Salmon Smolts Research Articles

Tagger effects in aquatic telemetry: Short‐term and delayed effects of surgery in Atlantic Salmon smolts

AbstractObjectiveAn assumption of biotelemetry is that animal performance is unaffected by the tagging process and tag burden, yet this assumption is often untested or not thoroughly explored. Our objective was to explore how transmitter implantation procedures influenced Atlantic Salmon Salmo salar smolt survival and migratory performance.MethodsWe monitored radio‐tagged smolts, first in the hatchery and then in a river with a receiver array. We assessed survival and in‐river performance in relation to surgeon, surgery duration, processing order, and fish size.ResultMortality was 13.3% during an 8‐day hatchery‐observation period but was higher for fish that were processed by one of two experienced surgeons (25% vs. 2%). Mortality peaked 3 days postsurgery and was higher for smaller fish and fish that were tagged during morning tagging sessions (versus afternoons). The size effect changed over time, being greatest during the first 2 days postsurgery and continuing thereafter at a diminished level. Fish performance once released into a river also differed between surgeons (migration initiation 66% vs. 82%; to‐lake migration success 22% vs. 43%) and, consistent with hatchery observations fish that were tagged in the morning by one surgeon, performed poorly once released.ConclusionWe highlight the immediate and lingering effects of surgical procedures on smolt survival that, if not accounted for, could bias inferences about the study population. Researchers should anticipate tagger effects during study design to ensure potential tagger effects (i.e., surgeon, order tagged, conditions during tagging) are balanced across study groups of interest. Testing for a fixed tagger effect in analyses may not always be adequate because a tagger effect may covary with processing order and fish size and may change over time.

Acoustic telemetry tracking of Coho Salmon smolts released from a community‐run hatchery into a marine inlet reveals low early ocean survival

AbstractObjectiveHatcheries are used to help supplement populations of Pacific salmon Oncorhynchus spp. for conservation and fisheries purposes and to provide stewardship opportunities. In British Columbia, small community‐run hatcheries typically focus on the latter and generally do not know the efficacy of their release approaches, unlike larger production hatcheries.MethodsWe used acoustic telemetry to track the survival and behavior of 90 hatchery‐reared Coho Salmon O. kisutch smolts, released from a community hatchery, through Burrard Inlet and into the Strait of Georgia. This is the first study to track Coho Salmon smolts released directly into a marine environment and one of very few studies to track them in an oceanic setting.ResultSmolt survival was lowest through the first 3 km of the migration at 37%, and the estimated cumulative survival to the final array (~20 km from release) was 10–23%. The presence of numerous predators in the region suggests that predation is a possible explanation for poor survival over the relatively short migration distance. Travel rates ranged from ~4 to 18 km/day, depending on the migration segment, which is slower than the marine migration of smolts from other species, likely increasing exposure to predators. However, we found potential evidence of “predator swamping,” as survival estimates across migration segments were highest for the final of our three release groups and poorest for the first release group, with intergroup survival estimates increasing by 7–12% in the final three migration segments. Releasing smolts at high densities and at night, as our partner hatchery currently does, likely improves smolt survival.ConclusionOur work demonstrates how acoustic telemetry can be used to examine hatchery practices and propose potential improvements. Saltwater readiness and the release location are factors that our results suggest may warrant future attention to potentially improve marine survival of Coho Salmon smolts.

Exposure to a winter photoperiod to produce large Atlantic salmon smolts (Salmo salar) increases energetic costs and reduces hypoxia tolerance during seawater transfer

A goal of salmon aquaculture is to produce larger, physiologically robust smolts to improve performance at time of seawater transfer and reduce time spent in marine net pens. However, little is known about whether standard processes to induce smoltification provide benefits in the production of large smolts. In Atlantic salmon (Salmo salar) reared under continuous light (24 L:0D) we applied an artificial photoperiod treatment (8 weeks of 12 L:12D followed by 4 weeks of 24 L:0D before seawater transfer) at three sizes to produce fish of ∼150–200 g, ∼400–600 g, and ∼ 1200–1300 g at seawater transfer. We then measured O2 consumption (a proxy for metabolic rate) of photoperiod treated and control salmon (kept in constant 24 L:0D until time of seawater transfer) at three time points (in freshwater prior to seawater entry, 24 h after seawater entry, and 1 month after seawater entry) to assess impacts of photoperiod treatment and seawater transfer on performance. Consistent with known higher metabolic rates in smolts vs parr, photoperiod treatment (12 L:12D) increased minimum O2 consumption (ṀO2min) by ∼13% across all three sizes and at all time points measured. Increased ṀO2min was not solely a result of increased growth rates and was associated with lower hypoxia tolerance at time of seawater transfer in ∼500 g and ∼ 1200 g salmon. Transfer to seawater reduced maximum O2 consumption (ṀO2max) acutely in ∼200 g and ∼ 500 g salmon regardless of the photoperiod treatment (12 L:12D or 24 L:0D), consistent with potential ion and osmotic disturbances leading to an osmo-respiratory compromise 24 h after seawater transfer. For both sizes of ∼200 g and ∼ 500 g, ṀO2max recovered to freshwater levels 1 month after seawater transfer. In contrast, ∼1200 g salmon showed no significant changes in ṀO2max following seawater transfer. Overall, we showed little benefits of traditional photoperiod treatments (12 L:12D) on performance of large salmon following seawater transfer. In fact, photoperiod treatment (12 L:12D) may lead to negative impacts on large salmon by increasing ṀO2min and sensitivity to environmental stress such as hypoxia. Our results, combined with other recent studies, suggest that traditional photoperiod manipulations may be unnecessary for the development of larger Atlantic salmon smolts. However, further research is needed to fully understand impacts of photoperiod manipulations on physiological performance of larger salmon at time of seawater transfer.

Dietary electrolyte imbalance alters drinking rate and gastrointestinal tract water fluxes of Atlantic salmon (Salmo salar) smolt in seawater

It was hypothesized that dietary electrolyte balance (dEB) would influence the dynamics of water, ions, and nutrient fluxes in the gastrointestinal tract (GIT) of Atlantic salmon (Salmo salar) smolts differently depending on water salinity. To date, a comparative study on how dEB alters these dynamics in freshwater- and seawater-adapted fish is lacking. The test diets were low versus high dEB (−100 versus 500 mEq kg−1 DM−1) and the test water salinities were 0 versus 30 ppt. Furthermore, the effect of the interaction between dEB and salinity on blood pH and osmolality were investigated. The experiment lasted for 6.5 weeks. Chyme was collected from four GIT segments (stomach, proximal, middle, and distal intestine) and analysed for dry matter (DM), pH, osmolality, crude protein, and ion (Ca2+, Mg2+, Na+, and K+) content. Water, ion, and nutrient fluxes were measured using yttrium oxide (Y2O3) as an inert marker. It was found that there was a diet effect on chyme pH in the stomach, being lower in fish fed the low dEB diet than the high dEB diet. Furthermore, the diet altered ion and nutrient fluxes in the stomach. Water salinity had the largest effect on chyme pH in all the GIT segments and on chyme osmolality in the stomach, which significantly increased in seawater conditions. The interaction between dEB and salinity had an effect on chyme DM, water and ion fluxes in the stomach, proximal and middle intestine. Our results showed that, depending on water salinity, dEB altered water fluxes differently. In freshwater-adapted fish, water influx to the stomach was higher in fish fed the high dEB diet than the low dEB diet, but the difference was neglectable. In contrast, in seawater-adapted fish, water influx into the stomach and proximal intestine was higher in fish fed the low dEB than the high dEB diet, and the amplitude was much larger. Additionally, in seawater conditions, drinking rate was 50% higher in fish fed the low dEB diet (3.07 ml kg−1 h−1) than the high dEB diet (2.04 ml kg−1 h−1). As a result, it was concluded that, in seawater conditions, a diet with a low dEB has a higher flux of water in the stomach and proximal intestine of Atlantic salmon smolts as well as enhanced drinking rate.

Persistent immune responses in the heart determine the outcome of cardiomyopathy syndrome in Atlantic salmon (Salmo salar)

Cardiomyopathy syndrome (CMS) caused by piscine myocarditis virus (PMCV) is a severe cardiac disease in Atlantic salmon (Salmo salar) and one of the leading causes of morbidity and mortality in the Norwegian aquaculture industry. Previous research suggest a variation in individual susceptibility to develop severe disease, however the role of the immune response in determining individual outcome of CMS is poorly understood particularly in cases where fish are also challenged by stress. The present study's aim was therefore to characterize cardiac transcriptional responses to PMCV infection in Atlantic salmon responding to infection under stressful conditions with a high versus low degree of histopathological damage.The study was performed as a large-scale controlled experiment of Atlantic salmon smolts from pre-challenge to 12 weeks post infection (wpi) with PMCV, during which fish were exposed to intermittent stressors. RNA sequencing (RNAseq) was used to compare the heart transcriptome of high responders (HR) with atrium histopathology score ‘4’ and low responders (LR) with score ‘0.5’ at 12 wpi. A high-throughput quantitative PCR (qPCR) analysis was used to compare immune gene transcription between individuals sampled at 6, 9 and 12 wpi. Based on RNAseq and qPCR results, RNAscope in situ hybridization (ISH) was performed for visualization of IFN-γ - and IFNb producing immune cells in affected heart tissue.Compared to LR, the transcription of 1592 genes was increased in HR at 12 wpi. Of these genes, around.40 % were immune-related, including various chemokines, key antiviral response molecules, and genes.associated with a Th1 pro-inflammatory immune response. Further, the qPCR analysis confirmed.increased immune gene transcription in HR at both 9 and 12 wpi, despite a decrease in PMCV.transcription between these time points. Interestingly, increased IFNb transcription in HR suggests the.presence of high-quantity IFN secreting cells in the hearts of these individuals. Indeed, RNAscope.confirmed the presence of IFN-γ and IFNb-positive cells in the heart ventricle of HR but not LR.To conclude, our data indicate that in severe outcomes of PMCV infection various chemokines attract leucocytes to the salmon heart, including IFN-γ and IFNb-secreting cells, and that these cells play important roles in maintaining persistent antiviral responses and a sustained host immunopathology despite decreasing heart viral transcription.

Smoltification, seawater performance, and maturation in Atlantic salmon (Salmo salar) fed different fat levels

IntroductionThe use of recirculating aquaculture systems (RAS) to produce Atlantic salmon smolts has resulted in exceptionally high fish growth rates. However, there are potential negative trade-offs between fast growth and key physiological processes, such as inadequate smoltification and early sexual maturation, which can both be linked to body energy reserves.MethodsThis study determined the effect of i) dietary fat levels on Atlantic salmon whole-body fat and fatty acids composition, growth performance and smoltification and ii) a previous dietary regime on seawater growth performance and male early sexual maturation. In freshwater RAS, salmon parr (~19g) were fed 3 fat levels (20, 24, and 28%) over a 14-week period at 12°C. Subsequently, in seawater flow-through systems, smolt (~96g) were fed a control diet (26%) for 12 weeks at 2 temperatures (12 and 16°C).ResultsDietary fat levels resulted in differences in k-factor, fish whole-body fat and fatty acids composition; the low fat diet resulted in fish with a 23% lower whole-body fat when compared to the other two dietary groups at the end of the freshwater phase. These differences in whole-body fat faded at the end of the seawater phase. During the freshwater phase, all three feeds resulted in a comparable growth performance and smoltification indicator values, including k-factor, gill Na+, K+-ATPase, blood serum chloride and smolt index score. In contrast, water temperature resulted in fish body weight differences, where groups reared at 16°C were larger than those reared at 12°C. Nevertheless, both temperature regimes supported a similar thermal growth coefficient. Smolts grown at 16°C showed a higher level of the sex steroids androstenedione compared to those at 12°C. However, 11-ketotestosterone and testosterone levels did not differ despite a trend for higher levels at 16°C. Furthermore, testis histology at the final sampling indicated that some individuals showed initial signs of maturation (stage 3).DiscussionIn conclusion, varying dietary fat levels (20 - 28%) during the freshwater phase did not influence smoltification or male early sexual maturation during the subsequent grow-out phase. However, a temperature increase from 12 to 16°C resulted in larger fish and appeared to stimulate early male maturation in some fish individuals.

The use of predator tags to explain reversal movement patterns in Atlantic salmon smolts (Salmo salar L.).

Acoustic telemetry has seen a rapid increase in utility and sophistication in recent years and is now used extensively to assess the behavior and survival rates of many aquatic animals, including the Atlantic salmon. As part of the salmon's complex life cycle, salmon smolts are thought to make a unidirectional migration from fresh water to the sea, which is initiated by changes in their physiology. However, some tag movement patterns do not conform with this and can be difficult to explain, particularly if the tagged fish has been eaten by a predator. This study combines the use of predator tags with machine learning techniques to understand the fate of migrating salmon smolts and thereby improve estimates for migration success. Over 3 years between 2020 and 2022, 217 salmon smolts (including wild and hatchery-reared ranched fish) were acoustically tagged and released into an embayment on the west coast of Ireland. Some tagged smolts were observed to return from the estuary back into a saline lagoon through which they had already migrated. To distinguish between the movement of a salmon smolt and that of a predator, predator tags were deployed in migrating smolts in 2021 and 2022. The addition of a temperature sensor in 2022 enabled the determination of predator type causing the returning movement. A significant number of predator tags were triggered, and the patterns of movement associated with these triggered tags were then used with two types of machine learning algorithms (hierarchical cluster analysis and random forest) to identify and validate the behavior of smolts tagged without extra sensors. Both models produced the same outputs, grouping smolts tagged with predator tags with smolts tagged without the additional sensors but showing similar movements. A mammalian predator was identified as the cause of most reversal movement, and hatchery-reared ranched smolts were found to be more likely predated upon by this predator than wild smolts within the lake and the estuary. However, overall migration success estimates were similar for both wild and hatchery-reared ranched fish. This study highlights the value of predator tags as an essential tool in the overall validation of detection data.

Interannual variation in survival of wild Atlantic salmon smolts through a dynamic estuarine habitat

AbstractMigration timing is critical for diadromous fish, especially for survival. Migration through fresh water and estuarine transitional waterbodies may be an important early life survival bottleneck through potential exposure to anthropogenic pressures and predators. Monitoring smolt movement and survival through riverine and estuarine habitats is important for identifying causes of smolt mortality and the potential for directing focused, mitigating management actions. Acoustic telemetry was used to track 186 Atlantic salmon smolts during migration from the river Boyne to the Irish Sea on the east coast of Ireland during 2019–2021. Migration success from release to sea entry that ranged from 47% to 81% was linked to biotic (predation) and abiotic effects (water flow and water temperature). Predation was associated with interannual variation in smolt survival, with 5.4% of smolts consumed by marine mammals and 26.3% lost to unknown fates. Our findings contribute to the understanding of predation on smolts and also how water temperature, flow and tidal phase can influence this critical life stage of Atlantic Salmon, a species in decline across its range.

Lipid profile of parr and smolts of Atlantic salmon (Salmo salar L.) reared in aquaculture under various lighting regimes

The study of the biochemical mechanisms of the effect of additional lighting on the growth and early development of salmonids in aquaculture allows to identify patterns of slow or accelerated growth, as well as the onset of the smoltification. In this study, the results of changes in lipid status in parr and smolts of Atlantic salmon after the “winter window” (December-January) are presented. These results were obtained from three experimental groups exposed to different lighting and feeding regimes at the fingerlings stage. The changes in lipid composition of the studied young salmon indicate the onset of smoltification in all experimental groups. However, this effect is the most pronounced in the fish previously reared under continuous lighting during the summer-autumn season.

Lipid Profile of Parr and Smolts of Atlantic Salmon (Salmo salar L.) Reared in Aquaculture Under Various Lighting Regimes

Lipid Profile of Parr and Smolts of Atlantic Salmon (Salmo salar L.) Reared in Aquaculture Under Various Lighting Regimes

Physiological condition infers habitat choice in juvenile sockeye salmon.

The amount of time that juvenile salmon remain in an estuary varies among and within populations, with some individuals passing through their estuary in hours while others remain in the estuary for several months. Underlying differences in individual physiological condition, such as body size, stored energy and osmoregulatory function, could drive individual variation in the selection of estuary habitat. Here we investigated the role of variation in physiological condition on the selection of estuarine and ocean habitat by sockeye salmon (Oncorhynchus nerka) smolts intercepted at the initiation of their 650-km downstream migration from Chilko Lake, Fraser River, British Columbia (BC). Behavioural salinity preference experiments were conducted on unfed smolts held in fresh water at three time intervals during their downstream migration period, representing the stage of migration at lake-exit, and the expected timing for estuary-entry and ocean-entry (0, 1 and 3 weeks after lake-exit, respectively). In general, salinity preference behaviour varied across the three time periods consistent with expected transition from river to estuary to ocean. Further, individual physiological condition did influence habitat choice. Smolt condition factor (K) and energy density were positively correlated with salinity preference behaviour in the estuary and ocean outmigration stages, but not at lake-exit. Our results suggest that smolt physiological condition upon reaching the estuary could influence migratory behaviour and habitat selection. This provides evidence on the temporally dependent interplay of physiology, behaviour and migration in wild juvenile Pacific salmon, with juvenile rearing conditions influencing smolt energetic status, which in turn influences habitat choice during downstream migration. The implication for the conservation of migratory species is that the relative importance of stopover habitats may vary as a function of initial condition.

Use of whole-genome sequence data for fine mapping and genomic prediction of sea louse resistance in Atlantic salmon.

Sea louse (Lepeophtheirus salmonis) infestation of Atlantic salmon (Salmo salar) is a significant challenge in aquaculture. Over the years, this parasite has developed immunity to medicinal control compounds, and non-medicinal control methods have been proven to be stressful, hence the need to study the genomic architecture of salmon resistance to sea lice. Thus, this research used whole-genome sequence (WGS) data to study the genetic basis of the trait since most research using fewer SNPs did not identify significant quantitative trait loci. Mowi Genetics AS provided the genotype (50k SNPs) and phenotype data for this research after conducting a sea lice challenge test on 3,185 salmon smolts belonging to 191 full-sib families. The 50 k SNP genotype was imputed to WGS using the information from 197 closely related individuals with sequence data. The WGS and 50k SNPs of the challenged population were then used to estimate genetic parameters, perform a genome-wide association study (GWAS), predict genomic breeding values, and estimate its accuracy for host resistance to sea lice. The heritability of host resistance to sea lice was estimated to be 0.21 and 0.22, while the accuracy of genomic prediction was estimated to be 0.65 and 0.64 for array and WGS data, respectively. In addition, the association test using both array and WGS data did not identify any marker associated with sea lice resistance at the genome-wide level. We conclude that sea lice resistance is a polygenic trait that is moderately heritable. The genomic predictions using medium-density SNP genotyping array were equally good or better than those based on WGS data.

Determining early marine survival and predation by endothermic predators on acoustically tagged Atlantic salmon (Salmo salar) post-smolts

Many Atlantic salmon ( Salmo salar) populations have experienced significant declines for decades throughout North America and Europe. Mortality due to marine mammal predation during their early marine life could be an important factor contributing to these declines and limiting their population recoveries. However, quantifying predation events, and particularly the extent of marine mammal predation on Atlantic salmon, remains a challenge. In this study, we estimated the contribution of mesothermic and endothermic species predation to the mortality of Atlantic salmon post-smolts during their early marine life using acoustic telemetry. Predation events were inferred from changes in temperatures and depths experienced by acoustically tagged hatchery-reared Atlantic salmon smolts. No salmon were consumed by mesothermic predators, with most endothermic predation events being classified as marine mammals. Post-smolt mortality during the study period was low overall in both years (13.1%–16.7%), with endothermic predation accounting for 33.1%–42.9% of all marine mortality events (5.2%–5.6% mortality). Our results suggest that the current low return of adult Atlantic salmon observed in this area in recent years was not heavily influenced by endothermic predation on post-smolts in the first weeks at sea.

Evaluation of the protective efficiency of an autogenous Vibrio anguillarum vaccine in lumpfish (Cyclopterus lumpus) under controlled and field conditions in Atlantic Canada

Lumpfish (Cyclopterus lumpus) have become the predominant cleaner fish species used in North American salmon aquaculture. Vibrio anguillarum is a frequent pathogen of lumpfish in Atlantic Canada, and current vaccines against local isolates conferred low to moderate efficacy. This study evaluated the safety and efficiency of a V. anguillarum autogenous vaccine under controlled and field conditions. Two safety trials were conducted following the Canadian Food Inspection Agency (CFIA) regulations before field trial testing. The first safety trial was a common garden assay, and 250 PIT-tsgged lumpfish were used for five treatments per tank (PBS-negative control, Lab vaccine-positive control, autogenous intraperitoneal (IP) vaccine, autogenous dip vaccine, autogenous dip-IP boosted vaccine) in triplicates. Weight, gross pathology, and IgM titers were evaluated to determine the vaccine’s safety. After 10 weeks post-vaccination (wpv), lumpfish were bath-challenged with a lethal dose (1.24 x 106 CFU/mL) of V. anguillarum serotype O2. Survival rate, IgM titers, memory immune response, and cross-immune protection were evaluated. The second safety trial was conducted using a double vaccine dose in two groups, PBS and autogenous IP. The gross pathology score indicated a normal immune response without tissue damage, and no mortality nor fish health issues were observed in the immunized animals. The IP route of administration conferred the highest protection against the V. anguillarum challenge. The autogenous vaccine conferred long-term immunity but did not confer cross-protection against V. anguillarum serotype O1. To evaluate the efficiency of the autogenous vaccine under field conditions, 58,976 naïve lumpfish (8-10 g) were IP immunized with either the autogenous vaccine or the commercial Forte micro IV vaccine. After 7 wpv, the lumpfish were equally distributed into eight sea cages and co-cultivated with Atlantic salmon smolts (60,000 per cage). Lumpfish IgM titers, tissue bacterial loads, mortality, and sea lice count on salmon were monitored. The autogenous vaccine and Forte vaccine confer acceptable protection to the lumpfish in sea cages. No significant difference was observed between lumpfish vaccinated with autogenous and commercial vaccines under field conditions.

Estimating multiple years, tributary-specific, and overall Atlantic salmon smolt abundance in a large Canadian catchment using capture-mark-recapture experiments.

Population monitoring of Atlantic salmon (Salmo salar L.) abundance is an essential element to understand annual stock variability and inform fisheries management processes. Smolts are the life stage marking the transition from the freshwater to the marine phase of anadromous Atlantic salmon. Estimating smolt abundance allows for subsequent inferences on freshwater and marine survival rates. Annual abundances of out-migrating Atlantic salmon smolts were estimated using Bayesian models and an 18-year capture-mark-recapture time series from two to five trapping locations within the Restigouche River (Canada) catchment. Some of the trapping locations were at the outlet of large upstream tributaries, and these sampled a portion of the total out-migrating population of smolts for the watershed, whereas others were located just above the head of tide of the Restigouche River and sampled the entire run of salmon smolts. Due to logistic and environmental conditions, not all trapping locations were operational each year. Additionally, recapture rates were relatively low (<5%), and the absolute number of recaptures was relatively few (most often a few dozen), leading to incoherent and highly uncertain estimates of tributary-specific and whole catchment abundance estimates when the data were modeled independently among trapping locations and years. Several models of increasing complexity were tested using simulated data, and the best-performing model in terms of bias and precision incorporated a hierarchical structure among years on the catchability parameters and included an explicit spatial structure to account for the annual variations in the number of sampled locations within the watershed. When the best model was applied to the Restigouche River catchment dataset, the annual smolt abundance estimates varied from 250,000 to 1 million smolts, and the subbasin estimates of abundance were consistent with the spatial structure of the monitoring programme. Ultimately, increasing the probabilities of capture and the absolute number of recaptures at the different traps will be required to improve the precision and reduce the bias of the estimates of smolt abundance for the entire basin and within subbasins of the watershed. The model and approach provide a significant improvement in the models used to date based on independent estimates of abundance by trapping location and year. Total abundance and relative production in discrete spawning, nesting, or rearing areas provide critical information to appropriately understand and manage the threats to species that can occur at subpopulation spatial scales.

Impact and recovery of water quality in a river with salmon aquaculture

AbstractFreshwater aquaculture can have negative impacts on chemical, physical and biological water quality. To contextualise the impact that these activities may have, a case study site in Ireland was monitored for 1 year. The study aims were to investigate the impact of freshwater aquaculture on water quality (biological, physical, and chemical) and the degree of the impact and recovery in water quality, within 1000 m downstream of a salmon smolt hatchery. The hatchery was monitored, at four locations (upstream, 10, 100, and 1,000 m downstream of the discharge point). Data on macroinvertebrate, macrophyte, hydromorphology and chemical characteristics were collected. Results for the physical monitoring indicated no significant changes in hydromorphology between up and downstream sites. Chemical water quality did decrease downstream in terms of nitrogen (N) and phosphorus (P). The greatest concentrations in N and P were found at 1000 m though it is likely additional contributions from agriculture impacted this site. Dissolved oxygen was steady throughout the monitoring campaign with a 13% variance between upstream and downstream locations. Changes in biological indices were statistically significant for taxa at all downstream sites. There were positive trends which indicated a degree of recovery in biotic indices.

Upcycling of carbon, nitrogen, and phosphorus from aquaculture sludge using the polychaete Hediste diversicolor (OF Müller, 1776) (Annelida: Nereididae)

IntroductionFocusing on circular bioeconomy and finding solutions for managing finite and scarce resources such as phosphorus are issues that need to be addressed across sectors to attain food security. Land-based aquaculture of Atlantic salmon (Salmo salar L.) smolt and post-smolt results in a large output of aquaculture sludge. These nutrient-rich side streams from aquaculture production constitute a valuable resource for cultivation of polychaetes Hediste diversicolor. The aim of this study was to evaluate the capacity of H. diversicolor to upcycle carbon (C), nitrogen (N), and phosphorus (P) contained in aquaculture sludge from Norwegian salmon production.Material and methodsWe investigated the effect of two diets, smolt sludge and post-smolt sludge, which were supplied at four different feed levels, on the gain of C, N, and P in H. diversicolor and established uptake rates for the upcycling of these nutrients. Additionally, we examined how different feed composition (type of diet) and supply (feel level) influenced C, N, and P content as well as the elemental ratios of H. diversicolor.Results and discussionPolychaetes fed with both types of sludge were shown to incorporate C, N, and P from their diets into biomass. Incorporation of C, N, and P in polychaete biomass was highest at the highest feed levels, following specific growth rates. Uptake of diet C, N, and P into polychaete biomass was similar for both diets and across feed levels. On average, C and N uptake rates amounted to 2–10% and 2–15%, respectively, while values for P uptake were lower at 0.5–1.5%. C, N, and P content of polychaetes was not affected by different diets or feed levels while the C:N ratio increased with increasing feed supply. Overall, we found that H. diversicolor can successfully take up nutrients contained in aquaculture sludge and thus can be beneficial for upcycling of side streams from Atlantic salmon production.

Partitioning survival during early marine migration of wild and hatchery-reared Atlantic salmon (Salmo salar L.) smolts using acoustic telemetry

The marine migration of Atlantic salmon (Salmo salar) has been prioritised for research internationally as populations of S. salar have declined significantly throughout the species’ range. The main objectives of this study were to use acoustic telemetry to partition survival during the early migration phase, investigate potential causes of mortality and establish diurnal and tidal influences on movements. In 2017 and 2018 wild (n = 49) and hatchery (n = 81) S. salar smolts were tagged with acoustic transmitters. Migration was monitored through a brackish tidal lake, which discharges through a short estuary into northeast Clew Bay in Ireland situated in the northeast Atlantic. Partitioned survival through each area was similar for both wild and hatchery smolts and both groups followed the same migration routes, travelling along the main current out of Clew Bay. Total survival was high within the brackish lake (wild = 98% ± 1.21 and hatchery = 100%) and estuary (wild = 98% ± 1.01 and hatchery = 89% ± 9.28) compared to the marine environment (wild = 67% ± 4.39 and hatchery = 66% ± 7.91). Leaving the brackish lake during an ebb tide and entering the marine environment during daylight increased the probability of survival through the early marine period. The majority of smolts transited through the study area during ebb tides. Migration of hatchery smolts occurred mainly during hours of daylight while wild smolt migration showed no diel patterns. High mortality rates during the initial stages of the marine migration have consequences for the persistence of salmon populations and should be addressed through regionally tailored management measures and conservation efforts aimed at increasing the resilience of salmon stocks.

Dams facilitate predation during Atlantic salmon (Salmo salar) smolt migration

Diadromous fish populations have incurred precipitous declines across the globe. Among many stressors, these species are threatened by anthropogenic barriers that impede movement, alter riverine habitat, and augment predator communities. In this study, we used acoustic transmitters ( n = 220) with predation and temperature sensors to characterize Atlantic salmon ( Salmo salar) smolt predation risk in the Penobscot River, Maine, USA. Across two seasons, we documented 79 predation events through a 170 km migratory pathway, which included three hydropower projects and a large estuary. We detected tagged smolts that were predated by fish ( n = 42), marine mammals ( n = 28), and birds ( n = 9). Using a multistate mark-recapture framework, we estimated that 46% of smolts were predated during downstream migration, which accounted for at least 55% of all mortality. Relative predation risk was greatest through impoundments and the lower estuary, where on average, predation rates were 4.8-fold and 9.0-fold greater than free-flowing reaches, respectively. These results suggest that predation pressure on Atlantic salmon smolts is exacerbated by hydropower projects and that predation in the lower estuary may be greater than expected.

Light pollution from illuminated bridges as a potential barrier for migrating fish–Linking measurements with a proposal for a conceptual model

Illuminated bridges have become important assets to navigable aquatic systems. However, if artificial light at night (ALAN) from illuminated bridges reaches aquatic habitats, such as rivers, it can threaten the river's natural heterogeneity and alter the behavioural responses of migratory fish. Here, via a pilot study, we quantified levels of ALAN at illuminated bridges that cross a river and, propose a conceptual model to estimate its potential implications on two migrating fish species with contrasting life histories. Night-time light measurements on the river Spree in Berlin were performed continuously along a transect and in detail at seven illuminated bridges. Photometric data of the pilot study showed rapidly increased and decreased light levels at several illuminated bridges from which we derived several model illumination scenarios. These illumination scenarios and their potential effect on migrating Atlantic salmon smolts (Salmo salar) and European silver eel (Anguilla anguilla) are presented as a conceptual model, considering illuminated bridges as behavioural barriers to fish migration. ALAN's adverse effects on freshwater habitats must be better researched, understood, managed, and properly communicated to develop future sustainable lighting practices and policies that preserve riverscapes and their biodiversity.