Migration pathways,speed and mortality of Atlantic salmon (Salmo salar) smolts in a Scottish river and the near‐shore coastal marine environment

Long‐distance migration of Atlantic salmon (Salmo salar) is known to result in high levels of mortality. For a species experiencing global population decline, it is thus vital to better understand migration behaviour, both in the river and marine stages. Atlantic salmon smolts (n = 50) were tracked using acoustic telemetry in the River Deveron, Scotland, and adjacent coastal area. Higher rates of mortality were observed in the river (0.77% per km) than the early marine stage of migration (0.0% per km). Mortality likely resulted from predation. Higher swim speeds were recorded in the early marine stage compared with the river (marine = 7.37 ± 28.20 km/day; river = 5.03 ± 1.73 km/day [mean ± SD]), a potential predator avoidance behaviour. The majority of smolts leaving the river did so in darkness and on a flooding tide. Overall river and marine migration success were linked to nights of lower lunar brightness. Marine migration speed decreased with increasing environmental noise levels, a finding with implications for fisheries management. The migration pathway in the early marine environment did not follow obvious geographical features, such as the coastline. Thus, we suggest that early marine environment pathways are more influenced by complex water currents. These findings highlight factors that influence smolt migration survival and behaviour, areas on which future research should focus.     

Choline supplementation increased total body lipid gain,while surplus methionine improved growth and amino acid retention in adult Atlantic salmon (Salmo salar)

Methionine–choline‐deficient (MCD) mammals are known to accumulate liver TAG probably due to phosphatidylcholine (PC) deficiency and thus assembly of VLDL and transport of lipids from liver to peripheral organs. To assess whether supplementation of choline could spare methionine and secure a healthy liver metabolism, by reducing the endogenous PC synthesis without interfering with lipid transport and distribution, Atlantic salmon with initial BW of 700 g were fed adequate (1.9 g Met/16 gN) or surplus methionine (2.5 g Met/16 gN) diets of which were supplemented with choline or not for a period of 19 weeks. Fish fed the low‐methionine diets had reduced growth (p = .013) due to reduced protein gain (p = .007), while lipid gain slightly improved in fish fed the choline‐supplemented diets (p = .047). Also, feed conversion improved when fed surplus methionine (p < .001), while choline supplementation had no impact on feed conversion. No interaction between choline and methionine on growth performance or retention existed. Phospholipid status in liver and muscle was not affected by treatments, and no liver TAG accumulation occurred at the methionine levels used. Gene expression of ApoB100 necessary for assembling VLDL or pemt necessary for endogenous PC synthesis was unaffected by treatments. Capacity of methylation (MAT, BHMT) within the liver was not affected by treatment nor was the gene expression of enzymes in liver transsulfuration (CBS or CDO). Methionine status within liver was unaffected by treatments, while free methionine reduced in those fish fed the low‐methionine diets in muscle and plasma. Cystathionine and taurine were elevated when fed surplus methionine. Choline supplementation had no impact on sulphur amino acid metabolites in either tissue. Neither did choline supplementation improve TAG mobilization from liver to muscle. To conclude, choline does not improve endogenous phospholipid synthesis or transport of TAG from liver to muscle depot when added to diets containing 1.9 g Met/16 gN, while surplus methionine improved growth and protein retention, indicating that 1.9 g Met/16 gN is enough to support a healthy liver metabolism, but too low to support muscle protein deposition in adult salmon fed high plant protein diets for longer periods of time.     

What the past tells us about the future of Pacific salmon research

The Larkin lectures are held every two years at the University of British Columbia in recognition of Dr. Peter Larkin's contributions to fisheries science. The lecture I presented in November 2015 coincided with an announcement that the Institute of Fisheries that Peter Larkin founded in the 1960s would be restructured as the “Institute for Oceans and Fisheries” with an emphasis on fisheries and oceans issues important to British Columbia as well as the rest of the world. I decided to look back at research issues that Peter Larkin thought would be important for Pacific salmon (Oncorhynchus spp.) in the future and see what has happened as a way of identifying the complexity that the Institute for Oceans and Fisheries in particular and the science community in general will face. I chose five themes from Peter Larkin's talks: 1 understanding marine survival, 2 ocean carrying capacity, 3 aquaculture, 4 climate, Pacific salmon and climate change and 5 informing the public, and then added my opinion about research priorities for the future. Peter Larkin recognized the future relevance of these examples, but he probably could not have imagined how these and related issues will challenge his renamed institute and the rest of the research community over the next 50 years.     

Interactions of baseflow habitat constraints: Macroinvertebrate drift,stream temperature,and physical habitat for anadromous salmon in the Calapooia River,Oregon

1. The Calapooia River in western Oregon supports a small winter steelhead trout (Oncorhynchus mykiss) population and historically supported spring Chinook salmon (Oncorhynchus tshawytscha). Early timber harvesting removed the riparian forest, and log transportation practices simplified the channel. Those disturbance legacies continue to affect fish habitat by limiting shade and channel complexity, complicating conservation efforts.
2. To evaluate juvenile salmonid rearing potential, macroinvertebrate drift, thermal regime and physical habitat were measured at eight sites in 24 km of the upper river during late summer baseflow.
3. Overall physical habitat was simple, with few functioning instream structures or pools. During the 22‐day drift study, flows declined and maximum site stream temperatures ranged from 23.1°C at the lower end to 16.4°C 24 km upstream.
4. Macroinvertebrate drift concentrations ranged from 0.7–13.7 ind. m^?3 with biomasses from 0.02–1.23 mg m^?3. Drift concentration biomass was higher upstream (P  = 0.006) than downstream and declined overall (P  < 0.001) during the study. Drift biomass was dominated by five taxon groups – Baetis tricaudatus, Calineuria californica, Hesperoperla pacifica, Simulium spp., and Chironomidae, which were 65% of total biomass. During twilight, total biomass and biomass of B. tricaudatus, Simulium spp., and Chironomidae (both larvae and adults) were higher.
5. Total drift declined dramatically over the study period owing to decreases in drift concentration and a 58% decline in discharge, greatly reducing overall drift and available food resources for juvenile‐rearing salmonids.
6. The upper catchment, both with cooler temperatures and higher food availability, provided the best conditions for juvenile anadromous salmonids to survive late summer conditions. Conservation consequences of climate change‐induced alterations in flow and temperature may further affect habitat quality for juvenile salmonids in this catchment in the coming decades.

     

Temperature modulates liver lipid accumulation in Atlantic salmon (Salmo salar L.) fed low dietary levels of long‐chain n‐3 fatty acids

Atlantic salmon (Salmo salar) were fed five graded levels of eicosapentaenoic acid (EPA, 20:5n‐3) and docosahexaenoic acid (DHA, 22:6n‐3), from 1.4 to 5.2% of total fatty acids (FA, 5–17 mg kg^?1 feed), and grew from ~160 g to ~3000 g, with the period from 1450 g onwards conducted both at 6 °C and at 12 °C. All fish appeared healthy, and there were no diet‐related differences in haematological or plasma parameters, as well as intestinal histological or gut microbiota analysis. Fish reared at 6 °C had higher accumulation of storage lipids in the liver compared to fish reared at 12 °C. Liver lipids also increased with decreasing dietary EPA + DHA at 6 °C, while there was no such relationship at 12 °C. Gene expression of SREBP1 and 2, LXR, FAS and CPT1 could not explain the differences in liver lipid accumulation. In liver polar lipids, DHA was found to be reduced when dietary EPA + DHA was <2.7% of FAs, while the level of EPA in the membranes was not affected. In conclusion, reducing dietary EPA + DHA from 5.2 to 1.4% of total FAs had a minor impact on fish health. Temperature was the factor that most affected the liver lipid accumulation, but there was also an interaction with dietary components.     

Riparian defoliation by the invasive green alder sawfly influences terrestrial prey subsidies to salmon streams

Invasive species in riparian forests are unique as their effects can transcend ecosystem boundaries via stream‐riparian linkages. The green alder sawfly (Monsoma pulveratum) is an invasive wasp whose larvae are defoliating riparian thin‐leaf alder (Alnus tenuifolia) stands across southcentral Alaska. To test the hypothesis that riparian defoliation by this invasive sawfly negatively affects the flow of terrestrial prey resources to stream fishes, we sampled terrestrial invertebrates on riparian alder foliage, their subsidies to streams and their consumption by juvenile coho salmon (Oncorhynchus kisutch). Invasive sawflies altered the composition of terrestrial invertebrates on riparian alder foliage and as terrestrial prey subsidies to streams. Community analyses supported these findings revealing that invasive sawflies shifted the community structure of terrestrial invertebrates between seasons and levels of energy flow (riparian foliage, streams and fish). Invasive sawfly biomass peaked mid‐summer, altering the timing and magnitude of terrestrial prey subsidies to streams. Contrary to our hypothesis, invasive sawflies had no effect on the biomass of native taxa on riparian alder foliage, as terrestrial prey subsidies, or in juvenile coho salmon diets. Juvenile coho salmon consumed invasive sawflies when most abundant, but relied more on other prey types selecting against sawflies relative to their availability. Although we did not find effects of invasive sawflies extending to juvenile coho salmon in this study, these results could change as the distribution of invasive sawflies expands or as defoliation intensifies. Nevertheless, riparian defoliation by these invasive sawflies is likely having other ecological effects that merits further investigation.     

Linking watershed and microhabitat characteristics: effects on production of Atlantic salmonids (Salmo salar and Salmo trutta)

The spatial scale of environmental factors influencing population dynamics ranges from microhabitat to continental or even global scales. Integration of multiple spatial scales is important in order to understand links between environmental variation and population processes. In the present study, we investigate how multiscale drivers influence the production of stream‐rearing Atlantic salmonids (Atlantic salmon, Salmo salar L. and brown trout, Salmo trutta L.) measured in terms of abundance. Variation in juvenile production was studied using data from single‐pass electrofishing surveys (measured as biomass per m²) from nine rivers. These data were combined with habitat data ranging from an important in‐stream microhabitat variable (shelter availability) to properties of the catchment. Variation in productivity within and among rivers was affected by both properties of in‐stream habitat and catchment properties. Shelter availability and the proportion of the catchment consisting of cultivated land and lakes influenced biomass positively, while catchment area had the opposite effect. For a different set of rivers (N = 20), river gradient and catchment area were shown to positively affect the amount of shelter. Finally, the variables identified in the two preceding analysis were included in the analysis of population productivity using catch statistics from 160 rivers. The proportion of cultivated land and lakes, estimated shelter availability were found to have positive effects. In addition, temperature had a positive effect, while river width had a negative effect. This study shows that combining multiple‐scale environmental factors can explain a substantial proportion of variation in population productivity among and within the populations of Atlantic salmonids.     

Release during night enhances survival of wild Atlantic salmon smolts

The effect of nocturnal and diurnal releases on survival and migration of wild and hatchery‐reared Atlantic salmon, Salmo salar, L., smolts (n = 82) was investigated by releasing acoustically tagged smolts at the lower end of the River Vosso, Norway. Hatchery smolts was registered in the estuary within hours of their release, whereas wild smolts migrated over a prolonged period. The time of estuary exit was affected by river discharge but not by time of release. Progression rates were slow through the estuary (0.25 BL/s ± 0.18 SD) and fast through the fjord (1.80 BL/s ± 0.69 SD), and they were not affected by the time of release or origin. Survival to the fjord was low (0%–15%). Survival was not affected by body length but was lower for wild smolts than for hatchery smolts, and survival of the former was lower when the fish were released in daylight.