Atlantic Salmon Salmo Salar Populations Research Articles

The effect of surveillance fishing on migration distance of Atlantic Salmon during the spawning period

AbstractObjectiveSurveillance fishing surveys can be performed to estimate the proportion of farmed salmon represented in the spawning stock of native Atlantic Salmon Salmo salar populations. These surveys take place after the recreational fishing period and therefore closer to the spawning period than the open recreational fishing season. Although catch‐and‐release angling has been demonstrated to affect salmon migration during the summer months, surveillance fishing that is conducted close to the spawning time could have more severe effects.MethodsTo test this, the migration distance of Atlantic Salmon (n = 74) caught in the Orkla River, Norway, was tracked by use of radiotelemetry. One group was tagged during the regular fishing season in the summer (control group), whereas another group was tagged in autumn during surveillance fishing (surveillance group).ResultSixty‐one salmon remained for analysis after we excluded fish that were recaptured, died, or migrated to other rivers. Relocation of the salmon during autumn (October 11–31) was used to compare movements and test for differences in migration using negative binomial regression because distances were nonnegative integers. During the tracking period, the surveillance group moved 12 ± 14 km (mean ± standard deviation) and the control group moved 13 ± 15 km; both groups moved 1 ± 2 km/day on average. There was no evidence that surveillance fishing impacted movement of the salmon compared to controls. However, one salmon died after tagging and three were not released due to injuries; total mortality of 9% during surveillance fishing could be unsustainable for smaller populations. Consequently, factors such as surveillance sample size, the status of the salmon population, and the population size should be assessed for each river individually when deciding the necessity of and approach to surveillance fishing.ConclusionThe results support existing recommendations to use careful handling and to end surveillance at least 2 weeks prior to the expected onset of spawning, thus providing a sufficiently long period for recovery after surveillance fishing.

Global warming is projected to lead to increased freshwater growth potential and changes in pace of life in Atlantic salmon Salmo salar.

Global warming has been implicated in widespread demographic changes in Atlantic salmon Salmo salar populations, but projections of life-history responses to future climate change are lacking. Here, we first exploit multiple decades of climate and biological data from the Burrishoole catchment in the west of Ireland to model statistical relationships between atmospheric variables, water temperature, and freshwater growth of juvenile Atlantic salmon. We then use this information to project potential changes in juvenile growth and life-history scheduling under three shared socioeconomic pathway and representative concentration pathway scenarios from 1961 to 2100, based on an ensemble of five climate models. Historical water temperatures were well predicted with a recurrent neural network, using observation-based atmospheric forcing data. Length-at-age was in turn also well predicted by cumulative growing degree days calculated from these water temperatures. Most juveniles in the Burrishoole population migrated to sea as 2-year-old smolts, but our future projections indicate that the system should start producing a greater proportion of 1-year-old smolts, as increasingly more juveniles cross a size-based threshold in their first summer for smoltification the following spring. Those failing to cross the size-based threshold will instead become 2-year-old smolts, but at a larger length relative to 2-year-old smolts observed currently, owing to greater overall freshwater growth opportunity. These changes in age- and size-at-seaward migration could have cascading effects on age- and size-at-maturity and reproductive output. Consequently, the seemingly small changes that our results demonstrate have the potential to cause significant shifts in population dynamics over the full life cycle. This workflow is highly applicable across the range of the Atlantic salmon, as well as to other anadromous species, as it uses openly accessible climate data and a length-at-age model with minimal input requirements, fostering improved general understanding of phenotypic and demographic responses to climate change and management implications.

Growth variation of Atlantic salmon Salmo salar at sea affects their population-specific reproductive potential

Understanding the processes shaping the dynamics of anadromous fish populations is essential for their management and conservation. Yet, little is known about how variation in performance at sea affects their population dynamics. Here we show that variation in body growth at sea contributes to explaining variation in the reproductive potential for 2 Atlantic salmon Salmo salar populations, but to a varying extent. To this end, we assembled data collected during 50 yr for 2 Baltic salmon populations of hatchery origin, including annually released smolts, survival at sea estimates, size-specific growth at sea, annual length distributions of returning adult females and their reproductive potential. The regression models fitted to explain the reproductive potential of our 2 study populations improved when growth at sea was included as an explanatory variable, in addition to smolt year class abundance and estimates of their survival at sea. This link between body growth at sea and population-level reproductive potential suggests that growth at sea can be important to consider when resolving variation in recovery and dynamics among salmon populations sharing the same sea.

Stable isotopes suggest the location of marine feeding grounds of South European Atlantic salmon in Greenland

AbstractHistorical data on the oceanic distribution and migration routes of southernmost Atlantic salmon Salmo salar populations from Europe are almost non-existent, as no rigorous tagging initiatives have been conducted. Here, we used stable isotope data (δ13C and δ15N) of historic scale collections to identify the potential marine feeding areas of the largest salmon population in the Iberian Peninsula. Data were compared with published datasets from Northern Ireland, Wales, south England, and northeast UK coast, which correspond to series between 15- and 33-year long within the time period from 1958 to 2009. Temporal covariation in sea surface temperature, primary productivity, and δ13C values suggests that feeding areas of Iberian salmon are located around Greenland, both in the Labrador and the Irminger seas. Furthermore, δ13C values of Atlantic salmon from Canadian rivers reported in the literature are similar to those found in individuals from Spanish rivers. Our results suggest that Iberian salmon follow a westerly migration route towards Greenland instead of following the easterly branch of the North Atlantic current into the Norwegian Sea. Characterization of feeding patterns and migration routes might help to understand the causes of ongoing population decline and establish targeted conservation programmes for threatened Iberian salmon.

Long-term stocking practices threaten the original genetic diversity of the southernmost European populations of Atlantic salmon Salmo salar

Many Atlantic salmonSalmo salarpopulations in Europe are threatened by previous stocking with foreign hatchery strains. Temporal patterns of genetic characteristics of salmon from northern Spain, the southernmost European populations, were compared before and after species decline and heavy stocking with specimens from northern Europe. Eleven microsatellite loci were analysed in archival (scales from 1958-1960) and contemporary (2007-2008) samples from the River Sella. Temporal analyses revealed a similar heterozygosity between archival and contemporary samples, despite a drastic decrease in population abundance, while the contemporary sample showed a higher allelic richness due to the occurrence of foreign alleles. Considering only the alleles with at least 4% frequency in the archival sample, 2 alleles exclusive to the River Sella were absent in the contemporary sample, and 14 alleles showed a decrease of at least 4% frequency. Four alleles common in Scotland showed a high occurrence in the contemporary sample, so they are good candidates as markers of introgression of foreign genes. The heavy stocking with non-native Scottish broodstocks between 1970 and 1990 caused the introgression found in the contemporary sample when compared with the pristine population. An abrupt decrease was evident when the estimates of effective number of breeders were adjusted to take into account overlapping generations (NbAdj), effective population size (NeAdj) estimated fromNbAdj, and number of breeders estimated using the sibship assignment method (NbSIB). The very low effective size values found in the contemporary sample, together with the detrimental synergy between genetic drift and high rates of introgression, represent a severe risk for the conservation of native salmon.

Population and size-specific distribution of Atlantic salmon Salmo salar in the Baltic Sea over five decades.

Population‐specific assessment and management of anadromous fish at sea requires detailed information about the distribution at sea over ontogeny for each population. However, despite a long history of mixed‐stock sea fisheries on Atlantic salmon, Salmo salar, migration studies showing that some salmon populations feed in different regions of the Baltic Sea and variation in dynamics occurs among populations feeding in the Baltic Sea, such information is often lacking. Also, current assessment of Baltic salmon assumes equal distribution at sea and therefore equal responses to changes in off‐shore sea fisheries. Here, we test for differences in distribution at sea among and within ten Atlantic salmon Salmo salar populations originating from ten river‐specific hatcheries along the Swedish Baltic Sea coast, using individual data from >125,000 tagged salmon, recaptured over five decades. We show strong population and size‐specific differences in distribution at sea, varying between year classes and between individuals within year classes. This suggests that Atlantic salmon in the Baltic Sea experience great variation in environmental conditions and exploitation rates over ontogeny depending on origin and that current assessment assumptions about equal exploitation rates in the offshore fisheries and a shared environment at sea are not valid. Thus, our results provide additional arguments and necessary information for implementing population‐specific management of salmon, also when targeting life stages at sea.

Atlantic salmon return rate increases with smolt length

AbstractRecent declines in Atlantic salmon Salmo salar populations are generally attributed to factors in their marine life-phase. However, it is postulated that factors affecting their freshwater life-phase might impact their marine survival, such as the influence of body size. While larger smolts are widely hypothesized to have higher marine survival rates, empirical support remains scant, in part due to inadequate data and ambiguous statistical analyses. Here, we test the influence of smolt body size on marine return rates, a proxy for marine survival, using a 12-year dataset of 3688 smolts tagged with passive integrated transponders in the River Frome, Southern England. State-space models describe the probability of smolts surviving their marine phase to return as 1 sea-winter (1SW) or multi-sea-winter adults as a function of their length, while accounting for imperfect detection and missing data. Models predicted that larger smolts had higher return rates; the most parsimonious model included the effect of length on 1SW return rate. This prediction is concerning, as freshwater juvenile salmon are decreasing in size on the River Frome, and elsewhere. Thus, to maximize adult returns, restoration efforts should focus on freshwater life-stages, and maximize both the number and the size of emigrating smolts.

Demographic and genetic description of Greenland's only indigenous Atlantic salmon Salmo salar population.

A survey of the Kapisillit River system was conducted in 2005 and 2012 to study the only indigenous Atlantic salmon Salmo salar population in Greenland. Little is known about its characteristics or its relationship with other S. salar populations across the species range. Juvenile S. salar were captured in all stations surveyed within the lower river with the highest densities lower in the river and decreasing densities with increasing distance from the river mouth. Captured juveniles ranged from 0+ to 7+ years old and the predominant smolt age was between 4 and 6 years. Median length of 0+ and 1+ juveniles in August-September was 38.8 and 70.4 mm, respectively. The proportion of mature male parr increased from 4% for 1+ year old fish to 95% for fish greater than 2 years old. Genetic analysis using 96 single nucleotide polymorphisms (SNP) revealed a high degree of genetic similarity between collections, extremely low genetic diversity and low estimates of effective population size (Ne = 28.7; 95% CI = 19.7-42.4). Genetic comparison to range-wide S. salar populations demonstrated that the Kapisillit River S. salar is an outgroup of the eastern Atlantic stock complex, which is consistent with the hypothesised colonisation from the east. River morphology and the absence of glacier runoff are hypothesised to be the main reasons for the relatively high river temperatures supporting this self-sustaining population of S. salar. Given its uniqueness and persistence, this population represents an important part of range-wide biodiversity of S. salar.

Estimating the effective number of breeders from single parr samples for conservation monitoring of wild populations of Atlantic salmon Salmo salar.

This study assesses whether the effective number of breeders (Nb ) can be estimated using a time and cost-effective protocol using genetic sibship reconstruction from a single sample of young-of-the-year (YOY) for the purposes of Atlantic salmon Salmo salar population monitoring. Nb was estimated for 10 consecutive reproductive seasons for S. salar in the River Nivelle, a small population located at the rear-edge of the species distribution area in France, chronically under its conservation limit and subjected to anthropogenic and environmental changes. Subsampling of real and simulated data showed that accurate estimates of Nb can be obtained from YOY genotypes, collected at moderate random sampling intensity, achievable using routine juvenile electrofishing protocols. Spatial bias and time elapsed since spawning were found to affect estimates, which must be accounted for in sampling designs. Nb estimated in autumn for S. salar in the River Nivelle was low and variable across years from 23 (95% C.I. 14-41) to 75 (53-101) and was not statistically correlated with the estimated number of returning adults, but it was positively correlated with the estimated number of YOY at age 9months. Nb was found to be lower for intermediate levels of redd aggregation, suggesting that the strength of the competition between males to access females affects reproductive success variance depending on redd spatial configuration. Thus, environmental factors such as habitat availability and quality for spawning and YOY development predominate over demographic ones (number of returning adults) in driving long-term population viability for S. salar in the River Nivelle. This study showcases Nb as an integrated parameter, encompassing demographic and ecological information about a reproductive event, relevant to the assessment of both short-term effects of management practices and long-term population conservation status.

Assessment of Early Migration Dynamics of River‐Specific Hatchery Atlantic Salmon Smolts

AbstractMany Atlantic Salmon Salmo salar populations within the southern extent of the species’ range are at critically low abundances, while others have been extirpated. The focus of many ongoing recovery efforts is on maximizing the number of smolts that reach the ocean, where marine survival is low, primarily through hatchery supplementation and improvement of in‐river hydropower system management. However, estuaries also are known to be sites of high mortality, although in many cases the correlates of this mortality are poorly characterized. We acoustically tagged hatchery smolts (n = 666) during 2001–2005 to evaluate migration performance through freshwater, estuarine, and bay reaches of the small but tidally dynamic Dennys River, Maine, USA, to investigate potential drivers contributing to low returns within the system. Migration behaviors (e.g., migration timing and tidal use) were typical for Atlantic Salmon smolts, but reversals during migration were observed upon entry into the estuary environment more frequently in this system than in regional rivers. We used Cormack–Jolly–Seber mark–recapture models to estimate apparent survival within the study environment during this period. We found two distinct periods of survival, with much lower survival in 2003–2005 than in 2001 and 2002 (more than a 40% decrease). Among the variables considered, temperature and migratory behavior had the largest effects on survival. We found that survival increased by about 28% across the range of temperatures observed during these years; additionally, survival increased by about 15% for fish that reversed direction during migration compared to fish that did not. These results indicate that extreme environmental changes during the transition through this coastal system constitute a significant obstacle to Atlantic Salmon restoration stocking efforts.

Gene flow from domesticated escapes alters the life history of wild Atlantic salmon.

Interbreeding between domesticated and wild animals occurs in several species. This gene flow has long been anticipated to induce genetic changes in life-history traits of wild populations, thereby influencing population dynamics and viability. Here, we show that individuals with high levels of introgression (domesticated ancestry) have altered age and size at maturation in 62 wild Atlantic salmon Salmo salar populations, including seven ancestral populations to breeding lines of the domesticated salmon. This study documents widespread changes to life-history traits in wild animal populations following gene flow from selectively bred, domesticated conspecifics. The continued high abundance of escaped, domesticated Atlantic salmon thus threatens wild Atlantic salmon populations by inducing genetic changes in fitness-related traits. Our results represent key evidence and a timely warning concerning the potential ecological impacts of the globally increasing use of domesticated animals.

Skjern River Valley, Northern Europe’s most expensive wetland restoration project: benefits to breeding waterbirds

After circa 35 years of drainage and intensive arable tillage, the lower Skjern River, Den-mark was re-engineered to its original meanders and flooding regime, creating 22 km 2 of lakes, shallow wetlands and seasonally flooded grazed wet grassland costing €38 million. The primary motivation was to restore the sediment/nutrient retention capacity of the river valley to reduce eutrophication of Ringkøbing Fjord at its efflux. Secondary objectives were to (i) restore breeding and staging bird habitat, (ii) enhance the self-sustaining Atlantic Salmon Salmo salar population and (iii) improve recreational and tourist activities. Despite lack of specific success criteria, breeding waterbird numbers increased from 134 ± 22.9 SE (n = 3) pairs before to 1,744 ± 153 SE (n = 5) after restoration (although on average 1,004 of these were Black-headed Gulls Chroicocephalus ridibundus), species richness and diversity also increased. Twenty-nine waterbird species returned to breed, 10 of national or international significance (Danish Red List/European Union Birds Directive Annex 1 species) now ranking Skjern River amongst the top 10 most important breeding waterbird sites in Denmark. Currently, agriculture supports cost-neutral management of the restoration area, but whilst most expected wet meadow and marsh species had returned, lack of goal-orientated management targets resulted in some additional rare and threatened species remaining absent. Breeding pair density and diversity of other species could have been greatly improved by prior planning and management intervention but at additional cost.

Evolutionary hypotheses for a constraint to life-history resilience in depleted Salmo salar populations.

Phenotypic plasticity underlies much of the variation in life-history expression in fishes. An understanding of potential constraints on life-history plasticity thus may be critical for assessing the resiliency of populations or species to environmental change. Here, several evolutionary hypotheses are formulated for why a depleted lineage of Canadian Atlantic salmon Salmo salar populations continues to express an apparently maladaptive life history in the face of severe marine mortality. These hypotheses include: (1) reduced genetic variability for expressing plasticity, (2) constraints from genetic architecture, (3) constraints from gene flow, (4) phylogenetic constraints or irreversible evolutionary transitions, (5) environmental constraints to plasticity and (6) a restriction to population rescue from evolutionary-demographic feedbacks. This S. salar lineage is intriguing to consider for understanding resilience or the lack thereof, because it has life-history attributes that should favour resilience (e.g. a high degree of iteroparity, variable age at maturity and the presence of both long- and short-distance migration ecotypes). In particular, the discussion centres on the question of why S. salar females, in contrast to males, do not adopt a non-anadromous life history and mature in fresh water, given extremely high marine mortality among anadromous individuals of both sexes. A salient implication, with possibly significant conservation ramifications, is that fishes may exhibit substantial plasticity and potential for adapting to environmental change, but still be incapable of responding to certain environmental changes due to sex-specific constraints to life-history plasticity.

Surface trawl survey for U.S. origin Atlantic salmon Salmo salar

A surface trawl survey (2001-2005) in Penobscot Bay, Maine, U.S.A., and the nearshore Gulf of Maine waters was conducted to investigate early marine dynamics of a hatchery-dependent Atlantic salmon Salmo salar population from a severely modified river system. Data generated were used to evaluate the effect of stocking location and time on migration success and to describe the migratory pathways and environments that post-smolts traverse. Significant differences in early migration success were detected among different stocking groups, but subsequent marine survival was independent of stocking group. While the post-smolt population was primarily composed of hatchery origin smolt-stocked fish, other life-stage stocking strategies (i.e. parr stocking) represented a higher proportion of the population than previously assumed. Catch distribution suggests evidence of an initial marine migratory pathway out of the dynamic Penobscot Bay environment. The hypothesized benefits of a predator refuge based on the co-occurring species complex is considered minimal for emigrating post-smolts given a mismatch in the size overlap among species and low abundance of other co-occurring diadromous populations. These data can be used to modify current management actions to optimize S. salar recovery and inform future research agendas.

Migratory behaviour and survival rates of wild northern Atlantic salmon Salmo salar post‐smolts: effects of environmental factors

To study smolt behaviour and survival of a northern Atlantic salmon Salmo salar population during river descent, sea entry and fjord migration, 120 wild S. salar were tagged with acoustic tags and registered at four automatic listening station arrays in the mouth of the north Norwegian River Alta and throughout the Alta Fjord. An estimated 75% of the post-smolts survived from the river mouth, through the estuary and the first 17 km of the fjord. Survival rates in the fjord varied with fork length (LF), and ranged from 97.0 to 99.5% km(-1). On average, the post-smolts spent 1.5 days (36 h, range 11-365 h) travelling from the river mouth to the last fjord array, 31 km from the river mouth. The migratory speed was slower (1.8 LF s(-1)) in the first 4 km after sea entry compared with the next 27 km (3.0 LF s(-1)). Post-smolts entered the fjord more often during the high or ebbing tide (70%). There was no clear diurnal migration pattern within the river and fjord, but most of the post-smolts entered the fjord at night (66%, 2000-0800 hours), despite the 24 h daylight at this latitude. The tidal cycle, wind-induced currents and the smolts' own movements seemed to influence migratory speeds and routes in different parts of the fjord. A large variation in migration patterns, both in the river and fjord, might indicate that individuals in stochastic estuarine and marine environments are exposed to highly variable selection regimes, resulting in different responses to environmental factors on both temporal and spatial scales. Post-smolts in the northern Alta Fjord had similar early marine survival rates to those observed previously in southern fjords; however, fjord residency in the north was shorter.

Growth-history perspective on the decreasing age and size at maturation of exploited Atlantic salmon

Decreases in age and size at maturation have been reported for several exploited fish populations, but little is known about possible changes in individual growth trajectories in the course of exploitation. We investigated changes in growth, age and size at maturation in a declining Atlantic salmon Salmo salar population subjected to strong fishing pressure. Based on historical catch records and scale samples collected between years 1948 and 2003, we estimated age and size at maturation and back-calculated individual growth trajectories. The age of the returning salmon declined, smolt size increased and post-smolt growth decreased during the study period. Reductions in age at matu- ration and shifts in growth trajectories occurred in parallel, suggesting that they were not indepen- dent of each other. Although our results cannot distinguish between plastic and genetic effects of exploitation on size and age at maturation, they support the contention that analyses of individual growth trajectories can provide important clues for understanding causes of life-history changes in exploited fish populations.

Changes in scale circulus spacings of an endangered Atlantic salmon Salmo salar population: evidence of a shift in marine migration?

Post‐smolt scale circulus spacing patterns for two Atlantic salmon Salmo salar populations from the Southern Upland (SU) of Nova Scotia, Canada, were compared with spacings from two endangered populations from the inner Bay of Fundy (iBoF) Nova Scotia and New Brunswick, to determine if growth patterns differed among these populations, and if growth patterns had changed as the abundance of these populations declined. An analysis of numbers of marine circuli from scales of post‐smolts and one‐sea‐winter adults of known age indicated that circuli were deposited at a rate of about one circulus per week in summer and slowed to one every 2 weeks in winter. During the summer and the autumn, mean circulus spacing in the iBoF populations, known to have occupied the outer Bay of Fundy during these seasons, was lower than in the SU populations, which are known to migrate to the North Atlantic. Similar circulus spacing patterns within SU populations is suggestive of a common marine distribution for these populations. In contrast, a cluster analysis revealed that within the geographically intermediate Big Salmon River (iBoF), some individuals exhibited wider spacing patterns that resemble the distant migrating SU populations, while others exhibited narrower spacing similar to other iBoF S. salar. Within the Big Salmon River, the relative abundance of the wider and the narrower spacing patterns varied in the earlier years, but all fish sampled since 1999, exhibited wider spacings similar to distant migrating SU S. salar. The apparent disappearance of the narrower pattern, characteristic of localized migration and indicative of historical iBoF populations, suggests that local migration may not presently be a successful strategy for these populations.

Reproductive strategies in small populations: using Atlantic salmon as a case study

Abstract – Wild salmonid populations with only a few breeding adults may not exhibit a significant reduction in genetic variability compared with larger populations. Such an observation suggests that effective population sizes are larger than population size estimates based on direct adult counts and/or the mating strategy maximises outbreeding, contributing to increased heterozygosity. In the case of wild Atlantic salmon Salmo salar populations, stratification by age classes and sexes on the spawning grounds avoids inbreeding and increases genetic variability. We studied the breeding composition of four Spanish salmon populations. Over a 7‐year period we concluded that the probability of within‐cohort mating is very low: females generally reproduce after two sea‐winters whereas males reproduce mostly as one sea‐winter (grilse) and/or mature parr. Considering different levels of contribution of mature parr to spawning derived from field surveys, we developed a simple model for estimating effective population sizes and found that they doubled with 65% parr contribution expected for rivers at this latitude (43°N), and ranged from 100–800 individuals. The effect of between‐cohort mating was modelled considering different ranges of differences in allele frequencies between cohorts and resulted in 28–50% increases in heterozygosity when considering a 65% parr contribution. The complex mating strategy of Atlantic salmon contributes to explain the high levels of genetic variability found for small populations of this species. This model can probably be extended to other animal species with mating strategies involving different cohorts.

Assessing temporal and spatial variation in wild populations of Atlantic salmon with particular reference to Asturias (Northern Spain) rivers

The extent of genetic variation and levels of temporal and spatial heterogeneity was investigated, at six polymorphic protein‐coding loci, in wild Atlantic salmon Salmo salar populations from six rivers of Asturias (Northern Spain). Also, stocks from northern Europe that were among those introduced to repopulate Asturian Rivers, and other wild Spanish and European populations were characterized. The lack of temporal variation observed suggests that effective population sizes of Asturian populations are sufficiently large to prevent extreme levels of genetic drift and that the introduced fish had a negligible contribution to the fisheries of Asturian rivers.

Allozyme and mtDNA divergence between Atlantic salmon populations in North Wales

Allozyme variation differentiates between Atlantic salmon Salmo salar populations in North Wales to a greater extent than mtDNA diversity.