Juvenile salmon in estuaries: comparisons between North American Atlantic and Pacific salmon populations

In Atlantic salmon, as in most salmonids, males can mature early in the life cycle, as small freshwater fish, termed parr, and/or undergo a sea migration before maturing as full-size adults. The alternative life histories are contingent on environmental and social circumstances, such as growth rate, territory quality or any other factor that affects the individual's state. In order to model the choice of life history in this group of commercially valuable species, it is necessary to understand not only the relative contribution of the different male types to subsequent generations, but also to know the factors that affect reproductive success in each type. In this paper we present the results of a study designed to investigate the factors that affect the reproductive success of mature parr. We used highly polymorphic minisatellite DNA markers to analyse paternity in a series of mating experiments where the number and body size of parr were manipulated. The fraction of eggs fertilized by mature parr ranged from 26 to 40 per cent, with individual parr fertilizing up to 26 per cent of the eggs. A strong positive correlation was found between parr size and reproductive success. The relative success of parr decreased with increasing parr number. Data from this and other studies on variation in the timing and degree of parr reproductive success are discussed in relation to the evolution of male mating strategies and life history in salmonids.

Competition with non‐native species may impede the restoration of native species, but differences in competitive abilities among intraspecific native populations may make some populations more suitable for reintroduction than others. Here, juvenile Atlantic salmon (Salmo salar) from two allopatric populations (LaHave and Sebago) being used for reintroduction into Lake Ontario were placed into two natural stream sites differing in the presence of ecologically similar rainbow trout (Oncorhynchus mykiss). We assessed the effects of competition in the natural streams on fitness‐related traits and habitat use of the Atlantic salmon. We then compared these effects to those observed in artificial streams from a previous study. Atlantic salmon in natural streams had reduced fitness‐related traits that were associated with suboptimal microhabitats in the presence of rainbow trout, but utilised optimal microhabitats in their absence. In the presence of rainbow trout, the two Atlantic salmon populations exhibited comparable recapture proportions to each other, but the individuals from the Sebago population had better performance (body size and condition) than those from the LaHave population. Responses of both Atlantic salmon populations to competition with rainbow trout were generally similar in both direction and magnitude when compared to results from the artificial stream study. The combined results suggest that native species restoration efforts should be focused on candidate populations that are ecologically suitable to reintroduction environments, as well as on suitable habitats that do not contain exotic competitors. Moreover, this study highlights the value of controlled experiments in artificial environments for predicting fitness‐related performance in natural environments.

Salmon hold an iconic status along the Atlantic and Pacific coasts of North America, historically providing critical ecosystem services and substantial economic benefits to these regions. Overharvest, fish passage barriers and habitat destruction, in combination with other factors, have resulted in extirpation of approximately 30 % of Pacific (Oncorhynchus spp.) and 90 % of Atlantic salmon (Salmo salar) populations in the contiguous United States (Parrish et al. 1998; Gustafson et al. 2007). Many of the remaining native populations of Atlantic salmon, and Pacific salmon are protected under the U.S. Endangered Species Act (Ford 2011). Significant population declines are also occurring on both coasts in southern Canada (Irvine et al. 2005), where conservation actions are ongoing. This conservation crisis has resulted in extensive research to inform management decisions associated with recovery of endangered salmon populations. Collectively, there is a large and productive research effort in North America focused on conservation of endangered salmon populations. Numerous partnerships are in place to facilitate collaborations among researchers within each of the respective Pacific and Atlantic salmon research communities. In contrast, opportunities for sharing information across these two communities are less structured and usually occur on a small scale—e.g., at international meetings. Publications from these international meetings have typically been collections of concept papers each focused on Atlantic salmon or Pacific salmon (e.g., Lynch et al. 2002; Waples and Hendry 2008). Our goal was to help establish new collaborations between these highly productive research communities by teaming up Atlantic and Pacific salmon biologists. We organized a ‘‘Teaming Up’’ symposium that was held at the 2012 Annual Meeting of the American Fisheries Society in St. Paul, Minnesota, USA. This meeting helped connect scientists with similar interests and it was the catalyst for many new collaborative papers in this special issue. These new teams of Pacific and Atlantic salmon biologists identified areas where collaboration between these research communities W. R. Ardren (&) U.S. Fish and Wildlife Service, Western New England Complex, 11 Lincoln St., Essex Junction, VT 05452, USA e-mail: william_ardren@fws.gov

Three separate effects on refuge use by signal crayfish Pacifastacus leniusculus and Atlantic salmon Salmo salar were examined: (1) the effect on Atlantic salmon of an addition of signal crayfish (doubling the total number of animals), (2) the effect on signal crayfish of an addition of Atlantic salmon and (3) intraspecific compared with interspecific competition, compared by holding total density of animals constant and varying the proportion of signal crayfish and Atlantic salmon in trials. Observations were made during winter, when both species are nocturnal. The proportion of Atlantic salmon sheltering was significantly lower in the presence than in the absence of signal crayfish when the interspecific treatment (Atlantic salmon plus signal crayfish) effected a doubling in density compared to the intraspecific treatment (Atlantic salmon alone). The proportion of signal crayfish sheltering was independent of the presence of Atlantic salmon. When total density was constant, the proportion of Atlantic salmon sheltering was significantly higher in intraspecific (52·8%) than interspecific trials (27·3%). Atlantic salmon out of shelter during the day in winter are believed to be very vulnerable to predators and the capacity for fish to share shelters with one another is known to be very low. Therefore, competition from crayfish for winter shelters may lead to detrimental effects on Atlantic salmon populations.

Abstract–The utilization of lakes, and inlet and outlet streams by juvenile Atlantic salmon (Salmo salarL.), brown trout (Salmo truttaL.) and Arctic charr (Salvelinus alpinus(L.), were investigated in 16 watercourses northern Norway, all known to inhabit salmon stocks. In lakes, fish were caught by small mesh size gill nets, while in rivers fish were caught electrofishing. In the shallow littoral (0‐3 m depth) there were juvenile salmon in 15 of 19 investigated lakes, juvenile trout in 17 and juvenile charr in seven. Trout dominated significantly in numbers in the shallow littoral of seven lakes, while salmon and charr dominated in three lakes each. When trout and salmon were frequent in the shallow littoral, charr was usually not present in this habitat, but were found in the profundal zone in most of the lakes. Atlantic salmon parr utilized both shallow and deep lakes, and used both stones and macrophytic vegetation as shelter. The utilization of lakes by salmon parr seemed to be closely related to utilization of small inlet streams for spawning. In most inlet and outlet streams salmon dominated over trout in numbers, while charr were absent. This is the first documentation of lake‐use by naturally occurring salmon parr in Scandinavia.

Atlantic salmon (Salmo salar) are an economically valuable species and have inhabited the lakes and rivers of Eastern Canada since the last post-glacial period. However, since the end of the 19th century, Atlantic salmon populations have collapsed in Lake Ontario due to land clearance and dam and mill construction. In 2006, the Atlantic Salmon Reintroduction Program (ASRP) was established in Lake Ontario in an effort to recreate a self-sustaining salmon population. The ASRP aims to rebuild the Lake Ontario Atlantic salmon population by stocking fish from genetically diverse broodstock, restoring Atlantic salmon habitat and monitoring water quality. The ASRP also includes a public education component to emphasize the importance of maintaining the Atlantic salmon population. However, re- introduction programs like the ASRP can be challenged by interspecific competition. Chinook salmon are known to induce male agonistic behavior in Atlantic salmon. In addition, Atlantic salmon juveniles are out-competed by Rainbow trout. Furthermore, the prospective diet of Atlantic salmon in Lake Ontario is thiamin-deficient, which may lead to neurological, developmental and reproductive problems as well as Early Mortality Syndrome (EMS). We will conduct research in partnership with Bring Back the Salmon Lake Ontario, Atlantic Salmon Conservation Fund and the Atlantic Salmon Federation to investigate the potential threats to both Atlantic salmon and existing species that may result from the ASRP for Lake Ontario. Findings will be presented as well as recommendations for alterations to the program to ensure that the ASRP is a safe and successful endeavor.

Increased mortality of juvenile Atlantic salmon (Salmo salar L.), related to lowered levels of stored energy following the loss of ice cover during winter, has been observed after hydropower development in the subarctic River Alta, northern Norway. Drift samples were compared to examine if drift densities, and thus drift prey availabilities for juvenile salmon, were lower in the ice-free than the ice-covered area. In addition, juvenile salmon stomach contents were compared to benthos and drift in the ice-free area to examine salmon winter feeding habitat. Zooplankton, originating from the reservoir, dominated drift at the ice-free site but had lower densities at the downstream ice-covered site. Excluding zooplankton, Chironomidae comprised most of the remaining drift at both the ice-free and ice-covered site, followed by Ephemeroptera, Plecoptera and Simuliidae. No Trichoptera were found in the drift samples. There was no consistent diel periodicity in drift. Benthos was dominated by Chironomidae, followed by Ephemeroptera, Plecoptera and Trichoptera. Other invertebrates occurred in low numbers. Juvenile salmon demonstrated size-selective feeding and fed mainly on Ephemeroptera, followed by Trichoptera and Plecoptera. No zooplankton and few Chironomidae were found in the stomach samples. Stomach content was more similar to benthos than to drift, indicating a larger extent of benthic than drift feeding. No evidence was found for the hypothesis that lack of ice cover reduced the invertebrate drift or caused diel periodicity in the drift. Differences in drift between areas with and without ice could not account for the observed differences in mortality of juvenile salmon during the winter in these areas.

ABSTRACTInteractive segregation is considered to be the main mechanism for reducing competition, limiting the size and overlap of niches when juvenile Atlantic salmon (Salmo salar Linnaeus, 1758) and brown trout (Salmo trutta Linnaeus, 1758) co‐occur. However, the differences observed between allopatric and sympatric populations of these fishes in their habitat utilisation are not always evident. We propose to use degree of the parasite communities divergence as an integral indicator of segregation, as it is known that similar infection is found in fish with similar behaviour and habitat use and vice versa. The parasite communities were studied in juvenile Atlantic salmon (Salmo salar Linnaeus, 1758) and brown trout (Salmo trutta Linnaeus, 1758) inhabiting the rivers South Esk and Spey (north‐eastern Scotland, flowing into the North Sea) and the Annan (south‐western Scotland, flowing into the Irish Sea). The parasitological data were used to test the interactive segregation hypothesis, which is thought to be the main mechanism of avoiding competition and determining the degree of niche overlap between sympatric fish species. In all the rivers, parasite communities of brown trout were richer than those of salmon. Ordination of infracommunities using non‐metric multidimensional scaling (NMDS) indicated the greatest and the smallest overlap between parasite communities of salmon and brown trout in the South Esk and the Spey, respectively. The analysis of similarity (ANOSIM) confirmed that intraspecific differences in the parasite communities of salmon and brown trout were consistently lower than interspecific differences, increasing from the South Esk via the Annan to the Spey. The trend of increasing segregation of the host populations was supported by NMDS‐coordination of the component communities of the parasites based on parasite prevalence, comparison of lists of core species and species accumulation curves. Accumulation models indicated the lowest expected species richness of infracommunities in both hosts in the South Esk (a small mountain river), and the highest expected species richness in the Spey (the largest of the studied rivers with a high habitat diversity). In the Annan, a southern river flowing in the plains but similar to the South Esk in length and catchment area, the parasite infracommunities of salmon were as poor as in the South Esk, but those of brown trout were as rich as in the Spey. This differentiation is presumably associated with a high trophicity of the Annan and thus an improved food resources for fish. Juvenile brown trout uses these food resources more actively than juvenile salmon, which is more narrowly specialised and prefers habitats with fast‐flowing water. A smaller divergence of the model curves in the Annan as compared to the Spey, indicating a lesser segregation of the host populations, can be explained by a much higher rate of parasite species accumulation in salmon. The potential of using fish parasites as biological indicators in freshwater ecosystems is discussed.

The presence of ecologically similar nonnative species may impede recovery efforts for native species. We assessed the survival and growth of juvenile Atlantic Salmon Salmo salar from three populations (LaHave River, Sebago Lake, and Lac Saint‐Jean) in the presence of four naturalized nonnative salmonid competitors. The three populations are being used for reintroduction efforts in Lake Ontario, where Atlantic Salmon are extirpated. Juvenile Atlantic Salmon were placed into artificial stream tanks with combinations of juvenile Brown Trout S. trutta, Rainbow Trout Oncorhynchus mykiss, Chinook Salmon O. tshawytscha, and Coho Salmon O. kisutch. Survival of all three Atlantic Salmon populations was lower in the presence of Brown Trout; growth was lower in the Brown Trout treatment and in the multispecies treatment. In contrast, Atlantic Salmon survival and growth were not negatively impacted by the presence of Chinook Salmon, Rainbow Trout, or Coho Salmon. Based on measurements of circulating hormones, Atlantic Salmon were not chronically stressed and did not show a change in social status after 10 months in the artificial stream tanks. Our results support the theory that differences in aggression and niche overlap can influence competitive outcomes and suggest that tributaries containing Brown Trout should be avoided during Atlantic Salmon reintroduction into Lake Ontario.

Salmonids possess straying abilities that allows them to exploit open territory and establish new populations. Atlantic salmon (Salmo salar) are expanding their distribution primarily as first-generation escapes from aquaculture, whereas Pacific salmon (Oncorhynchus spp.) are expanding their range in polar and temperate regions due to both anthropogenic and natural influences. Here we utilize artificial intergeneric reciprocal crosses to assess the ability of seven species of Pacific salmon to hybridize with Atlantic salmon. Most cross types were found to produce low numbers of hatched embryos, but none survived to sexual maturation. Survivors consisted of diploids and triploids containing both Atlantic and Pacific salmon parental genomes. Thus, introgression of DNA between Pacific and Atlantic salmon may occur to form F1 hybrids, but transmission to subsequent generations is expected to be rare and occur only over evolutionary time scales. Further, the low viability observed for the most part at early stages of development and in both reciprocal crosses indicates that intergeneric crosses in nature between Atlantic and Pacific salmon are expected to have severe fitness consequences for both dams and sires.

A laboratory experiment was conducted to assess the potential impacts of surgically implanted 23 and 32 mm passive integrated transponder (PIT) tags on survival, growth, and body condition of juvenile Atlantic salmon Salmo salar. Rate of tag retention and healing of the tagging incision were also evaluated. Atlantic salmon of three different size classes (I: 80 to 99 mm fork length (FL), II: 100 to 119 mm FL, III: 120 to 135 mm FL) were allocated to each of five experimental treatment groups: control, sham-operated (surgery without PIT-tag implantation), 23 mm PIT-tag implantation with and without suture closure of the incision, and 32 mm PIT-tag implantation without suture closure. Over the 35-day experiment, mortality occurred only among fish tagged with 32 mm PIT tags (14%) and all fish larger than 103 mm FL survived. Non-sutured Atlantic salmon between 80 and 99 mm FL implanted with 23 mm PIT tags had a significantly lower mean specific growth rate of mass compared with untagged (control and sham-operated) and sutured conspecifics. However, no significant difference in growth was found between untagged fish and 23 mm PIT-tagged fish 100 to 135 mm FL. Implantation of 32 mm PIT tags decreased growth in all size classes. Regardless of size class, body condition of the fish was not affected by PIT tagging. Retention rates of 23 mm PIT tags with and without suture closure were 100% and 97%, respectively; retention of 32 mm PIT tags without suture closure was 69%. At the end of the experiment, tagging incisions without suture closure were generally well-healed. Fungal infection and inflammation around the incision site occurred only when suture was used, in 46% of size class I, 21% of size class II and 38% of size class III. Although suture closure of the incision following 23 mm PIT-tag implantation had a positive impact on growth of fish smaller than 100 mm FL, we advise against the use of sutures due to high rates of fungal infection around the incision site. Hence, results suggest that surgical implantation of 23 mm PIT tags without suture closure of the incision is a feasible method for marking juvenile Atlantic salmon 100 to 135 mm FL. Further, we caution researchers about the use of 32 mm PIT tags in juvenile Atlantic salmon 80 to 135 mm FL due to high rate of tag rejection and reduced survival and growth.

Diets to which testosterone (1 or 10 ng/g diet) had been added were fed to juvenile Atlantic salmon (Salmo salar) for nine months beginning for months after hatching (Experiment 1) and upon completion of yolk absorption (Experiment 2) to determine the effects on growth, gonadal development, and sex ratio. Dietary testosterone at 10 ng/g fed to juvenile salmon at four months after hatching (Exp. 1) induced significant changes in condition factor (0.69±0.01) compared to controls (0.79±0.01) at the end of the test period. In both experiments, salmon treated with 10 ng/g diet induced a significantly higher percentage of male fish compared to controls. Dietary testosterone at 1 ng/g fed to juvenile salmon beginning four months after hatching induced significant increases in weight (18.95±0.99) and length (13.58±0.23) compared to controls (14.55±1.50 and 11.94±0.43, respectively). In Experiment 1 or 2, there was no apparent influence of dietary testosterone on precocious male sexual development. Dietary testosterone at 1 or 10 ng/g fed to juvenile salmon upon completion of yolk absorption (Exp. 2) induced no consistent changes in growth in juvenile Atlantic salmon. These studies indicate that low levels of dietary testosterone may influence physiological responses in juvenile Atlantic salmon dependent upon timing of treatment.

Very few studies have investigated the effect of genetic diversity on the behavioral and phenotypic traits linked to the competitive ability of individuals. In this study, we reared juvenile Atlantic salmon (Salmo salar) alone or with the competitive rainbow trout (Oncorhynchus mykiss) in order to: (1) to assess correlations between heterozygosity and traits related to individual competitive ability [i.e., heterozygosity–fitness correlations (HFCs)] in Atlantic salmon, and (2) to evaluate the effect of the competitive rainbow trout on any such HFCs. We also tested whether a few loci had a disproportionately large effect (i.e., the local effect hypothesis) or, on the contrary, if all loci contributed equally (i.e., the global effect hypothesis) in explaining the observed HFCs. We found significant HFCs for phenotypic traits related to the competitive ability of juvenile Atlantic salmon, i.e., the growth rate and the distance to the feeding source. Some HFCs were nonlinear, suggesting that individuals with intermediate levels of heterozygosity were favored. In addition, we found that the competition exerted by rainbow trout only weakly modified these HFCs as the relationships were highly consistent across treatments. We demonstrated that the local-effect hypothesis best explained both linear and nonlinear HFCs. Overall, our results illustrated the importance of genetic diversity in explaining the behavioral variability observed within populations. Moreover, we provide evidence that, even if a competitive species can have strong ecological effects, the relationships between genetic diversity and fitness-related traits in juvenile Atlantic salmon were not influenced by such effects.

The effects of hydropeaking and intra‐ and interspecific competition on the growth performance (growth in length, mass and lipid content) of juvenile Atlantic salmon Salmo salar and brown trout Salmo trutta were studied in six experimental channels (three experiencing hydropeaking and three controls with a stable discharge of water). Changes in the water‐covered area in the hydropeaking channels were small to avoid fish stranding. Each channel was divided into three similar‐sized sections and stocked with either low or high density of Atlantic salmon, or a mix of Atlantic salmon and brown trout, with the density of the latter equalling the high‐density treatment of Atlantic salmon. A marked effect of competition was visible as salmon in the low‐density treatment were significantly larger (27–33%) and had a higher mass (30–38%) than salmon in both the high‐density salmon treatment and the high‐density salmon and trout treatment. Hydropeaking had only minor and insignificant effects on the growth performance: overall final length, mass and body lipid content in the salmon experiencing hydropeaking differed by −9%, −7% and +2% compared with controls. Furthermore, there was no indication that the competitive regime influenced hydropeaking effects. The increase in both intra‐ and interspecific competition among the juvenile salmon had a pronounced and significant effect on growth. Our study adds to the growing evidence that energetic consequences of hydropeaking are likely to be small for Atlantic salmon and that stress and mortality associated with stranding represent the main source of population impact.

Habitat use of aquatic organisms is essential to evaluate effects of many environmental challenges like effects of hydropower regulation, where stranding may occur under hydropeaking (rapid dewatering of shallow river areas). Experimental studies as well as observations from nature with juvenile Atlantic salmon (Salmo salar) show that the proportion of the population that uses shallow depth was both independent of population density and decreases with fish size. Experiments were conducted both in the presence and in absence of older fish, during day and night, and during all four seasons. Juvenile salmon from deep areas may therefore distribute into the shallow areas even when fish density becomes reduced. Thus, low density does not lead to reduced stranding risk, and shallow areas may therefore function as a sink in a within-generation source–sink dynamic under a repeated hydropeaking scenario. The sink effect of rapid dewatering in shallow areas may be mitigated by seasonal and diurnal regulation of hydropeaking activity, but dewatering may still result in extinction of weak populations.