Baltic Salmon Research Articles

Thiamine Deficiency M74 Developed in Salmon (Salmo salar) Stocks in Two Baltic Sea Areas after the Hatching of Large Year-Classes of Two Clupeid Species—Detected by Fatty Acid Signature Analysis

Lipid-related thiamine (vitamin B1) deficiency of Baltic salmon (Salmo salar), the M74 syndrome, is generally caused by feeding on abundant young sprat (Sprattus sprattus) in the Baltic Proper, the main foraging area of these salmon. In 2014, a strong year-class of sprat was hatched in the Baltic Proper, and a strong herring (Clupea harengus) year-class was hatched in the Gulf of Bothnia, where herring is the dominant salmon prey. The fatty acid (FA) signatures of prey fish in muscle or eggs of second sea-year spawners suggested that 27% of wild River Simojoki and 68% of reared River Dal salmon remained in the Gulf of Bothnia in 2014 instead of continuing to the Baltic Proper. In 2016, 23% of the M74 females of the River Simojoki and 58% of the River Dal originated from the Gulf of Bothnia, and 13% and 16%, respectively, originated from the Baltic Proper. Some salmon from the River Neris in the southern Baltic Proper had also been feeding in the Gulf of Bothnia. In general, low free thiamine (THIAM) concentration in eggs was associated with high lipid content and high docosahexaenoic acid (DHA, 22:6n−3) and n−3 polyunsaturated FA (n−3 PUFA) concentrations in muscle but not in eggs. A higher THIAM concentration and lower proportions of DHA and n−3 PUFAs in Arctic Ocean salmon eggs, despite higher egg lipid content, indicated that their diet contained fewer fatty fish than the Baltic salmon diet. Hence, M74 originated by foraging heavily on young fatty sprat in the Baltic Proper or herring in the Gulf of Bothnia.

Thiamin dynamics during the adult life cycle of Atlantic salmon (Salmo salar).

Thiamin is an essential water-soluble B vitamin known for its wide range of metabolic functions and antioxidant properties. Over the past decades, reproductive failures induced by thiamin deficiency have been observed in several salmonid species worldwide, but it is unclear why this micronutrient deficiency arises. Few studies have compared thiamin concentrations in systems of salmonid populations with or without documented thiamin deficiency. Moreover, it is not well known whether and how thiamin concentration changes during the marine feeding phase and the spawning migration. Therefore, samples of Atlantic salmon (Salmo salar) were collected when actively feeding in the open Baltic Sea, after the sea migration to natal rivers, after river migration, and during the spawning period. To compare populations of Baltic salmon with systems without documented thiamin deficiency, a population of landlocked salmon located in Lake Vänern (Sweden) was sampled as well as salmon from Norwegian rivers draining into the North Atlantic Ocean. Results showed the highest mean thiamin concentrations in Lake Vänern salmon, followed by North Atlantic, and the lowest in Baltic populations. Therefore, salmon in the Baltic Sea seem to be consistently more constrained by thiamin than those in other systems. Condition factor and body length had little to no effect on thiamin concentrations in all systems, suggesting that there is no relation between the body condition of salmon and thiamin deficiency. In our large spatiotemporal comparison of salmon populations, thiamin concentrations declined toward spawning in all studied systems, suggesting that the reduction in thiamin concentration arises as a natural consequence of starvation rather than to be related to thiamin deficiency in the system. These results suggest that factors affecting accumulation during the marine feeding phase are key for understanding the thiamin deficiency in salmonids.

Boldness, activity, and aggression: Insights from a large-scale study in Baltic salmon (Salmo salar L).

Atlantic salmon (Salmo salar) display high levels of agonistic behavior in aquaculture farms, resulting in fin damage and chronic stress. Aggression affects fish growth and performance negatively, and presents a serious welfare problem. Indeed, it would be beneficial to identify, separate or exclude overly aggressive individuals. Research on behavioral syndromes suggests that aggressive behavior may correlate with other behavioral traits, such as boldness and locomotory activity. We aimed to develop a high-throughput method to quantify and predict aggressive behavior of individual parr in hatchery-reared Baltic salmon (Salmo salar L.). We screened approximately 2000 parr in open field (OF) and mirror image stimulation (MIS) tests. We extracted seven variables from video tracking software for each minute of the tests; distance moved and duration moving (activity), the duration in and number of entries to the center of the arena (boldness), the distance moved in and duration spent in the area adjacent to the mirror during the MIS test (aggressiveness) and head direction (lateralization). To investigate the relationship between activity, boldness and aggression we first correlated the first six variables to one another. Second, we assigned individuals to high, medium, low or zero aggression groups based on the MIS test and quantified activity and boldness in each group. Third, we analyzed whether the fish viewed the mirror with the left or right eye. Our results show that medium and low aggressive fish were the most active, while highly aggressive fish showed average activity. Aggressive groups did not differ in boldness. Activity and boldness were positively correlated. Finally, we detected a preference for fish to view the mirror with the left eye. We conclude that aggressiveness cannot be predicted from the results of the OF test alone but that the MIS test can be used for large-scale individual aggression profiling of juvenile salmon.

Effect of the trap net emptying method on release mortality of Atlantic salmon estimated by a Bayesian system model

AbstractWe evaluated the effect of emptying techniques of a pontoon trap on mortality of Atlantic salmon on their spawning migration along the Finnish coast of the northern Baltic Sea by tagging and releasing fish from commercial trap nets. Two methods currently used, a traditional emptying chute and a lifting bag, were compared. The lifting bag has been developed primarily for minimising injuries for fish during emptying. A custom‐built Bayesian system model was used for mortality estimation. We also explored external injuries of salmon and compared the probability of getting an injury between the emptying methods. The mean probability of salmon dying due to release from a trap with an emptying chute was 24% (90% HDI 3%–43%), whereas that for salmon released from a trap with a lifting bag was 13% (90% HDI 1%–24%). Results showed a higher probability of injuries and longer air exposure time in salmon caught with a trap with an emptying chute, which may explain the observed difference in mortality. Manual handling of a lifting bag appears to pose a clear work safety risk for fishers. It is noteworthy that the role of release mortality for northern Baltic salmon stocks is small because the number of salmon released in coastal fishery is currently low.

Feasibility and effectiveness of seal deterrent in coastal trap-net fishing – development of a novel mobile deterrent

With the rapid growth of grey seal (Halichoerus grypus) population in the Baltic Sea, seal-induced catch losses have increased dramatically in coastal fisheries. There have been various attempts to mitigate these damages, such as modification of fishing gear, but solutions have proven inadequate. Promising research results have recently been obtained by using acoustic deterrent devices (ADDs) to keep seals away from the immediate vicinity of the gear. We tested the feasibility and effectiveness of a raft-mounted and a novel mobile ADD in Baltic salmon (Salmo salar) trap-net fisheries along the Finnish coast in collaboration with 13 commercial fishers. Fishers operated trap-nets (pontoon traps) that were equipped with and without an ADD. Our results indicate that a fisher with a trap-net equipped with an ADD can expect to catch on average 3.4 salmon per day whereas in trap-net without an ADD a fisher can expect on average 2.1 salmon per day. The deployment of ADDs over the two years testing period indicated an average increase of 64% in salmon catches. Our study suggests that in salmon trap-net fisheries an ADD is a useful and economically viable (given that the investment is subsided) mitigation tool for reducing seal-induced catch losses. Furthermore, the mobile ADD developed in this study provides unique practical opportunities and notable potential for a wider use of an ADD.

Release mortality of wild Atlantic salmon in coastal pontoon-trap fishery in the northern Baltic Sea

The survival and injuries of wild Atlantic salmon released by coastal fishers from pontoon traps was examined in the early summer of 2020 in the Gulf of Bothnia in the northern Baltic Sea. A total of 491 salmon caught with pontoon traps was tagged with dart tags. By the end of December 2020, a total of 62 tag recoveries (13%) were received. Sixty-two per cent of the tag recoveries were obtained from the sea area, mainly from the commercial fishers in the Bay of Bothnia near northern river mouths. The remaining tags were received from river fishers. It took an average of 22 days from tagging to the capture (range 1–91 days), and the longest distance a tagged salmon travelled was more than 700 km. The mortality of salmon caused by capture and release was estimated to be a maximum of 25–31% for multi-sea-winter (MSW) salmon and a maximum of 24% for one-sea-winter (1SW) salmon. Most of the tagged fish were visually in good condition, although some injuries were observed. When two trap emptying techniques were compared, salmon caught in the Bothnian Sea with an emptying chute showed more injuries than salmon caught in the Bay of Bothnia with a lifting bag. Our results suggest relatively low release mortality for Baltic salmon. Furthermore, we show that disentangling release mortality from combined tagging and handling mortality is crucial in tagging studies.

Determining Baltic salmon foraging areas at sea using stable isotopes in scales—a tool for understanding health syndromes

AbstractManaging fish stocks for species migrating between freshwater and the sea is challenging when ecological information for life stages at sea is poorly known. Oceans offer increased opportunity for growth but also morbidity and mortality. By improving our understanding of foraging at sea we can better identify factors driving stock health and recruitment. We analysed stable isotopes of carbon and nitrogen (δ13C and δ15N) in scales from tagged Baltic salmon (Salmo salar) caught at sea over four decades. We found consistently divergent patterns in δ13C and δ15N between the two main basins of the Baltic Sea, which can be used to determine foraging area. Isotope analysis of amino acids revealed differences in source δ15N as the main reason for observed differences between basins. We also analysed isotopes in scales and thiamin concentrations in roe (thiamin deficiency can cause substantial fry mortality) from adult female salmon returning to a river to spawn in 2017 and 2018. Individuals with low thiamin levels were associated with offshore feeding in both basins, suggesting the deficiency syndrome is widespread in the Baltic Sea.

Trade-offs among spatio-temporal management actions for a mixed-stock fishery revealed by Bayesian decision analysis

Abstract Management and conservation of populations that are harvested simultaneously present a unique set of challenges. Failure to account for differences in productivity and spatio-temporal abundance patterns can lead to over-exploitation of depleted populations and/or loss of potential yield from healthy ones. Mixed-stock fisheries (where a stock may comprise one or more populations of reared or wild origin) harvest multiple stocks, often in unknown proportions, and lack of tools for estimation of stock-specific harvest rates can hamper status evaluations and attainment of management goals. We present a method for evaluating stock-specific impacts of alternative harvest strategies, using coastal trap net fisheries for Atlantic salmon (Salmo salar) in the Baltic Sea as a case study. Our results demonstrate a large variation among stocks in coastal mixed fishery harvest rates, as well as large differences in harvest rates relative to stock-specific maximum sustainable yield (MSY) and recovery levels. Bayesian decision analysis showed that spatio-temporal management actions, such as delayed fishery opening and closed areas may be effective in improving probabilities of meeting management objectives for Baltic salmon. However, stocks did not respond uniformly to different management actions, highlighting the potential for trade-offs in reaching stock-specific targets that must be considered by managers.

Gut Microbiome of Wild Baltic Salmon (Salmo salar L.) Parr.

Gut microbiota of wild Baltic salmon (a sub-population of Atlantic salmon Salmo salar L.) parr was first analyzed using microbial profiling of the 16S rRNA gene (V3-V4 region) and high taxonomic richness was revealed. At the phylum level, the gut microbiota was dominated by Firmicutes, Actinobacteria, and Proteobacteria, the most numerous of which were Firmicutes. The phylum Tenericutes (mainly assigned to Mycoplasmataceae), which is common both in wild North- and East- Atlantic salmon parr, was not detected in Baltic salmon parr. Across all samples, unique amplicon sequence variants (ASVs) belonging to the unclassified Bacilli, Actinomycetales, and Rhizobiales were identified as the major taxa. Fifteen ASVs at the family level were found in all gut samples of Baltic salmon parr, the majority of which were Mycobacteriaceae, Cryptosporangiaceae, Microbacteriaceae, and Planctomycetaceae. At the genus level, Mycobacterium, Clostridium sensu stricto, and Hyphomicrobium were dominant but at low levels in all gut samples. Our study has revealed that the gut microbial community of wild Baltic salmon parr differs from those of wild North- and East-Atlantic salmon parr. This can be due to biogeographical differences or host-selective pressures, as the Baltic salmon population is believed to have split from the Atlantic salmon population in the Ancylian period.

Model for estimating thiamine deficiency-related mortality of Atlantic salmon (Salmo salar) offspring and variation in the Baltic salmon M74 syndrome

ABSTRACT Thiamine (vitamin B1) deficiency of salmonines, caused by an abundant lipid-rich fish diet and consequently, the abundance of polyunsaturated fatty acids, is called the M74 syndrome in the Baltic Sea. Because of its deleterious effects on wild Atlantic salmon (Salmo salar) stocks and progeny production in fish cultivation, a model was developed to derive the annual female-specific mortality percentages of yolk-sac fry (YSFM) from the free thiamine concentrations of unfertilized eggs. In years with a high M74 incidence, thiamine-deficient females were larger, with a larger condition factor (CF) than non-M74 females. Otherwise, M74 females were generally smaller. The mean CF of M74 females was in most years higher than that of non-M74 females. The model compiled enables the cost-effective estimation of YSFM of individual female salmon, without the incubation of eggs and hatched yolk-sac fry for several months, thus benefitting the management of salmon stocks and their efficient utilization.

Salmonids as Natural Functional Food Rich in Omega-3 PUFA

Salmonids are valuable fish in the human diet due to their high content of bioactive omega-3 very long-chain polyunsaturated fatty acid (VLC PUFA). The aim of this study was to assess the omega-3 VLC PUFA content in selected salmonid fish present on the food market regarding whether they were farm-raised or wild. It was assumed that farm-raised fish, by eating well-balanced feed enriched with omega-3 PUFA, might contain omega-3 VLC PUFA in levels similar to that of wild fish. Fat content, fatty acid composition and omega-3 VLC PUFA content in fish fillets were measured. Farm-raised salmon from Norway, wild Baltic salmon, farm-raised rainbow trout and brown trout were bought from a food market whereas wild trout (rainbow and brown) were caught alive. The fat content in fish ranged from 3.3 to 8.0 g/100 g of fillet. It was confirmed that although wild salmonid fish contain 10–25% more omega-3 VLC PUFA in lipid fraction, the farm-raised ones, due to the 60–100% higher fat content, are an equally rich source of these desirable fatty acids in the human diet. One serving (130 g) of salmonid fish fillets might provide a significant dose of omega-3 VLC PUFA, from 1.2 to 2.5 g. Thus, due to very high content of bioactive fatty acids eicosapentaenoic (EPA), docosapentaenoic (DPA) and docosahexaenoic (DHA) in their meat, salmonid fish currently present on the food market, both sea and freshwater as well as wild and farm-raised, should be considered as natural functional food.

Growth variation of Atlantic salmon Salmo salar at sea affects their population-specific reproductive potential

Understanding the processes shaping the dynamics of anadromous fish populations is essential for their management and conservation. Yet, little is known about how variation in performance at sea affects their population dynamics. Here we show that variation in body growth at sea contributes to explaining variation in the reproductive potential for 2 Atlantic salmon Salmo salar populations, but to a varying extent. To this end, we assembled data collected during 50 yr for 2 Baltic salmon populations of hatchery origin, including annually released smolts, survival at sea estimates, size-specific growth at sea, annual length distributions of returning adult females and their reproductive potential. The regression models fitted to explain the reproductive potential of our 2 study populations improved when growth at sea was included as an explanatory variable, in addition to smolt year class abundance and estimates of their survival at sea. This link between body growth at sea and population-level reproductive potential suggests that growth at sea can be important to consider when resolving variation in recovery and dynamics among salmon populations sharing the same sea.

A large wild salmon stock shows genetic and life history differentiation within, but not between, rivers

Anadromous salmonid fishes frequently exhibit strong geographic population structuring. However, population genetic differentiation of Atlantic salmon (Salmo salar) at fine geographic scales differs across equivalent spatial extents in different regions. So far, fine-scale genetic differentiation has not been assessed in rivers of the Baltic Sea, a region that contains an evolutionarily distinct Atlantic salmon lineage. Thus, Baltic salmon are currently managed on the river level, without focus on potential genetic structure and diversity within rivers. Here, we used microsatellites to characterize the genetic structure of wild juvenile salmon sampled throughout the interconnected, northern Baltic Tornio and Kalix Rivers. We found genetic differentiation within the two rivers, but not between them: salmon in the upper reaches differed from individuals in the lower reaches, regardless of river system. Further, examining smolts migrating from the river to the sea and adults returning from the sea to spawn, we found an association between the genetic structure and seasonal migration timing. Out-migrating smolts genetically assigned to upper river reaches were older and tended to reach the sea later in the season than smolts from the lower reaches. In contrast, mature adults originating from the upper reaches returned to the river early in the season. Our observation of genetic population structuring between downstream and upstream reaches of the large Tornio and Kalix rivers, and its association with migration timing, implies that careful temporal management of the northern Baltic fisheries would help to preserve the diversity and sustainability of the wild salmon stocks of these rivers.

Effects on the Liver Transcriptome in Baltic Salmon: Contributions of Contamination with Organohalogen Compounds and Origin of Salmon.

Hatchery-reared Atlantic salmon (Salmo salar) has been released to support the wild salmon stocks in the Baltic Sea for decades. During their feeding migration, salmon are exposed to organohalogen compounds (OHCs). Here, we investigated the OHC levels and transcriptome profiles in the liver of wild and hatchery-reared salmon collected from the Baltic main basin (BMB), the Bothnian Sea (BS), and the Gulf of Finland (GoF) and examined whether salmon origin and OHC levels contributed to the hepatic transcriptome profiles. There were no differences in the OHC concentrations between wild and reared fish but larger differences between areas. Several transcript levels were associated with non-dioxin-like polychlorinated biphenyls, polybrominated diphenylethers, chlordanes, and dichlorodiphenyltrichloroethane in a concentration-dependent manner. Between wild and reared salmon, lipid metabolism and related signaling pathways were enriched within the BMB and BS, while amino acid metabolism was altered within the GoF. When comparing the different areas, lipid metabolism, environmental stress and cell growth, and death-related pathways were enriched. Class coinertia analysis showed that the covariation in the OHC levels and the transcriptome were significantly similar. These results suggest that the hepatic transcriptomes in wild and hatchery-reared salmon are more affected by the OHC levels rather than the origin of salmon.

Environmentally driven changes in Baltic salmon oxidative status during marine migration

The fitness and recruitment of fish stocks can be markedly affected by environmental disturbances including global warming, eutrophication and contamination. Understanding the effects of environmental stressors on salmon physiology during marine residence is of a global concern as marine survival has decreased. We present a unique combination of physiological responses - antioxidant defence and oxidative damage biomarkers, stable isotopes and contaminant exposure biomarkers - measured from adult Atlantic salmon (Salmo salar) collected at the Baltic Sea and studied in relation to environmental variables and fitness estimates. The results demonstrate that feeding populations of salmon display marked temporal and spatial variation in oxidative status. Better oxidative status of salmon was characterized by a higher amount of reduced glutathione (GSH) and decreased lipid peroxidation (LPX), when the weight-at-age of 3–4-year old sprats was higher and contaminant exposure biomarker (EROD) was lower. Summer season conditions, which included cooler sea surface temperature (SST), higher bottom O2 and less cyanobacteria also indicated conditions for better oxidative status. Summer SST was additionally shown to affected glutathione metabolism enzyme activities. Oxidative status was associated with stable isotopes δ13C and δ15N indicating indirect effect of abiotic conditions and lower levels of the food web. Differences in condition factor and growth were associated with oxidative status in one and two sea winter salmon, respectively. Wild salmon survival was higher in years when they had higher GSH and catalase activity and lower LPX. Enhanced glutathione metabolism and increased protein carbonyls were associated with higher occurrence of yolk-sac fry mortality (M74). Our results show that oxidative status can provide information on exposure to complex combinations of environmental conditions and stressors in the wild and provide a link of physiological function to individual and population level fitness effects.

The effects of climate change on Baltic salmon: Framing the problem in collaboration with expert stakeholders

In the Baltic Sea region, salmon are valued for the ecological, economic, and cultural benefits they provide. However, these fish are threatened due to historical overfishing, disease, and reduced access to spawning rivers. Climate change may pose another challenge for salmon management. Therefore, we conducted a problem-framing study to explore the effects climate change may have on salmon and the socio-ecological system they are embedded within. Addressing this emerging issue will require the cooperation of diverse stakeholders and the integration of their knowledge and values in a contentious management context. Therefore, we conducted this problem framing as a participatory process with stakeholders, whose mental models and questionnaire responses form the basis of this study. By framing the climate change problem in this way, we aim to provide a holistic understanding of the problem and incorporate stakeholder perspectives into the management process from an early stage to better address their concerns and establish common ground. We conclude that considering climate change is relevant for Baltic salmon management, although it may not be the most pressing threat facing these fish. Stakeholders disagree about whether climate change will harm or benefit salmon, when it will become a relevant issue in the Baltic context, and whether or not management efforts can mitigate any negative impacts climate change may have on salmon and their fishery. Nevertheless, by synthesizing the stakeholders' influence diagrams, we found 15 themes exemplifying: (1) how climate change may affect salmon, (2) goals for salmon management considering climate change, and (3) strategies for achieving those goals. Further, the stakeholders tended to focus on the riverine environment and the salmon life stages occurring therein, potentially indicating the perceived vulnerability of these life stages to climate change. Interestingly, however, the stakeholders tended to focus on traditional fishery management measures, like catch quotas, to meet their goals for these fish considering climate change. Further, social variables, like “politics,” “international cooperation,” and “employment” comprised a large proportion of the stakeholders' diagrams, demonstrating the importance of these factors for salmon management.

Population and size-specific distribution of Atlantic salmon Salmo salar in the Baltic Sea over five decades.

Population‐specific assessment and management of anadromous fish at sea requires detailed information about the distribution at sea over ontogeny for each population. However, despite a long history of mixed‐stock sea fisheries on Atlantic salmon, Salmo salar, migration studies showing that some salmon populations feed in different regions of the Baltic Sea and variation in dynamics occurs among populations feeding in the Baltic Sea, such information is often lacking. Also, current assessment of Baltic salmon assumes equal distribution at sea and therefore equal responses to changes in off‐shore sea fisheries. Here, we test for differences in distribution at sea among and within ten Atlantic salmon Salmo salar populations originating from ten river‐specific hatcheries along the Swedish Baltic Sea coast, using individual data from >125,000 tagged salmon, recaptured over five decades. We show strong population and size‐specific differences in distribution at sea, varying between year classes and between individuals within year classes. This suggests that Atlantic salmon in the Baltic Sea experience great variation in environmental conditions and exploitation rates over ontogeny depending on origin and that current assessment assumptions about equal exploitation rates in the offshore fisheries and a shared environment at sea are not valid. Thus, our results provide additional arguments and necessary information for implementing population‐specific management of salmon, also when targeting life stages at sea.

Making the most of mental models: Advancing the methodology for mental model elicitation and documentation with expert stakeholders

Eliciting stakeholders' mental models is an important participatory modeling (PM) tool for building systems knowledge, a frequent challenge in natural resource management. Therefore, mental models constitute a valuable source of information, making it imperative to document them in detail, while preserving the integrity of stakeholders’ beliefs. We propose a methodology, the Rich Elicitation Approach (REA), which combines direct and indirect elicitation techniques to meet these goals. We describe the approach in the context of the effects of climate change on Baltic salmon. The REA produced holistic depictions of mental models, with more variables and causal relationships per diagram than direct elicitation alone, thus providing a solid knowledge base on which to begin PM studies. The REA was well received by stakeholders and fulfilled the substantive, normative, instrumental, and educational functions of PM. However, motivating stakeholders to confirm the accuracy of their models during the verification stage of the REA was challenging.

How to improve governance of a complex social-ecological problem? Dioxins in Baltic salmon and herring

ABSTRACTThis article focuses on the dioxin problem of Baltic herring and salmon fisheries and its governance that is based on natural scientific knowledge. The dioxin problem weakens the perceived quality of Baltic salmon and herring as food and affects the way the catches can be used. This influences negatively the fishing livelihood, the coastal culture, and the availability of the fish for consumers. We explored how the governance of the dioxin problem could be improved, to better address its socio-economic and cultural implications. We identified four main actions: (1) adopt environmental, economic and social sustainability, and food security and safety as shared principles between the environmental, food safety/public health, and fisheries policies, (2) establish collaboration between the environmental, public health, and fisheries sectors at the regional level, (3) enhance interaction around the dioxin problem within the fisheries sector, and (4) support the participation of the Baltic fisheries stakeholders in the EU-level food safety governance. Viewing dioxins in fish not only as a natural scientific problem but as a multidimensional one would enable a wider toolbox of governing instruments to be developed to better address the different dimensions. This would support steps towards collaborative governance and a food system approach.

Endoparasitic helminths in Baltic salmon Salmo salar: ecological implications.

Parasites in fish are ecological indicators, as they reflect the host's migration routes, feeding behavior and immune status. We performed a parasitological investigation of sea-running Baltic salmon to study the use of parasites as indicators for this fish stock. The host-a strain of Atlantic salmon Salmo salar-has been isolated for several millennia in the semi-enclosed brackish Baltic Sea, with limited migration to and from the North Sea. Twenty-four salmon (total body weight: 4.2-14.2 kg; total body length: 80-105 cm) were caught by spoon bait in the southern Baltic Sea during feeding migrations, necropsied shortly afterwards and internal organs subjected to parasitological investigation focusing on endoparasitic helminths. The pyloric region was heavily parasitized by the cestode Eubothrium crassum (100% prevalence; intensity: 97-273 parasites per infected fish), reflecting a diet of smaller pelagic fishes. The stomach contained the hemiurid digeneans Brachyphallus crenatus (95.8% prevalence; intensity: 8-151) and Hemiurus luehei (58.3% prevalence; intensity: 2-13), indicating a diet of clupeids. Schistocephalus solidus (25% prevalence; intensity: 1-2), liberated from ingested sticklebacks, the acanthocephalan Echinorhynchus truttae (54% prevalence; intensity: 1-13) and the adult nematode Hysterothylacium aduncum (29% prevalence; intensity: 1-13) were found in the intestine. The liver was parasitized by third-stage nematode larvae of Contracaecum osculatum (45.8% prevalence; intensity: 1-4), but these were growth-stunted and encapsulated. The parasite fauna differs markedly from salmon in North Atlantic waters, and the lack of purely marine parasite species indicates that the Baltic salmon has remained in the Baltic Sea during its life history.