significant difference in mean body mass when fed restricted ration, but the individual variation was considerably higher in the farmed fish. Both Bleke and farmed S. salar grew significantly faster when fed to satiation, but the farmed S. salar showed much higher gain in mass and were three times heavier (201.5 g vs 66.7 g) and possessed twice as many fast muscle fibres (179,682 vs 84,779) compared with landlocked S. salar after 10 months. Farmed fish fed full ration displayed both hypertrophic and hyperplasic muscle growth, while the increased growth in Bleke S. salar was entirely associated with a larger fibre diameter. The landlocked Bleke strain has apparently adapted to low food availability by minimising the metabolic costs of maintenance and growth through reduced dominance hierarchies and by an increase in average muscle fibre diameter relative to the ancestral condition.     

Upstream migratory behaviour of wild and ranched Atlantic salmon Salmo salar at a natural obstacle in a coastal spate river

The upstream migratory behaviour of wild and ranched Atlantic salmon Salmo salar in a small Irish coastal spate river was investigated using acoustic telemetry. Prespawning migratory behaviour was investigated including movement patterns at a large natural waterfall in the lower reaches of the river. A strong diurnal pattern was observed for upstream migrants at the waterfall indicative of the need for daylight to ascend this complex natural obstacle to migration. Successful passage of the waterfall was also associated with distinct environmental conditions and no difference in migratory ability was detected between wild and ranched origin S. salar. Wild S. salar tended to exhibit a non‐erratic, stepwise upstream migration pattern after ascending the waterfall while ranched S. salar had an increased probability of displaying more erratic migratory behaviour. Wild S. salar penetrated further into the river catchment than ranched S. salar, although male ranched S. salar exhibited the greatest cumulative distance moved prior to the spawning period. The management implications of escaped or released ranched S. salar and movement at natural obstacles are discussed.     

The influence of aquaculture unit proximity on the pattern of Lepeophtheirus salmonis infection of anadromous Salmo trutta populations on the isle of Skye,Scotland

A total of 230 anadromous Salmo trutta (brown trout) were sampled in five sheltered coastal fjords (or sea lochs) on the Isle of Skye, Scotland, U.K., in 2016 at varying distances from active Atlantic salmon Salmo salar farms. Statistical models were developed to investigate potential correlations between salmon lice Lepeophtheirus salmonis burdens on S. trutta hosts and their proximity to S. salar farm cages. Significant correlations were found between lice burdens and fish fork length and proximity to the nearest S. salar farm. The probability of the presence of L. salmonis on fish hosts increased with fish host size and with distance from the nearest S. salar farm, but total lice burdens were highest in fish sampled near S. salar farms and decreased with distance. The proportion of different life‐cycle stages of L. salmonis were also dependent on S. salar farm proximity, with higher juvenile lice numbers recorded at sites near S. salar farm cages. These results highlight the complexity of the relationship between S. trutta and L. salmonis infections on wild fish and emphasize the requirement of further research to quantify these effects to better inform conservation and management strategies, particularly in areas of active S. salar farm facilities.     

Sperm variation within a single ejaculate affects offspring development in Atlantic salmon

It is generally believed that variation in sperm phenotype within a single ejaculate has no consequences for offspring performance, because sperm phenotypes are thought not to reflect sperm genotypes. We show that variation in individual sperm function within an ejaculate affects the performance of the resulting offspring in the Atlantic salmon Salmo salar. We experimentally manipulated the time between sperm activation and fertilization in order to select for sperm cohorts differing in longevity within single ejaculates of wild caught male salmon. We found that within-ejaculate variation in sperm longevity significantly affected offspring development and hence time until hatching. Whether these effects have a genetic or epigenetic basis needs to be further evaluated. However, our results provide experimental evidence for transgenerational effects of individual sperm function.     

Development of seawater tolerance and subsequent downstream migration in wild and stocked young‐of‐the‐year derived Atlantic salmon Salmo salar smolts

This study investigated the development of hypo‐osmoregulatory capacity and timing of downstream migration in wild Atlantic salmon Salmo salar smolts from the River Stjørdalselva and stocked young‐of‐the‐year (YOY), derived S. salar smolts from the tributary River Dalåa. Both wild and stocked S. salar smolts developed seawater (SW) tolerance in early May, persisting through June, measured as their ability to regulate plasma osmolality and chloride following 24 h SW (salinity = 35) exposure. Although the majority of downstream migration among the stocked S. salar smolts occurred later than observed in their wild counterparts, the development of SW tolerance occurred concurrently. The wild S. salar from Stjørdalselva and stocked YOY smolts from the River Dalåa started to migrate on the same cumulative day‐degrees (D°). The study revealed no downstream migration before development of SW tolerance. This emphasizes the importance of incorporating physiological status when studying environmental triggers for downstream migration of S. salar smolts. Overall, these findings suggest that the onset of smolt migration in stocked S. salar smolts was within the smolt window from an osmoregulatory point of view.     

Evolution of virulence under intensive farming: salmon lice increase skin lesions and reduce host growth in salmon farms

Parasites rely on resources from a host and are selected to achieve an optimal combination of transmission and virulence. Human‐induced changes in parasite ecology, such as intensive farming of hosts, might not only favour increased parasite abundances, but also alter the selection acting on parasites and lead to life‐history evolution. The trade‐off between transmission and virulence could be affected by intensive farming practices such as high host density and the use of antiparasitic drugs, which might lead to increased virulence in some host–parasite systems. To test this, we therefore infected Atlantic salmon (Salmo salar) smolts with salmon lice (Lepeophtheirus salmonis) sampled either from wild or farmed hosts in a laboratory experiment. We compared growth and skin damage (i.e. proxies for virulence) of hosts infected with either wild or farmed lice and found that, compared to lice sampled from wild hosts in unfarmed areas, those originating from farmed fish were more harmful; they inflicted more skin damage to their hosts and reduced relative host weight gain to a greater extent. We advocate that more evolutionary studies should be carried out using farmed animals as study species, given the current increase in intensive food production practices that might be compared to a global experiment in parasite evolution.     

Reduced anti-predator responses in multi-generational hybrids of farmed and wild Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis> L.)

Cultured organisms undergo genetically-based behavioural changes that may reduce their ability to survive in the wild. This has raised concerns that interbreeding between escaped cultured and wild organisms will generate hybrids exhibiting maladaptive behaviours which may ultimately reduce the fitness of the wild counterpart. We compared anti-predator responses in Atlantic salmon (Salmo salar) from two wild North American populations, the major farmed strain used in regional aquaculture, and their wild-farmed hybrids (F₁, F₂, and wild backcross). Anti-predator responses of fry (age 0+ parr) were measured under common environmental conditions, using a model of a natural predator (belted kingfisher, Ceryle alcyon). Farmed fry exhibited significantly reduced anti-predator responses relative to fry from both wild populations. The anti-predator responses of wild-farmed hybrid fry were intermediate to those of the parental populations (pure farmed or wild). The magnitude by which wild-farmed hybrids differed in anti-predator responses from pure wild fish also depended on the wild population. These results suggest that: (1) the observed behavioural differences have a genetic basis; (2) wild-farmed hybrids have, on average, reduced anti-predator responses relative to wild fish; and that (3) the effects of wild-farmed interbreeding on anti-predator responses will differ between wild populations. Our study is consistent with the general hypothesis that continual farmed-wild interbreeding may have detrimental effects on the fitness of wild organisms.     

Does size matter? A test of size‐specific mortality in Atlantic salmon Salmo salar smolts tagged with acoustic transmitters

Mortality rates of wild Atlantic salmon Salmo salar smolts implanted with acoustic transmitters were assessed to determine if mortality was size dependent. The routinely accepted, but widely debated, ‘2% transmitter mass: body mass’ rule in biotelemetry was tested by extending the transmitter burden up to 12·7% of body mass in small [mean fork length (L_F) 138·3 mm, range 115–168 mm] downstream migrating S. salar smolts. Over the short timescale of emigration (range 11·9–44·5 days) through the lower river and estuary, mortality was not related to S. salar size, nor was a relationship found between mortality probability and transmitter mass: body mass or transmitter length: L_F ratios. This study provides further evidence that smolt migration studies can deviate from the ‘2% rule’ of thumb, to more appropriate study‐specific measures, which enables the use of fishes representative of the body size in natural populations without undue effects.     

Differences in the energetic cost of swimming in turbulent flow between wild,farmed and domesticated juvenile Atlantic salmon Salmo salar

Domestication has been shown to have an effect on morphology and behaviour of Atlantic salmon (Salmo salar). We compared swimming costs of three groups of juvenile Atlantic salmon subject to different levels of domestication: (1) wild fish; (2) first generation farmed fish origination from wild genitors; and (2) seventh generation farmed fish originating from Norwegian aquaculture stocks. We assessed swimming costs under two types of turbulent flow (one mean flow velocity of 23 cm s^?1 and two standard deviations of flow velocity of 5 and 8 cm s^?1). Respirometry experiments were conducted with fish in a mass range of 5–15 g wet at a water temperature of 15° C. Our results confirm (1) that net swimming costs are affected by different levels of turbulence such that, for a given mean flow velocity, fish spent 1·5‐times more energy as turbulence increased, (2) that domesticated fish differed in their morphology (having deeper bodies and smaller fins) and in their net swimming costs (being up to 30·3% higher than for wild fish) and (3) that swimming cost models developed for farmed fish may be also be applied to wild fish in turbulent environments.     

No evidence of sperm conjugate formation in an Australian mouse bearing sperm with three hooks

Sperm conjugation occurs when two or more sperm physically unite for motility or transport through the female reproductive tract. In many muroid rodent species, sperm conjugates have been shown to form by a single, conspicuous apical hook located on the sperm head. These sperm “trains” have been reported to be highly variable in size and, despite all the heads pointing in roughly the same direction, exhibit a relatively disordered arrangement. In some species, sperm “trains” have been shown to enhance sperm swimming speed, and thus have been suggested to be advantageous in sperm competition. Here, we assessed the behavior of sperm in the sandy inland mouse (Pseudomys hermannsburgensis), a muroid rodent that bears sperm with three apical hooks. First, we accrued genetic evidence of multiple paternity within “wild” litters to unequivocally show that sperm competition does occur in this species. Following this we utilized both in vitro and in vivo methodologies to determine whether sandy inland mouse sperm conjugate to form motile trains. Our observations of in vitro preparations of active sperm revealed that sandy inland mouse sperm exhibit rapid, progressive motility as individual cells only. Similarly, histological sections of the reproductive tracts of mated females revealed no in vivo evidence of sperm conjugate formation. We conclude that the unique, three‐hooked morphology of the sandy inland mouse sperm does not facilitate the formation of motile conjugates, and discuss our findings in relation to the different hypotheses for the evolution of the muroid rodent hook/s.     

Environmental change influences the life history of salmon Salmo salar in the North Atlantic Ocean

Annual mean total length (L_T) of wild one‐sea‐winter (1SW) Atlantic salmon Salmo salar of the Norwegian River Imsa decreased from 63 to 54 cm with a corresponding decrease in condition factor (K) for cohorts migrating to sea from 1976 to 2010. The reduction in L_T is associated with a 40% decline in mean individual mass, from 2 to 1·2 kg. Hatchery fish reared from parental fish of the same population exhibited similar changes from 1981 onwards. The decrease in L_T correlated negatively with near‐surface temperatures in the eastern Norwegian Sea, thought to be the main feeding area of the present stock. Furthermore, S. salar exhibited significant variations in the proportion of cohorts attaining maturity after only one winter in the ocean. The proportion of S. salar spawning as 1SW fish was lower both in the 1970s and after 2000 than in the 1980s and 1990s associated with a gradual decline in post‐smolt growth and smaller amounts of reserve energy in the fish. In wild S. salar, there was a positive association between post‐smolt growth and the sea survival back to the River Imsa for spawning. In addition, among smolt year‐classes, there were significant positive correlations between wild and hatchery S. salar in L_T, K and age at maturity. The present changes may be caused by ecosystem changes following the collapse and rebuilding of the pelagic fish abundance in the North Atlantic Ocean, a gradual decrease in zooplankton abundance and climate change with increasing surface temperature in the Norwegian Sea. Thus, the observed variation in the life‐history traits of S. salar appears primarily associated with major changes in the pelagic food web in the ocean.     

Differences in pathogen resistance within and among cultured,conservation-dependent,and endangered populations of Atlantic salmon, <Emphasis Type="Italic">Salmo salar</Emphasis> L.

We report genetic differences for resistance to the pathogen Listonella anguillarum within and among one cultured and two wild Canadian populations of Atlantic salmon, Salmo salar, using a common-garden experimental protocol. Following exposure to the causative agent for vibriosis, parr originating from the endangered Stewiacke River population experienced significantly higher mortality than cultured parr, four generations removed from the Saint John River population, and wild parr from Tusket River. Pathogen resistance differed between sexes; males consistently experienced higher survival than females. There was no evidence that maturity influenced pathogen resistance in male parr. The population and sex differences in pathogen resistance documented here have implications for risk assessments of the demographic consequences of interbreeding between wild and farmed Atlantic salmon.     

Inbreeding depression of sperm traits in the zebra finch Taeniopygia guttata

Inbreeding depression, or the reduction in fitness due to mating between close relatives, is a key issue in biology today. Inbreeding negatively affects many fitness‐related traits, including survival and reproductive success. Despite this, very few studies have quantified the effects of inbreeding on vertebrate gamete traits under controlled breeding conditions using a full‐sib mating approach. Here, we provide comprehensive evidence for the negative effect of inbreeding on sperm traits in a bird, the zebra finch Taeniopygia guttata. We compared sperm characteristics of both inbred (pedigree F = 0.25) and outbred (pedigree F = 0) individuals from two captive populations, one domesticated and one recently wild‐derived, raised under standardized conditions. As normal spermatozoa morphology did not differ consistently between inbred and outbred individuals, our study confirms the hypothesis that sperm morphology is not particularly susceptible to inbreeding depression. Inbreeding did, however, lead to significantly lower sperm motility and a substantially higher percentage of abnormal spermatozoa in ejaculate. These results were consistent across both study populations, confirming the generality and reliability of our findings.     

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.     

Stable isotopes reveal contrasting trophic dynamics between host–parasite relationships: A case study of Atlantic salmon (Salmo salar) and parasitic lice (Lepeophtheirus salmonis and Argulus foliaceus)

Across existing fish host–parasite literature, endoparasites were depleted in δ¹⁵N compared to their hosts, while ectoparasitic values demonstrated enrichment, depletion and equivalence relative to their hosts. δ¹³C enrichment varied extensively for both endo- and ectoparasites across taxa and host tissues. In our case study, sea lice (Lepeophtheirus salmonis) were enriched in δ¹⁵N relative to their farmed Atlantic salmon (Salmo salar) hosts, although the value contradicted the average that is currently assumed across the animal kingdom. Common fish lice (Argulus foliaceus) did not show a consistent trend in δ¹⁵N compared to their wild S. salar hosts. Both parasitic species had a range of δ¹³C enrichment patterns relative to their hosts. Farmed and wild S. salar had contrasting δ¹³C and δ¹⁵N, and signals varied across muscle, fin and skin within both groups. L. salmonis and A. foliaceus subsequently had unique δ¹³C and δ¹⁵N, and L. salmonis from opposite US coasts differed in δ¹⁵N. Given the range of enrichment patterns that were exhibited across the literature and in our study system, trophic dynamics from host to parasite do not conform to traditional prey to predator standards. Furthermore, there does not appear to be a universal enrichment pathway for δ¹³C nor δ¹⁵N in parasitic relationships, which emphasizes the need to investigate host–parasite linkages across species.     

A primary phosphorus‐deficient skeletal phenotype in juvenile Atlantic salmon Salmo salar: the uncoupling of bone formation and mineralization

To understand the effect of low dietary phosphorus (P) intake on the vertebral column of Atlantic salmon Salmo salar, a primary P deficiency was induced in post‐smolts. The dietary P provision was reduced by 50% for a period of 10 weeks under controlled conditions. The animal's skeleton was subsequently analysed by radiology, histological examination, histochemical detection of minerals in bones and scales and chemical mineral analysis. This is the first account of how a primary P deficiency affects the skeleton in S. salar at the cellular and at the micro‐anatomical level. Animals that received the P‐deficient diet displayed known signs of P deficiency including reduced growth and soft, pliable opercula. Bone and scale mineral content decreased by c. 50%. On radiographs, vertebral bodies appear small, undersized and with enlarged intervertebral spaces. Contrary to the X‐ray‐based diagnosis, the histological examination revealed that vertebral bodies had a regular size and regular internal bone structures; intervertebral spaces were not enlarged. Bone matrix formation was continuous and uninterrupted, albeit without traces of mineralization. Likewise, scale growth continues with regular annuli formation, but new scale matrix remains without minerals. The 10 week long experiment generated a homogeneous osteomalacia of vertebral bodies without apparent induction of skeletal malformations. The experiment shows that bone formation and bone mineralization are, to a large degree, independent processes in the fish examined. Therefore, a deficit in mineralization must not be the only cause of the alterations of the vertebral bone structure observed in farmed S. salar. It is discussed how the observed uncoupling of bone formation and mineralization helps to better diagnose, understand and prevent P deficiency‐related malformations in farmed S. salar.     

Testing the phenotype‐linked fertility hypothesis in the presence and absence of inbreeding

The phenotype‐linked fertility hypothesis suggests that females can judge male fertility by inspecting male phenotypic traits. This is because male sexually selected traits might correlate with sperm quality if both are sensitive to factors that influence male condition. A recent meta‐analysis found little support for this hypothesis, suggesting little or no shared condition dependence. However, we recently reported that in captive zebra finches (Taeniopygia guttata) inbreeding had detrimental effects both on phenotypic traits and on measures of sperm quality, implying that variation in inbreeding could induce positive covariance between indicator traits and sperm quality. Therefore, we here assess empirically the average strength of correlations between phenotypic traits (courtship rate, beak colour, tarsus length) and measures of sperm quality (proportion of functional sperm, sperm velocity, sperm length) in populations of only outbred individuals and in mixed populations consisting of inbreds (F = 0.25) and outbreds (F = 0). As expected, phenotype sperm‐trait correlations were stronger when the population contained a mix of inbred and outbred individuals. We also found unexpected heterogeneity between our two study populations, with correlations being considerably stronger in a domesticated population than in a recently wild‐derived population. Correlations ranged from essentially zero among outbred‐only wild‐derived birds (mean Fisher's Zr ± SE = 0.03 ± 0.10) to moderately strong among domesticated birds of mixed inbreeding status (Zr ± SE = 0.38 ± 0.08). Our results suggest that, under some conditions, the phenotype‐linked fertility hypothesis might apply.