Widespread occurrence of pink salmon (Oncorhynchus gorbuscha) throughout Greenland coastal waters

Using social media, the Greenland Institute of Natural Resources collected data on the occurrence of pink salmon (Oncorhynchus gorbuscha) in 2019. Eighty-four pink salmon were reported from 22 locations across Greenland. This comprised 76 specimens from 2019 and 8 specimens from 2013 to 2018. Of these, 12 were caught in fresh water, and a single pink salmon was from the bottom of the Nuuk Fjord near the Kapisillit River – the only known river in Greenland where the Atlantic salmon (Salmo salar) spawn. It is unknown if pink salmon have reproduced in Greenland waters.     

Evidence for long‐term change in length,mass and migration phenology of anadromous spawners in French Atlantic salmon Salmo salar

This study provides new data on Atlantic salmon Salmo salar life‐history traits across France. Using a long‐term recreational angling database (1987–2013) covering 34 rivers in three regions (genetic units), a decline in individual length, mass and a delayed adult return to French rivers was reported. Temporal similarities in trait variations between regions may be attributed to common change in environmental conditions at sea. The relative rate of change in phenotypic traits was more pronounced in early maturing fish [1 sea‐winter (1SW) fish] than in late maturing fish (2SW fish). Such contrasted response within populations highlights the need to account for the diversity in life histories when exploring mechanisms of phenotypic change in S. salar. Such detailed life‐history data on returning S. salar have not previously been reported from France. This study on French populations also contributes to reducing the gap in knowledge by providing further empirical evidence of a global pattern in S. salar across its distribution range. Results are consistent with the hypothesis that the observed changes in life‐history traits are primarily associated with environmental changes in the North Atlantic Ocean. They also emphasize the presence of less important, but still significant contrasts between region and life history.     

Ontogenetic Behavior and Migration of Gulf of Mexico Sturgeon, Acipenser oxyrinchus desotoi, with Notes on Body Color and Development

We observed Suwannee River Gulf sturgeon, Acipenser oxyrinchus desotoi, in the laboratory and found free embryos (first interval after hatching) hid under rocks and did not migrate. Thus, wild embryos should be at the spawning area. Larvae (first interval feeding exogenously) initiated a slow downstream migration, and some juveniles (interval with adult features) continued to migrate slowly for at least 5 months, e.g., a 1-step long larva-juvenile migration. No other population of sturgeon yet studied has this migration style. A conceptual model using this result suggests wild year-0 sturgeon have a variable downstream migration style with short-duration (short distance) migrants and long-duration (long distance) migrants. This migration style should widely disperse wild fish. The model is supported by field studies that found year-0 juveniles are widely dispersed in fresh water to river km 10. Thus, laboratory and field data agree that the entire freshwater reach of river downstream of spawning is nursery habitat. Foraging position of larvae and early juveniles was mostly on the bottom, but fish also spent hours holding position in the water column, an unusual feeding location for sturgeons. The holding position of fish above the bottom suggests benthic forage in the river is scarce and fish have evolved drift feeding. The unusual migration and foraging styles may be adaptations to rear in a river at the southern limit of the species range with poor rearing habitat (low abundance of benthic forage and high summer water temperatures). Suwannee River Gulf sturgeon and Hudson River Atlantic sturgeon, A. o. oxyrinchus, are similar for initiation of migration, early habitat preference, and diel migration. The two subspecies differ greatly for migration and foraging styles, which is likely related to major differences in the quality of rearing habitat. The differences between Atlantic sturgeon populations show the need for geographical studies to represent the behavior of an entire species.     

Reduced Genetic Diversity and Effective Population Size in an Endangered Atlantic Salmon (Salmo Salar) Population from Maine, USA

Atlantic salmon (Salmo salar) populations in Maine, USA, are listed as a Distinct Population Segment under the U.S. Endangered Species Act due to reduced spawning runs and juvenile densities. Whenever possible, optimal conservation strategies for endangered populations should incorporate both present and historical knowledge of genetic variation. We assayed genetic diversity at seven microsatellite loci and at the mitochondrial ND1 gene in an endangered wild population of Atlantic salmon captured from the Dennys River from 1963 to 2001 using DNA’s extracted from archival scale and tissue samples. We examined temporal trends of genetic diversity, population structure, and effective population size (Ne). Overall temporal trends of diversity and Ne show significant reductions from 1963 to 2001 raising the possibility that current restoration efforts may be impacted by historical loss of diversity potentially critical to adaptation. Although our results suggest genetic stability in this population from 1963 to 1981, significant differentiation was observed for both the 1995 and 2001 samples compared with all other temporal samples. The presence of an ND1 mtDNA haplotype in this population, historically observed only in European and Newfoundland stocks, may represent previously unrecognized local wild diversity or, alternatively, may represent introgression from non-native fish.     

Influence of sea temperature and initial marine feeding on survival of Atlantic salmon Salmo salar post-smolts from the Rivers Orkla and Hals, Norway

The abundance of returning adult Atlantic salmon Salmo salar, in the River Orkla in mid‐norway (1 sea‐winter, SW, fish) and River Hals in north Norway (1–3 SW fish), was tested against the early marine feeding and the seawater temperature experienced by their corresponding year classes of post‐smolts immediately after entry into the Trondheimsfjord (Orkla smolts, 22 years of data) and Altafjord (Hals smolts, 17 years of data). In both river–fjord systems, there was a significant positive correlation between the abundance of returning S. salar and the mean seawater temperature at the time of smolts descending to the sea. The number of 1SW fish reported caught in River Orkla was positively correlated to the proportion of fish larvae in the post‐smolt stomachs in Trondheimsfjord. The abundance of returning S.salar was, however, neither correlated to forage ratio (R_F) nor other prey groups in post‐smolt stomachs in the two fjord systems. In the Altafjord, the post‐smolts fed mainly on pelagic fish larva (70–98%) and had a stable R_F (0·009–0·023) over the 6 years analysed. In the Trondheimsfjord, however, there was a higher variation in R_F (0·003–0·036), and pelagic fish larvae were dominant prey in only two (50 and 91%) of the 8 years analysed. These 2 years also showed the highest return rates of S. salar in River Orkla. These results demonstrate that the thermal conditions experienced by post‐smolts during their early sea migration may be crucial for the subsequent return rate of adults after 1–3 years at sea. Pelagic marine fish larvae seem to be the preferred initial prey for S. salar post‐smolts. As the annual variation in abundance of fish larvae is related to seawater temperature, it is proposed that seawater temperature at sea entry and the subsequent abundance of returning adult S. salar may be indirectly linked through variation in annual availability of pelagic fish larvae or other suitable food items in the early post‐smolt phase.     

Characterization of an Atlantic salmon Salmo salar stream at the southern limit of its eastern Atlantic distribution

Ten reaches of an Atlantic stream located in north‐west Spain were sampled intensively during one summer to characterize the conditions where Atlantic salmon Salmo salar have been re‐introduced along the stream. Fish species richness and diversity showed a downstream increase, which was mainly attributed to the higher number of cyprinid species found in the lower reaches. Moreover, except for brown trout Salmo trutta that appeared to be the most ubiquitous species, the densities of the other species was higher in the lower than in the upper stream reaches. Redundancy analysis showed that the pattern of fish assemblages observed along the studied stream was mainly related to the expected gradient observed in the levels of dissolved oxygen, discharge and mean current speed. There was a significant differentiation between midstream and downstream reaches, both in terms of the composition of their fish assemblages and the freshwater habitat. This study emphasizes the importance of describing the variations in fish assemblages and habitat characteristics along a river to explore its relation to potential changes in the survival of fish populations. In particular, the development of habitat–fish relationships may be a useful tool for water managers to assess the effects of development and restoration projects on the habitat of S. salar.     

Evidence for episodic acidification effects on migrating Atlantic salmon Salmo salar smolts

Field studies were conducted to determine levels of gill aluminium as an index of acidification effects on migrating Atlantic salmon Salmo salar smolts in the north‐eastern U.S.A. along mainstem river migration corridors in several major river basins. Smolts emigrating from the Connecticut River, where most (but not all) tributaries were well buffered, had low or undetectable levels of gill aluminium and high gill Na⁺/K⁺‐ATPase (NKA) activity. In contrast, smolts emigrating from the upper Merrimack River basin where most tributaries are characterized by low pH and high inorganic aluminium had consistently elevated gill aluminium and lower gill NKA activity, which may explain the low adult return rates of S. salar stocked into the upper Merrimack catchment. In the Sheepscot, Narraguagus and Penobscot Rivers in Maine, river and year‐specific effects on gill aluminium were detected that appeared to be driven by underlying geology and high spring discharge. The results indicate that episodic acidification is affecting S. salar smolts in poorly buffered streams in New England and may help explain variation in S. salar survival and abundance among rivers and among years, with implications for the conservation and recovery of S. salar in the north‐eastern U.S.A. These results suggest that the physiological condition of outmigrating smolts may serve as a large‐scale sentinel of landscape‐level recovery of atmospheric pollution in this and other parts of the North Atlantic region.     

The effect of stocking with 0+ year age‐class Atlantic salmon Salmo salar fry: a case study from the River Bush,Northern Ireland

An enhancement programme based on stocking 0+ year age‐class Atlantic salmon Salmo salar, conducted in the River Bush, Northern Ireland, U.K. over the period 1996–2005, was reviewed with reference to the performance and biological characteristics of wild fish. Wild ova to 0+ year fry (summer) survival was c. 8% with subsequent wild 0+ year fry‐to‐smolt survival c. 9%. Stocked unfed 0+ year juveniles gave c. 1% survival to smolt whilst fed 0+ year S. salar stocked in late summer exhibited survival at c. 5%. Stocking with unfed and fed fry contributed to increased smolt production and helped attain local management objectives between 2001 and 2005. Significant differences in biological characteristics were observed between wild and stocked‐origin fish. Wild‐smolt cohorts were dominated by 2+ year age‐class fish on the River Bush whilst smolts originating from fed fry mostly comprised younger 1+ year individuals. The mean mass of 1+ year smolts derived from stocked fed fry was significantly lower than that of wild 1+ year smolts, although these differences were not evident between older age classes. Differences in run timing between wild smolts and smolts derived from stocked fry were also apparent with the stocked‐origin fish tending to run earlier than wild fish. Although the stocking exercise was useful in terms of maximizing freshwater production, concerns over the quality of stocked‐origin recruits and the long term consequences for productivity are highlighted.     

Mitigation of acidified salmon rivers – effects of liming on young brown trout Salmo trutta

In southern Norway, 22 acidified rivers supporting anadromous salmonids were mitigated with lime to improve water quality and restore fish populations. In 13 of these rivers, effects on Salmo trutta and Salmo salar densities were monitored over 10–12 years, grouped into age 0 and age ≥ 1 year fish. These rivers had a mean annual discharge of between 4·9 and 85·5 m³ s^?1, and six of them were regulated for hydro‐power production. Salmo salar were lost in six of these rivers prior to liming, and highly reduced in the remaining seven rivers. Post‐liming, S. salar became re‐established in all six rivers with lost populations, and recovered in the seven other rivers. Salmo trutta occurred in all 13 study rivers prior to liming. Despite the improved water quality, both age 0 and age ≥ 1 year S. trutta densities decreased as S. salar density increased, with an average reduction of >50% after 10 years of liming. For age 0 year S. trutta this effect was less strong in rivers where S. salar were present prior to liming. In contrast, densities of S. trutta increased in unlimed streams above the anadromous stretches in two of the rivers following improved water quality due to natural recovery. Density increases of both age 0 and age ≥ 1 year S. salar showed a positive effect of river discharge. The results suggest that the decline in S. trutta density after liming is related to interspecific resource competition due to the recovery of S. salar. Thus, improved water quality through liming may not only sustain susceptible species, but can have a negative effect on species that are more tolerant prior to the treatment, such as S. trutta.     

Spawning-strategy-dependent diets in two North American populations of Atlantic salmon Salmo salar

The diet of repeat-spawner Atlantic salmon Salmo salar was investigated using carbon and nitrogen stable-isotope values from the outer growth band of scales, which reflect the fish's consumption and growth during their most recent marine phase. Isotope values for S. salar displaying different spawning strategies were compared between and within the Miramichi and Nashwaak Rivers, New Brunswick, Canada and a Bayesian mixing model was used to infer dietary contributions from potential prey items. Significant differences in the stable-isotope values were found among spawning strategies and between rivers, indicating differences in diet and feeding area, consistent with hypotheses. Bayesian mixing model results inferred the main prey items consumed during marine feeding by S. salar to consist of hyperiid amphipods and capelin Mallotus villosus for repeat alternate spawners from both rivers, sandlance Ammodytes sp. for repeat consecutive spawners from the Miramichi River and amphipods for repeat consecutive spawners from the Nashwaak River. These results demonstrate the diversity of feeding tactics among S. salar spawning strategies from the same river and between populations from different rivers. Accounting for differences in prey availability and the subsequent impact on S. salar diet and spawner return rates (i.e., marine survival) will facilitate the application of ecosystem-based management practices, such as ensuring that fisheries for forage species do not indirectly adversely affect S. salar return rates.     

Differences in growth between offspring of anadromous and freshwater brown trout Salmo trutta

In this study, individual growth of juvenile offspring of anadromous and freshwater resident brown trout Salmo trutta and crosses between the two from the River Imsa, Norway, was estimated. The juveniles were incubated until hatching at two temperatures (±S.D. ), either 4.4 ± 1.5°C or 7.1 ± 0.6°C. Growth rate was estimated for 22 days in August–September when the fish on average were c. 8 g in wet mass, and the estimates were standardized to 1 g fish dry mass. Offspring of anadromous S. trutta grew better at both 15 and 18°C than offspring of freshwater resident S. trutta or offspring of crosses between the two S. trutta types. This difference appears not to result from a maternal effect because anadromous S. trutta grew better than the hybrids with anadromous mothers. Instead, this appears to be an inherited difference between the anadromous and the freshwater resident fish lending support to the hypothesis that anadromous and freshwater resident S. trutta in this river differ in genetic expression. Egg incubation temperature of S. trutta appeared not to influence the later growth as reported earlier from the studies of Atlantic salmon Salmo salar.     

The gill maggot Salmincola salmoneus as an indicator of repeat spawning in Atlantic salmon Salmo salar

The potential of the gill maggot Salmincola salmoneus for use as an indicator of repeat spawning in Atlantic salmon Salmo salar was studied in adult fish captured during their return migration to the River Alta (n = 659) and River Namsen (n = 540) in Norway. Eighty‐eight and 49% of previous spawners identified by scale readings were infected with S. salmoneus in the two rivers, respectively. Salmincola salmoneus can be used as a reliable, rapid and objective field indicator of repeat spawning in S. salar as nearly all infected fish (99·4%) were identified as repeat spawners, although it is important to have appropriate background information on S. salmoneus prevalence on the postspawning individuals within the same population.     

Are antipredator behaviours of hatchery Salmo salar juveniles similar to wild juveniles?

This study explores how antipredator behaviour of juvenile Atlantic salmon Salmo salar developed during conventional hatchery rearing of eggs from wild brood stock, compared with the behaviour of wild‐caught juveniles from the same population. Juveniles aged 1+ years were tested in two unfamiliar environments; in one S. salar were presented with simulated predator attacks and in the other they were given the opportunity to explore an open‐field arena. No difference was found in their spontaneous escape responses or ventilation rate (reflex responses) after simulated predator attacks. Hatchery‐reared juveniles were more risk‐prone in their behaviours than wild‐caught individuals. Hatchery juveniles stayed less time in association with shelter. In the open‐field arena, hatchery juveniles were more active than wild juveniles. Hatchery juveniles were also immobile for less time and spent a shorter amount of time than wild juveniles in the fringe of the open‐field arena. Salmo salar size had no effect on the observed behaviour. Overall, this study provides empirical evidence that one generation of hatchery rearing does not change reflex responses associated with threats, whereas antipredator behaviour, typically associated with prior experience, was less developed in hatchery‐reared than in wild individuals.     

Seasonal variation in the thermal performance of juvenile Atlantic salmon (Salmo salar)

1. Experimental data on the maximum growth and food consumption of winter‐acclimatised Atlantic salmon (Salmo salar) juveniles from three Norwegian rivers situated at 59 and 70°N were compared with predictions from published models of growth and food consumption of summer‐acclimatised fish from the same populations. 2. All winter‐acclimatised fish maintained positive growth and a substantial energy intake over the whole range of experimental temperature (1–6 °C). This contrasted with predictions from growth models based on summer acclimatised Atlantic salmon, where growth and energy intake ceased at approximately 5 °C. 3. Growth and food consumption varied significantly among populations. Winter‐acclimatised fish from a Northern population had a higher mass‐specific growth rate, higher energy intake and higher growth efficiency than southern populations, which is contrary to predictions from models developed using summer‐acclimatised salmon, where fish from the Northern population had the lowest growth efficiency. 4. The experiment provides evidence that thermal performance varies seasonally and suggests adaptation to the annual thermal regime.     

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.     

Growth differences between the Atlantic salmon parr,Salmo salar,of nursery brooks and natal rivers in the River Teno watercourse in northern Finland

Synopsis A comparison was done between growth in Atlantic salmon,Salmo salar, parr from natal rivers and small tributaries in the subarctic River Teno watercourse in northern Finland (70° N, 28° E). Salmon do not spawn in these tributaries but juveniles enter the brooks from their spawning rivers. Parr from these brooks (age groups 1 + and 2 +) were larger, so were their recorded annual growth increments, than those caught simultaneously in the River Teno. First year growth was also better in brook parr, although both groups had spent their first year in the main river. The brook water temperature was lower than that in the main rivers through-out the growing season. On the other hand, drifting food resources were more abundant in brooks. It has been suggested that better growth and a longer residence in fresh water result in the larger smolts being produced in the brooks relative to the main river. Differences in habitat selection and subsequent differences in the smolt age and size of fish from the same place of origin reflect the distinct life histories of the salmon in the River Teno.     

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.     

Occurrence of Atlantic salmon in the Douro River: Recent evidence

Two adult Atlantic salmon, Salmo salar, were netted in 2015 and 2017 during a study conducted in the terminal stretch of the Douro River, after more than 20 years without confirmed records of the species occurrence in the southern limit of its European range. According to professional fisherman operating in the area, a few adult Atlantic salmon are caught every year in this river stretch. Further work should be conducted to assess if the species is reproducing in the Douro River and the overall status of its populations in the North of Iberia.     

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.     

Divergent trends in life‐history traits between Atlantic salmon Salmo salar of wild and hatchery origin in the Baltic Sea

Four Atlantic salmon Salmo salar stocks in the Baltic Sea, varying in their breeding history, were studied for changes in life‐history traits over the years 1972–1995. Total length (L_T) at age of captured (L_TC) fish had increased throughout the study period, partly due to increased temperature and increased L_T at release, (L_TR) but also due to remaining cohort effects that could represent unaccounted environmental or genetic change. Simultaneously, maturation probabilities controlled for water temperature, L_TC and L_TR had increased in all stocks. The least change was observed in the River Tornionjoki S. salar that was subject only to supportive stockings originating from wild parents. These results suggest a long‐term divergence between semi‐natural and broodstock‐based S. salar stocks. Increased L_T at age explained advanced maturation only marginally, and it remains an open question to what extent the generally increased probabilities to mature at early age reflected underlying genetic changes.