Spawning Run Research Articles

Handling effects on dispersal of PIT‐tagged Flannelmouth Sucker

AbstractObjectiveHandling and tagging migrating fish might alter their behavior, limiting inference from mark–recapture studies. Posthandling flight of tributary spawning Flannelmouth Sucker Catostomus latipinnis was previously identified in Coal Creek in the upper Colorado River basin. Our objective was to determine if similar issues were present at McElmo Creek in the San Juan River basin.MethodsWe compared emigration timing of Flannelmouth Sucker that had been handled and tagged with passive integrated transponder tags during their tributary spawning run to individuals tagged in previous years and detected both entering and exiting the tributary. Linear mixed‐effects models were used to examine intrinsic and extrinsic factors contributing to exit timing.ResultSex and tagging year were associated with emigration timing, but handling did not result in posthandling flight from McElmo Creek. Females exited the tributary ~3 days before males, and larger fish emigrated earlier than smaller adults.ConclusionDifferences in capture technique and timing, available spawning habitat, and fish motivation across river systems may contribute to differences in posthandling emigration of tributary spawning Flannelmouth Sucker.

Documentation of a probable spawning run of cisco Coregonus artedi in the Spanish River, Ontario, Canada

Coregonines were historically diverse and abundant in the Laurentian Great Lakes but declined throughout the 19th and 20th centuries due to multiple factors, including overfishing, habitat degradation, and interactions with invasive species. Great Lakes fishery managers are now working to restore coregonines across the basin. An important aspect of restoration efforts is the identification of key habitats, such as spawning habitats. Our understanding of the extent and diversity of coregonine spawning habitats in the Great Lakes is limited. Historical accounts suggest some cisco Coregonus artedi migrated to tributaries to spawn, but contemporary runs have not been reported. Moreover, cisco display considerable phenotypic variation, with different “forms” often occupying specific habitats or regions. We provide evidence for a previously undocumented spawning run of cisco in the Spanish River, a tributary to the North Channel of Lake Huron. We captured 31 mature and ripe fish (25 males, 6 females) approximately 52 km upstream of the river mouth on 15–16 November 2022. The fish ranged in age from 3-9 yr and total length from 287-394 mm. Of these 31 individuals, 27 were morphologically similar to C. artedi manitoulinus, a subspecies of cisco found in the North Channel; however, four individuals more closely resembled cisco captured in other areas of Lake Huron. These findings represent the best evidence of tributary spawning by cisco since the 1880s, and they have important implications for coregonine conservation and restoration efforts. Ongoing work aims to augment our understanding of the ecological and evolutionary importance of this population.

Ecological evolution of salmonids

For the first time, ecological evolution of salmonid fishes is analyzed using the method of historical biogeography, considering formation of ecology and reproductive biology for these species under planetary climatic and geological changes. Climate cooling in the Cenozoic and associated reconstruction of the food base for salmonids had a major impact on evolution of their ecology. This process began at the Asian coast of the North Pacific much earlier than at the American coast and was much more intense there. The salmonids developed marine and oceanic feeding that led to their stocks increasing and a subsequent imbalance with freshwater prey. Adapting to this imbalance, the species of gen. Oncorhynchus came to irreversible reduction of organs and functions in the fresh water and absolute mortality of producers. Egg development lengthened in cold environments, and anadromous and semi-anadromous salmonid species were forced to spawn earlier, so that the larvae would hatch during the spring bloom of their prey. Gradually, the timing of their spawning shifted from early spring to winter and then to fall; accordingly, the spawning run to the rivers began earlier, in particular in the northern areas. The spawning season of atlantic salmon Salmo salar and brown trout Salmo trutta shifted to a fall and spring-summer spawning run in the north of their range and to a winter-spring and fall spawning run in the south of their range, but there is currently no reason to consider these runs as seasonal races, given that both cases evolved from a single run in the process of evolutionary changes when the spawning season was interrupted by either winter cooling or summer warming in the lower river reaches and at the coast. The northern and southern populations of salmonid fishes in the Pacific separated, too, because of these ecological changes in the late Pliocene, in conditions of cold climate. Being in isolation, they diverged enough to be considered as separate taxa rather than seasonal races.

Spawning run estimates and phenology for an extremely small population of Atlantic Sturgeon in the Marshyhope Creek–Nanticoke River system, Chesapeake Bay

AbstractObjectiveOnce thought to be extirpated from the Chesapeake Bay, fall spawning runs of Atlantic Sturgeon Acipenser oxyrinchus have been rediscovered in the Marshyhope Creek (MC)–Nanticoke River (NR) system of Maryland, United States. High recapture rates in past telemetry surveys suggested a small population in the two connected tributaries. This study aims to generate estimates of abundance and understand within system connectivity for spawning runs in 2020 and 2021.MethodsData from mobile side‐scan sonar surveys and detections of acoustically tagged adults on stationary telemetry receivers were analyzed in an integrated model to estimate spawning season abundance and examine run timing and system connectivity for this population. An array of acoustic receivers was deployed throughout the MC–NR system to monitor the movement of tagged fish during the spawning run period from mid‐August to late October. Side‐scan sonar surveys were conducted weekly in September in an area of high spawner aggregation to generate count data on spawning run abundance.ResultIn 2020 and 2021, 32 (95% credible interval [CRI] = 23–47) and 70 (95% CRI = 49–105) Atlantic Sturgeon, respectively, used the MC–NR system. The lower estimate for 2020 coincided with an earlier end to the spawning run related to cooler September temperatures in that year.ConclusionIn both years, high spawning run connectivity between MC and the upper NR was observed. Overall, run estimates supported previous hypotheses that the MC–NR system supports a very small population and that both MC and the upper NR serve as important areas for spawning activity.

Results of monitoring on the spawning run of autumn chum salmon at the observation point in Tyr in 2023

Results of monitoring on the spawning run of autumn chum salmon at the observation point in Tyr in 2023

Comparative characteristics for producers of pink salmon <i>Oncorhynchus gorbuscha</i>in the basin of Lake Sopochnoe

Pink salmon spawners in the channel connecting Lake Sopochnoe (Iturup Island) with the sea are investigated. The samples were collected in July-August, 2016. The males dominated significantly during entire spawning run. The fish became more and more ready for spawning with approaching to the end of the run. The average length and weight of females were 50.0 ± 0.3 cm and 1.53 ± 0.03 kg, the average length and weight of males — 50.2 ± 0.2 cm and 1.56 ± 0.02 kg; the fish caught in the channel were significantly larger than the pink salmon spawners caught in the adjacent Prostor Bay at the same time (on average 48.5 ± 0.1 cm, 1.32 ± 0.01 kg and 49.0 ± 0.2 cm, 1.40 ± 0.02 kg for females and males, respectively).

Predictions about the Pink Salmon Oncorhynchus gorbuscha (Salmonidae) Catches on Iturup Island

The pink salmon Oncorhynchus gorbuscha spawners which otolith marks created at fish early age at the Kurils and Reidov fish-rearing stations have been studied; fish breeding data and fishing statistics have been analyzed. At the end of the spawning run of pink salmon, only fish of the fish-farm origin have been caught near the fish-rearing stations. More than that, the pink salmon spawners return to the stations of their release (although with rare exceptions). The catches of the pink salmon on Iturup Island are on average 1.6 times greater in even years than in odd years. The increase of the fish stock in odd years significantly depends on the breeding of juveniles in fish farms. There is a positive statistically significant relationship between the filling of spawning grounds with pink salmon spawners and its catches in the Prostor and Kurilsky bays two years later.

Catches, size and age composition of the spawning stock muksun Coregonus muksun in the Middle Ob (Tomsk region)

Muksun Coregonus muksun is one of the most valuable whitefish species. It lives in large rivers of Siberia. This is a typical semi-anadromous species. There are presents data on the catches of muksun in the Middle Ob (on spawning grounds and migration routes to them), which generally reflects the number of spawning stock of this species rising to spawn. It is shown that in the middle of the 20th century the catches of muksun averaged 47 tons per year. In the early 1970s, there was a sharp short-term increase in its catches to an average of 215 tons per year, which may be caused by the start of intensive work on collecting whitefish caviar in the area of their spawning run for obtain fish stock for pasture aquaculture in the lakes of Western Siberia. Since the early 1980s, muksun catches have fallen catastrophically, and its fishing is currently prohibited. On the spawning grounds of the Middle Ob, specimens under the age of 16+ were noted. Until the end of the 1990s, the age and size composition of the Muksun spawning stock remained relatively stable. In recent years, there has been an increase of younger fish and, accordingly, a decrease in the average size of fish in the catches. At present (2017–2019), specimens with a fishing length from 392 to 523 mm (on average 457 mm) and a weight of 747 to 1915 g (on average 1233 g) have been noted in the catches of muksun.

Time series covering up to four decades reveals major changes and drivers of marine growth and proportion of repeat spawners in an Atlantic salmon population.

Wild Atlantic salmon populations have declined in many regions and are affected by diverse natural and anthropogenic factors. To facilitate management guidelines, precise knowledge of mechanisms driving population changes in demographics and life history traits is needed.Our analyses were conducted on (a) age and growth data from scales of salmon caught by angling in the river Etneelva, Norway, covering smolt year classes from 1980 to 2018, (b) extensive sampling of the whole spawning run in the fish trap from 2013 onwards, and (c) time series of sea surface temperature, zooplankton biomass, and salmon lice infestation intensity.Marine growth during the first year at sea displayed a distinct stepwise decline across the four decades. Simultaneously, the population shifted from predominantly 1SW to 2SW salmon, and the proportion of repeat spawners increased from 3 to 7%. The latter observation is most evident in females and likely due to decreased marine exploitation. Female repeat spawners tended to be less catchable than males by anglers.Depending on the time period analyzed, marine growth rate during the first year at sea was both positively and negatively associated with sea surface temperature. Zooplankton biomass was positively associated with growth, while salmon lice infestation intensity was negatively associated with growth.Collectively, these results are likely to be linked with both changes in oceanic conditions and harvest regimes. Our conflicting results regarding the influence of sea surface temperature on marine growth are likely to be caused by long‐term increases in temperature, which may have triggered (or coincided with) ecosystem shifts creating generally poorer growth conditions over time, but within shorter datasets warmer years gave generally higher growth. We encourage management authorities to expand the use of permanently monitored reference rivers with complete trapping facilities, like the river Etneelva, generating valuable long‐term data for future analyses.

ГИДРОЛОГИЧЕСКОЕ ОБОСНОВАНИЕ ПРОЕКТИРОВАНИЯ ПРИГИДРОУЗЛОВЫХ РЫБОХОДНО-НЕРЕСТОВЫХ КАНАЛОВ НА ПРИМЕРЕ КОЧЕТОВСКОГО ГИДРОУЗЛА НА Р. ДОН

Purpose: to determine hydrological characteristics in the design of fish passage and spawning channels near waterworks. Materials and methods. The methodological basis of the development was the well-known approaches to the determination of the hydrological characteristics of the near-waterworks sections of the rivers, which describe the hydrological conditions for fish passage and spawning channel’ creation and operation. The factual basis was data on the values of the hydrological characteristics of the river Don near the Kochetovsky waterworks facility. The methodological basis was technologies for processing hydrological information. Results and discussion. As the determining conditions for the creation and functioning of near-waterworks fish passage and fish passage and spawning channels, the discharges and levels of the river Don along the damside of the upstream and downstream of the Kochetovsky waterworks facility are considered. The values of the specified hydrological characteristics are determined according to the data of their 44-year daily measurements for different levels of their availability at different durations of seasonal periods. As time periods for the analysis of hydrological characteristics, various options for the period of the spawning run and the periods of waterworks operation in the drop interstream mode are considered. Conclusions. The processing and systematization of hydrological data on seasonal periods of observation (of different durations) of the river flow of the river Don was carried out, and the corresponding graphical dependences for the values of water levels in the downstream of the Kochetovsky waterworks facility were made. The expediency of consideration and use in the design of fish passage and fish pass-spawning channels as the calculated level and flow hydrological characteristics of the river, determined by the period of the spawning run of anadromous fish species and the operational time of the waterworks in drop mode is substantiated.

КАЧЕСТВО ГИДРОЛОГИЧЕСКИХ УСЛОВИЙ ДЛЯ ПРИВЛЕЧЕНИЯ РЫБ В УСТЬ-МАНЫЧСКИЕ РЫБОХОДНЫЕ КАНАЛЫ

Purpose: quality assessment of hydrometric conditions for attracting anad-romous fish migratory along the river Western Manych in the fish passage channels of the Ust-Manych waterworks. Materials and methods. The factual basis of the study was the data of hydrological surveys carried out on the Ust-Manych fish passage channels, designed to allow fish spawners to pass to their spawning grounds. Generally accepted methods and techniques were used while conducting surveys, processing experimental data, their analysis and generalization. Results and discussion. Data have been obtained and calculations of hydrological characteristics have been made in the sections of the river Western Manych, corresponding to the location of the entrances to the fish channels, for two and three-month periods of the spring-summer spawning run of migratory fish. Differences in the values of water levels in the downstream of the Ust-Manych waterworks depending on the duration of the observation period were determined. Experimental data and calculated dependences to determine the average flow rates in the control sections for a wide range of changes in flow rates and water levels were obtained. The determined hydrological data of the river and the hydrometric data of fish passage channels constituted an empirical basis for assessing the quality of conditions for attracting fish from the river Western Manych to the entrances to the canals. It has been found that the quality indicator of conditions at different ratios of the values of the factors of influence for one fish passage channel was 27.3–45.3 %, and for the second channel it varies within 15.1–21.6 %. Conclusions. Data were obtained on hydrological values characterizing the section of the river Western Manych from the Ust-Manych waterworks to the alignment of its confluence with the river Don. An assessment of the hydrometric conditions quality for attracting fish migrating along the river to spawning grounds at the entrances to fish passage channels is made.

Evaluating the Effectiveness of Fyke‐Net Sampling to Characterize an Anadromous Rainbow Smelt Population in the Fore River, Massachusetts, USA

AbstractRainbow Smelt Osmerus mordax are a small anadromous species that is native to the northwest Atlantic Ocean, and their populations have declined rangewide. A small, mid‐channel survey fyke net is annually deployed to collect demographic information on Rainbow Smelt in several rivers in Massachusetts. Understanding the effectiveness of this fyke net for providing representative data on the spring spawning run is essential for accurate population assessment. To evaluate this effectiveness, we deployed an experimental fyke net that spanned the river channel concurrently with the survey fyke net in the Fore River, Massachusetts, from 2009 to 2012. The survey net CPUE increased as the total number of fish increased, but survey net catches are highly variable at lower daily abundances. The differences in the size distributions were slight and only significant for females in 1 of 4 years. The sex ratios were generally the same between both nets. These findings suggest that while fyke nets might not be ideal for monitoring rivers with very small populations and/or in wide river reaches, catches in larger populations should generally reflect the overall population demographics.

Combining fixed-location count data and movement data to estimate abundance of a lake sturgeon spawning run: a framework for riverine migratory species

Estimating abundance of migrating fishes is challenging. While sonars can be deployed continuously, improper assumptions about unidirectional migration and complete spatial coverage can lead to inaccurate estimates. To address these challenges, we present a framework for combining fixed-location count data from a dual-frequency identification sonar (DIDSON) with movement data from acoustic telemetry to estimate spawning run abundance of lake sturgeon (Acipenser fulvescens). Acoustic telemetry data were used to estimate the probability of observing a lake sturgeon on the DIDSON and to determine the probability that a lake sturgeon passing the DIDSON site had passed the site previously during the season. Combining probabilities with DIDSON counts, using a Bayesian integrated model, we estimated the following abundances: 99 (42–215 credible interval, CI) in 2017, 131 (82–248 CI) in 2018, and 92 (47–184 CI) in 2019. Adding movement data generated better inferences on count data by incorporating fish behavior (e.g., multiple migrations in a single season) and its uncertainty into abundance estimates. This framework can be applied to count and movement data to estimate abundance of spawning runs of other migratory fishes in riverine systems.

Optimal sampling designs for estimating salmonid run abundances from video recordings at a fishway on the Bois Brule River, Wisconsin, a tributary to Lake Superior

Using a simulation program and video census data (2004–2018) from the Bois Brule River fishway, Wisconsin, USA, we compared alternative sampling designs to estimate spawning run abundances of steelhead, coho salmon, Chinook salmon, and brown trout. We evaluated two types of two-stage sampling designs, comprising varying numbers of days sampled within a year (1st stage samples) and varying numbers of hours sampled within a day (2nd stage samples). While days were sampled using stratified random sampling under both types of sampling designs, hours were sampled using uniform (1/24) or non-uniform (proportional to hourly runs) selection probabilities under the first and second types of sampling designs, respectively. Number of days sampled within a year, comprising three strata, varied from 30 to 200 days in 10-day increments, and number of hours sampled within a day varied from 2 to 24 h in 2-hour increments. Spawning run sizes of the salmonids could be estimated with a relative root mean square error (RMSE) of less than 10% on average by employing a two-stage sampling design with samples of 100 days·yr−1 and 8 hrs·day−1, i.e., 800 hrs·yr−1; by contrast, full census involved reviewing 250 days·yr−1 and 24 hrs·day−1, i.e., 6000 hrs·yr−1, of video. Sampling more days (>100) resulted in greater reductions in estimation error than sampling more hours (>8). Non-uniform (vs. uniform) selection probabilities for hours sampled slightly reduced error of estimates. Our results underscore that optimal sampling designs could ensure a considerable reduction in survey resources while maintaining relatively low error in estimation of salmonid abundances.

Activity Patterns of Anadromous Fish below a Tide Gate: Observations from High‐Resolution Imaging Sonar

AbstractThe construction of dams and tide gates on waterways has altered the physical structure of many coastal, estuarine, and freshwater systems. These changes have come at a cost to fish populations, most notably diadromous species, which rely on connectivity between marine and freshwater systems. These anthropogenic structures can have direct effects on migrating fish, such as blocking fish passage, or have more subtle effects, such as changing movement patterns. This study used a high‐resolution Adaptive Resolution Imaging Sonar to examine the behavior of Striped Bass Morone saxatilis, a large coastal predator, and Alewife Alosa pseudoharengus and Blueback Herring Alosa aestivalis (collectively known as river herring), which are forage fish, below a tide gate structure on the Herring River in Wellfleet, Massachusetts, during the river herring spring spawning run. Striped Bass were persistently present downstream of the tide gate and exhibited strong diurnal and tidal patterns. Activity of Striped Bass was highest at night and during ebb tides. During peak outflow periods, river herring were observed milling downstream of the dam in a scour pool, indicating delayed upstream passage. River herring upstream migration was primarily associated with daytime and during incoming tides. Downstream‐migrating river herring were primarily observed during nighttime hours. While it was documented that the tide gates provided a physical impediment to migration, their effect on predator behavior could pose an additional challenge to migrating river herring, further complicating their recovery efforts. Due to the prevalence of obstructed waterways, studying the behavior of fish around anthropogenic structures is important in understanding the full range of impacts that these systems have under varying ecological conditions and on ecological relationships.

Quantifying Great Lakes sea lamprey populations using an index of adults

Effective control of aquatic invasive species requires knowledge of the population throughout the infested area. Lake-wide assessments of invasive sea lampreys (Petromyzon marinus) are used to assess their status in the Laurentian Great Lakes, informing fisheries managers and decision makers in the sea lamprey control program. Initially these assessments focused on an estimate of absolute abundance, but later switched to an estimate of relative abundance as an index. In this paper, we describe the recently developed index of sea lamprey abundance and the reasons for its use. Rather than trying to estimate spawning run sizes of all Great Lakes tributaries, the index instead estimates run sizes of a small subset of index streams. Streams chosen for the index had large spawning runs and a history of trapping operations that consistently yielded mark-recapture estimates. This change enabled the sea lamprey control program to abandon a previously used regression model that predicted run size on streams with no sea lamprey traps. Further research is needed to determine how strongly correlated the index is with actual patterns in the lake-wide population of adult sea lampreys.

The effects of resource subsidy duration in a detritus-based stream ecosystem: A mesocosm experiment.

Most resource subsidies are temporally variable, dynamically affecting the consumer populations, community structures and ecosystem functions of recipient ecosystems. Temporally variable resource subsidies are characterized by the duration, magnitude, timing and frequency of resource subsidy inputs. These different characteristics may have different mechanisms by which to affect recipient ecosystems. Few studies have examined the duration of resource subsidy inputs on recipient ecosystems, although there exist previous studies focusing on magnitude, timing and frequency. We provide the first experimental test of the effects of subsidy duration on a stream ecosystem by using an outdoor mesocosm experiment, in which we directly manipulated the subsidy duration (pulsed vs. prolonged) of terrestrial invertebrate input into the mesocosm. Given the same overall amount of terrestrial invertebrate subsidy was added, a prolonged subsidy allowed large-stage fish to effectively monopolize the subsidy over small-stage fish, which led small-stage fish to maintain their predation pressure on in-situ prey, that is, benthic invertebrates. On the other hand, a pulsed subsidy allowed small-stage fish to increase their feeding rate of the subsidy and to become away from foraging in-situ prey. Consequently, weaker indirect positive effects on in-situ benthic prey and leaf break-down rate were found with the prolonged versus pulsed subsidy. However, these indirect effects varied by the dominant benthic prey species, which differed in edibility for fish. Such predator-specific vulnerability of benthic prey can be important in mediating trophic cascades in detritus-based stream food webs. Phenological events that generate temporal subsidies (e.g. salmon spawning run and arthropod emergence) can be synchronized (pulsed) or desynchronized (prolonged) within and among species, depending on the degree of spatial and temporal environmental heterogeneity. The effects of subsidy duration would thus be important to better understand ecological processes in spatially and temporally coupled ecosystems.

Multiple spawning run contingents and population consequences in migratory striped bass Morone saxatilis.

Multiple spawning run contingents within the same population can experience varying demographic fates that stabilize populations through the portfolio effect. Multiple spawning run contingents (aka run timing groups) are reported here for the first time for striped bass, an economically important coastal species, which is well known for plastic estuarine and shelf migration behaviors. Adult Hudson River Estuary striped bass (n = 66) were tagged and tracked with acoustic transmitters from two known spawning reaches separated by 90 km. Biotelemetry recaptures for two years demonstrated that each river reach was associated with separate contingents. Time series of individual spawning phenologies were examined via nonparametric dynamic time warping and revealed two dominant time series centroids, each associated with a separate spawning reach. The lower spawning reach contingent occurred earlier than the higher reach contingent in 2017 but not in 2018. The majority (89%) of returning adults in 2018 showed the same contingent behaviors exhibited in 2017. Spawning contingents may have been cued differently by temperatures, where warming lagged 1-week at the higher reach in comparison to the lower reach. The two contingents exhibited similar Atlantic shelf migration patterns with strong summer fidelity to Massachusetts Bay and winter migrations to the southern US Mid-Atlantic Bight. Still, in 2017, differing times of departure into nearby shelf waters likely caused the early lower reach contingent to experience substantially higher mortality than the later upper reach contingent. Anecdotal evidence suggests that higher fishing effort is exerted on the early-departing individuals as they first enter shelf fisheries. Thus, as in salmon, multiple spawning units can lead to differential demographic outcomes, potentially stabilizing overall population dynamics.

A review of an electric weir and fishway in a Great Lakes tributary from conception to termination

A successful management plan requires clear goals and a process for evaluation. Without them, managers risk operational shifts in which continuous changes disguised as improvements may have little beneficial effect. The conception, design, and operation of an electric barrier and fishway on the Pere Marquette River of Lake Michigan serve as an illustration. The Great Lakes Fishery Commission operated an electric weir to stop migration of adult sea lampreys (Petromyzon marinus) and a fishway to provide upstream passage of rainbow trout (Oncorhynchus mykiss) during 2000–2009. The weir and fishway were successful in blocking some of the annual spawning run of sea lampreys (trapped an annual average of 439 sea lampreys) and it allowed passage of rainbow trout (an annual average of 6,091). Even with success that yielded an estimated density of 0.03 adult female sea lampreys per 100 m2 of larval habitat upstream of the weir, lampricide treatments continued because the weir still allowed establishment of substantial densities of larval sea lampreys. Our evaluation suggests that an in-stream barrier must approach 100% blockage of sea lampreys to eliminate large recruitment events. The failure of the weir to reduce lampricide treatments was due to an informally defined purpose and measures for success at the onset, the complexity of electric weir systems (and the operational problems created by such intricacy), and lack of recognition of the reduction in larval sea lamprey recruitment needed to succeed. Control of sea lampreys in the Pere Marquette River could have benefited from an adaptive management approach.

Changes in thiamine concentrations, fatty acid composition, and some other lipid-related biochemical indices in Baltic Sea Atlantic salmon (Salmo salar) during the spawning run and pre-spawning fasting

Salmonines in the Baltic Sea and North American lakes suffer from thiamine (vitamin B1) deficiency, which is connected to an abundant lipid-rich diet containing substantial amounts of polyunsaturated fatty acids (PUFAs). In the Baltic region, this is known as the M74 syndrome. It affects both adult salmon (Salmo salar) and especially their offspring, impairing recruitment. However, very little is known about the thiamine and lipid metabolism of salmon during feeding and spawning migrations in the Baltic Sea. In this study, salmon females were sampled along the spawning run from the southern Baltic Proper in four locations at sea and finally at spawning in a river at the Bothnian Bay in a year with insignificant M74 mortality. Changes in concentrations of thiamine and its components in muscle, ovaries, and the liver and other biochemical indices potentially relating to lipid and fatty acid metabolism were investigated. The results provide further evidence of the role of peroxidation of PUFAs in eliciting thiamine deficiency in salmon: During the entire spawning run, the muscle total lipid content decreased by 50%, palmitic acid (16:0) by 62%, and docosahexaenoic acid (DHA, 22:6n-3) by 45%. The concentration of total thiamine decreased significantly until the spawning in the liver and ovaries, 66 and 70% respectively. In the muscle, the proportion of thiamine pyrophosphate of total thiamine increased with the use of muscular lipid stores. There was no trend in the concentration of total carotenoids during the spawning run. The doubling of the concentration of hepatic malondialdehyde indicated peroxidation of PUFAs, and the mobilisation of body lipids suppressed the activity of glucose-6-phosphate dehydrogenase, as consumed dietary lipids would also have done.