On the tail of Atlantic salmon around Europe

• SMOLTRACK is an EU-wide project coordinated by NASCO. By conducting field research and acquiring new data, SMOLTRACK acts as a platform for salmonid telemetry knowledge.
• SMOLTRACK allows international lead researchers to come together and share knowledge and best practices, with the goal of establishing an EU strategic salmon telemetry advisory group.
• SMOLTRACK will help scientists to learn about survival rates of salmon smolts during their migration through the lower parts of rivers, estuaries and coastal areas, providing data on smolt run timing and migration behavior.
• Ultimately, SMOLTRACK will inform future salmon management and conservation work on an EU level.

SMOLTrack V

SMOLTrack V started in June 2023 and will end in April 2025.

Research conducted in Denmark, England, Ireland, Spain, Sweden and Northern Ireland through the EU-funded SMOLTrack and related initiatives, have indicated that smolt mortality during the early outward migratory phase from ‘source to sea’ is much higher than previously assumed. Thus, early smolt mortality may be more important than marine mortality. To support this hypothesis, various studies have demonstrated that a reduction in negative pressures (e.g. predation, aquaculture) on smolts in the relevant zone of influence can ultimately boost associated numbers of adult returns.

Therefore, it has become apparent that significant knowledge gaps remain to understand, quantify and partition the principal cumulative factors responsible for Atlantic salmon smolt survival during this critical life stage. In addition to the high mortality for smolts and post- smolts, there is significant mortality in the oceanic phase of Atlantic salmon. It is important to investigate if any part of this mortality may be managed.

The SMOLTrack V project will build on the ongoing and published work of the previous SMOLTrack projects, enabling studies of behaviour and survival rates of salmon during their migration through the lower parts of rivers, estuaries and coastal areas. Thus, the project will provide data on smolt run timing and migration behaviour, as well as generate ocean migration data through nano-DST tagging of smolt.

SMOLTrack V will further expand on the development of a ‘fit for purpose tagging programme’ to be able to follow the return migration of salmon from feeding areas in the Arctic Sea (East Greenland). Methodological refinements will be made to optimize representative results and improve experimental animal welfare. A part of the SMOLTrack V project will be allocated to analysing large datasets collected and publishing some of the overall results and conclusions.

SMOLTrack IV

Quantifying salmon survival from river exit to return as adult: Collecting thermal and behavioural data to refine smolt to adult survival indices

SMOLTrack IV started in January 2021 and will end in December 2023.

There is significant mortality in the oceanic phase of Atlantic salmon. The studies into causes and consequences have primarily been done by modelling because detailed information about salmon behaviour and migration routes in the Ocean is scarce. Classic expeditions by ship are prohibitively expensive and restricted both in time and space and are also to some extent catch-per-unit effort limited. Telemetry has proven to be the only viable method to get detailed knowledge of this behaviour.

In recent years a few studies using satellite tags have been published, giving information on salmon migration at sea. Unfortunately, the current PSAT tags are too large to deploy on smaller fish (i.e. smolts), hence the preferred animals to tag are large kelts and the PSATs are typically limited to a 12 month deployment. Hence, there is still a lack of behavioural data, especially for the smaller life stages, and there is a limitation in data describing the return migration from the feeding grounds in the Northern Atlantic.

Acquiring detailed information from salmon behaviour at sea is a challenge and will require a multifaceted approach. The project aims to investigate the post-smolt behaviour during migration from coastal areas to the feeding grounds in the north Atlantic. Additionally, the project aims to identify and evaluate the potential for ‘fit for purpose tagging’ in the North Atlantic, using insight gained from pilot studies undertaken in previous SMOLTRACK projects. Identifying ways to obtain detailed knowledge on coastal and ocean behaviour and migration will be a pre-requisite for a greater understanding of the factors governing behaviour and ocean survival for Atlantic salmon and can feed directly into fisheries management strategies that may optimise and naturally enhance the probability of wild adult returns.

SMOLTrack III

Quantifying smolt survival from source to sea: informing management strategies to optimise returns

SMOLTrack III started on January 2020 and ended in December 2022.

Despite conservation efforts, wild Atlantic salmon stocks have experienced declines throughout the global range of distribution. Maximising the number of wild smolts in good condition that leave the river to the ocean can help minimise the impacts of changing ecosystems and low marine survival. Based on the platform developed in SMOLTrack I and II, this project (SMOLTrack III) investigated several key factors potentially contributing to smolt mortality during their transition from freshwater to the marine environment.

The project achieved three objectives:

1. evaluation of wild smolt survival during migration
2. quantifying the influence of climate change on salmon production
3. evaluate telemetry-based assessments to provide accurate information on smolt migration and survival.

The findings from SMOLTrack III have advanced our understanding of how bottlenecks can influence the size of smolt runs and highlight the benefits of moving smolts past identified bottlenecks. These results are highly relevant for policy makers and managers seeking to reduce smolt mortalities during their riverine migration. The recent increase in awareness about negative effects of barriers and other bottlenecks is in part an outcome of the studies performed in SMOLTrack I – III. Identifying and removing/mitigating bottlenecks may represent one of the fastest and potentially easiest ways of increasing adult return of salmon populations. SMOLTrack III also provided the foundation for a temperature logger infrastructure with an extensive latitudinal distribution, which will be used to track the ongoing effects of climate change on wild salmon stocks. Finally, the findings from objective 3 indicate that trapping smolts during their seaward migration does not influence their survival, further validating the standard operating procedure (SOP) and the method of using trapped smolts for investigating behaviour and survival. These findings are highly relevant to improve methods used by the academic community and fisheries managers monitoring smolt runs, using fish traps.

SMOLTrack II

Comparing mortality of European salmon populations at sea using multiple method telemetry studies

SMOLTrack II started in January 2018 and ended in December 2019.

The project aimed to expand the platform and collaboration of SMOLTrack I by including more partners (Sweden is now included, taking the total number of study sites to eight). The geographical span of the project now ranges over all of the salmon distribution area in the EU from north to south and east to west.

The project aims to identify specific predators causing the documented loss of smolts from the SMOLTrack I project and make comparisons between survival of wild and hatchery-reared salmon smolts. Blood sampling is being used to evaluate smolt quality and sex as they exit rivers to test if gender and physiological background affect the chance of survival. The project carried out a pilot study to test the feasibility to tag genetically-assigned large salmon at the Faroe Islands or Greenland and track the return migration.

The first project tagging (smolts) in 2018 has been completed and overall results are shown below. The samples collected in 2018 for sexing the salmon are under analysis in Northern Ireland (at AFBI). After these analyses the samples will be reallocated to Cefas (England) and / or DTU Aqua (Denmark) for physiological analyses.

The radio tagging in 2019 has been undertaken, but data are not yet finally compiled (automatic listening station and manual tracking has to be completed first). A workshop was held in Pontevedra, Spain in March 2018 (in prolongation of the SMOLTrack I workshop), where studies and analysis were discussed and agreed and the Standard Operating Procedure for SMOLTrack I was updated and adapted for the present project, including procedures for blood sampling and genetic analyses.

The experimental fieldwork took place and the results are much in line with 2017. In general, the wild smolts have a slightly better survival than hatchery smolts (except for the River Erriff), but the difference is less pronounced than may have been expected. The radio-telemetry work provided information about the exact cause of smolt loss and both avian and mammalian predators were found to prey on smolts.

The pilot study on salmon at sea has run into challenges of accessing suitable salmon for tagging. Work is being done to prepare for the potential tagging of adult salmon off East Greenland and, if this proves unfeasible, to discuss potential other solution with North American colleagues before their tagging operation in late Autumn 2019.

SMOLTrack I

Understanding and comparing early mortality of European salmon populations at sea.

SMOLTrack I was initiated in January 2017 and ended in December 2018.

Wild Atlantic salmon were tagged with acoustic transmitters and their subsequent migration followed via acoustic listening stations.

Wild salmon smolts were caught and carefully tagged with acoustic transmitters and then released. The tagged smolts were detected as they moved past Automatic Listening Station (ALS) arrays on the way from their natal river to the open sea. This project setup was carried out in five areas spanning almost the entire North – South distribution area in the EU (Denmark, England, Ireland, Northern Ireland and Spain).

Table 1. Fate of the released smolts for each study area. The percentage of smolts successfully crossing the study area is shown in brackets in the last column.

The SMOLTRACK I project revealed variable, but generally low survival rates in the lower freshwater and transition environments through study areas across the European area of distribution of the Atlantic salmon. The methodology applied worked well and it was possible to acquire highly valuable data sets about loss of salmon smolts in lower rivers / estuaries, comparable over several countries. The results directed attention towards the conditions under which the smolts must move from river to sea as well as the presence of multiple predators. The results provide a solid, comparable one-year estimate on the survival of salmon smolts through the lower river and estuary / fjord from 6 river systems in 5 countries. The results show that potential bottlenecks for Atlantic salmon exists already in the initial migratory phase, which may have a large impact on the overall return rate of adult salmon.

In addition to the scientific aims, the project brought together a group of experts to provide advice on best practice and to produce a Standard Operating Procedure for this type of study.