Baltic Sea River Research Articles

Rewetting effects on nitrogen cycling and nutrient export from coastal peatlands to the Baltic Sea

Coastal nutrient loads from point sources such as rivers are mostly well-monitored. This is not the case for diffuse nutrient inputs from coastal catchments unconnected to rivers, despite the potential for high inputs due to intensive land use. The German Baltic Sea coastline consists of numerous peatlands that have been diked and drained. However, some of the dikes have been removed in order to re-establish the hydrological connection to the Baltic Sea, restore local biodiversity, and promote natural CO2 uptake. Since these peatlands were used for agriculture, their rewetting may release accumulated nutrients, leading to nutrient export into the Baltic Sea and intensified coastal eutrophication. Data on these potential nutrient exports are mostly lacking. Therefore, this study investigated nutrient exports from two former agricultural, coastal peatlands: Drammendorfer Wiesen, rewetted in 2019, and Karrendorfer Wiesen, rewetted in 1993. Nutrients (NO3–, NO2–, NH4+, PO43–), nitrous oxide (N2O), particulate organic matter (POM, comprising POC and PON; δ13C-POC), chlorophyll-a, and nitrification rates were analyzed in surface water and porewater sampled weekly to monthly in 2019 and 2020 to compare the effects of different time scales after rewetting on nutrient cycling and potential exports. NH4+, NO2−, and PO43− concentrations were higher in the porewater than in the overlying water at both sites, while nutrient concentrations were generally higher at the recently rewetted Drammendorfer Wiesen than at the Karrendorfer Wiesen. NO3− concentrations in porewater, however, were lower than in the overlying water, indicating NO3− retention within the peat, likely due to denitrification. Nitrification rates and N2O concentrations were generally low, except for a high N2O peak immediately after rewetting. These results suggest that denitrification was the dominant process of N2O production at the study sites. Both peatlands exported nutrients to their adjacent bays of the Baltic Sea; however, N exports were 75% lower in the longer-rewetted peatland. Compared to major Baltic Sea rivers, both sites exported larger area-normalized nutrient loads. Our study highlights the need to monitor the impact of rewetting measures over time to obtain accurate estimates of nutrient exports, better assess negative effects on coastal waters, and to improve peatland management.

ANALYSIS OF THE AMPHIPOD FAUNA OF CONTINENTAL RESERVOIRS IN THE NORTHWEST OF RUSSIA’S EUROPEAN PART

The faunal composition of Amphipoda crustaceans of mainland water bodies in northwestern Russia is analyzed. To compile a checklist of amphipod species, data from original collections made in 1998–2021, the collections of the Zoological Institute, and literary information are used. To date, 29 species of amphipods have been recorded from the continental waters of the region. An arealogical analysis of the faunal composition reveals six groups or distribution patterns: Holarctic, West Palearctic, and North Atlantic estuaries, and further three groups of emigrants (settlers) from the Baikal-Siberian, Ponto-Caspian, and Arctic regions. The greatest species richness is recorded in estuarine sections of Baltic Sea rivers in the territories of the Leningrad and Kaliningrad regions, as well as fresh water bodies of the Republic of Karelia. In terms of species richness, estuarine species amphi-Atlantic in distribution and representatives of allochthonous faunas (invasive species) from the Ponto-Caspian basin dominate the region. The most common in the region are representatives of Holarctic and Palearctic patterns, as well as species of Arctic and Baikal-Siberian origins. Among them, the following species are most often to be recorded in lakes: Gammarus lacustris, Monoporeia affinis, Palaseopsis quadrispinosa and Gmelinoides fasciatus, vs G. zaddachi, G. oceanicus, G. tigrinus and Pontogammarus robustoides in estuarine brackish waters. Human-mediated introductions, both intentional and accidental, must have facilitated the appearance of invasive species in the fauna of Russia’s northwestern region. In the future, an increase in species richness in the region can be expected to occur in two ways: through the introduction of marine species into fresh waters and due to speciation.

Deriving VIIRS High-Spatial Resolution Water Property Data over Coastal and Inland Waters Using Deep Convolutional Neural Network

The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (SNPP) satellite has been a reliable source of ocean color data products, including five moderate (M) bands and one imagery (I) band normalized water-leaving radiance spectra nLw(λ). The spatial resolutions of the M-band and I-band nLw(λ) are 750 m and 375 m, respectively. With the technique of convolutional neural network (CNN), the M-band nLw(λ) imagery can be super-resolved from 750 m to 375 m spatial resolution by leveraging the high spatial resolution features of I1-band nLw(λ) data. However, it is also important to enhance the spatial resolution of VIIRS-derived chlorophyll-a (Chl-a) concentration and the water diffuse attenuation coefficient at the wavelength of 490 nm (Kd(490)), as well as other biological and biogeochemical products. In this study, we describe our effort to derive high-resolution Kd(490) and Chl-a data based on super-resolved nLw(λ) images at the VIIRS five M-bands. To improve the network performance over extremely turbid coastal oceans and inland waters, the networks are retrained with a training dataset including ocean color data from the Bohai Sea, Baltic Sea, and La Plata River Estuary, covering water types from clear open oceans to moderately turbid and highly turbid waters. The evaluation results show that the super-resolved Kd(490) image is much sharper than the original one, and has more detailed fine spatial structures. A similar enhancement of finer structures is also found in the super-resolved Chl-a images. Chl-a filaments are much sharper and thinner in the super-resolved image, and some of the very fine spatial features that are not shown in the original images appear in the super-resolved Chl-a imageries. The networks are also applied to four other coastal and inland water regions. The results show that super-resolution occurs mainly on pixels of Chl-a and Kd(490) features, especially on the feature edges and locations with a large spatial gradient. The biases between the original M-band images and super-resolved high-resolution images are small for both Chl-a and Kd(490) in moderately to extremely turbid coastal oceans and inland waters, indicating that the super-resolution process does not change the mean values of the original images.

The Baltic Sea and the adjacent North Sea silicate concentrations

From 1970 to 2000, the yearly Baltic Sea River Runoff (BSRR) together with the silicate concentrations in the Baltic (BS) and North Sea (NS) were observed. We found that the concentrations of silicates were consistently higher in the BS. Furthermore, long-term changes in both ecosystems indicated an overall decrease of Si with increased BSRR. Silicate concentrations were also modelled in two setups of Generalised Linear Mixed Models. Selected exceptional (high and low) BSRR events were used as a predictor in the models. The results suggest that the whole BS acts as a silicate source for the studied, adjacent NS areas. Modelling indicated that the periods with low BSRR show generally higher silicate levels in BS water. In the NS, however, the silicate concentrations decrease with low BSRR periods and increase at high. We point out that the increase of rainfall, and consequently the BSRR, has predominantly taken place in the northern parts of the BS basin, where the ground silicate concentrations are lower than in the southern BS. Therefore, we present, as a conceptual model and a subject for further research, that the BS potentially has acted as a concentrating basin for silicates during periods of normal and low rainfall, while episodic high runoff periods flush the BS surface water including the silicates through the Danish Sounds and into the adjacent parts of the NS. In a post hoc test, we found that also the reproductive success of the NS herring showed a correlation to silicate concentrations thus partially demonstrating the existence of a Si-related, bottom-up regulation.

Carbon and nutrient recycling ecotechnologies in three Baltic Sea river basins – the effectiveness in nutrient load reduction

There exist numerous ecotechnologies for recovery and reuse of carbon and nutrients from various waste streams before they are lost to runoff. However, it remains largely unknown how growing implementation of such ecotechnologies affect nutrient emissions to surface waters at catchment scale. Here, this knowledge gap is addressed by application of SWAT model in three case study catchments draining to the Baltic Sea: Vantaanjoki (Finland), Fyrisån (Sweden) and Słupia (Poland). Sustainability analysis with Multi-Criteria Analysis was applied in the stakeholder workshops in the case study areas to assess different ecotechnology alternatives. The following ecotechnologies received the highest sustainability scores: in Vantaanjoki anaerobic digestion, based on mostly agricultural residues; in Fyrisån source-separation of wastewaters; in Słupia nutrient extraction within the wastewater treatment process. The effect of application of digestate on agricultural soils in the Vantaanjoki catchment was simulated by adjusting the model parameters describing the organic carbon content and physical properties of soil. The results showed small reductions of nutrient loads to the Gulf of Finland. Larger reductions of nutrient loads to Lake Mälaren in Sweden and the Baltic Sea in Poland were achieved as a result of the wastewater treatment upgrades. In the Fyrisån catchment, higher relative reductions were simulated for TN than TP, and in dry years than in wet years. Although the studied ecotechnologies did not show as high effectiveness in nutrient load reduction as combinations of traditional Best Management Practices reported in literature, they do have other multiple benefits including crop yield increase, electricity, heat and bio-based fertilizer production.

Population genetic structure after 125 years of stocking in sea trout (Salmo trutta L.)

Stocking can be an effective management and conservation tool, but it also carries the danger of eroding natural population structure, introducing non-native strains and reducing genetic diversity. Sea trout, the anadromous form of the brown trout (Salmo trutta), is a highly targeted species that is often managed by stocking. Here, we assess the present-day population genetic structure of sea trout in a backdrop of 125 years of stocking in Northern Germany. The study area is characterized by short distances between the Baltic and North Sea river watersheds, historic use of fish from both watersheds for stocking, and the creation of a potential migration corridor between the Baltic and North Sea with the opening of the Kiel Canal 120 years ago. A survey of 24 river systems with 180 SNPs indicates that moderate but highly significant population genetic structure has persisted both within and between the Baltic and North Sea. This genetic structure is characterized by (i) heterogeneous patterns of admixture between the Baltic and North Sea that do not correlate with distance from the Kiel Canal and are therefore likely due to historic stocking practises, (ii) genetic isolation by distance in the Baltic Sea at a spatial scale of < 200 km that is consistent with the homing behaviour of sea trout, and (iii) at least one genetically distinct Baltic Sea river system. In light of these results, we recommend keeping fish of North Sea and Baltic Sea origin separate for stocking, and restricting Baltic Sea translocations to neighbouring river systems.

Watershed characteristics and climate factors effect on the temporal variability of mercury in the southern Baltic Sea rivers

Mercury (Hg) is a neurotoxic metal which can enter into the human organism mainly by fish consumption, skin and transpiration. In the coastal zone of the southern Baltic Sea, rivers are the main source of Hg. The Polish region represents the largest proportion of the Baltic Sea catchment and this research included four rivers of the Baltic watershed: the Reda, Zagórska Struga, Kacza and Gizdepka. The samples were collected in the years 2011–2013. Total and particulate Hg concentration in these rivers were measured. Due to intensive rain, deposited mercury on the catchment area was washed out into the riverines water and introduced into the Baltic Sea. Consequently, the load of Hg increased three times. Additionally, the intensive dry atmospheric deposition during heating season caused the increase of the concentration of particulate Hg in the river water even by 85%. The research confirmed the role of the river flow magnitude in the load of mercury introduced into the sea by rivers. Moreover, a high variability of mercury concentration was connected to the additional sources such as the chemicals containing Hg and no municipal sewage system. The analysis of stable isotopes indicated that the SPM contained terrestrial organic matter; however, there was no clear correlation between Hgtot, Corg and Ntot concentrations and δ13C, δ15N, C/N in particulate matter.

Riverine tot-P loading and seawater concentrations in the Baltic Sea during the 1970s to 2000-transfer function modelling based on the total runoff.

The signal of climate through the North Atlantic Oscillation (NAO) extends to westerly weather and to the Baltic Sea river runoff (BSRR) and further to the salinity and the marine fauna in the Baltic Sea. Our working hypothesis was that increased BSRR should also lead to increasing nutrient concentrations in the seawater. In rivers, transfer function (TF) models of the loading were constructed by time series of BSRR and tot-P concentrations. Based on the loading time series, we modelled, to our knowledge, first time, seawater tot-P concentrations in both the Northern Baltic Proper and in the Gulf of Bothnia, both on the surface (0–20 m) and deeper (21–70 m) waters. Our results further suggest a unifying mechanism by the BSRR that could explain most prominent ecological changes observed in the Baltic Sea during and after the 1970s. Such changes are eutrophication (as in this paper) and decreasing salinity and growth and reproduction of marine fauna, all of which have been separately described as due to different causes. BSRR is crucial when possible future developments of the Baltic Sea environment are considered because a general opinion exists that the rainfall (and the BSRR) is expected to increase in pace with proceeding climate change.

Nitrogen surface water retention in the Baltic Sea drainage basin

Abstract. In this paper, we estimate the surface water retention of nitrogen (N) in all the 117 drainage basins to the Baltic Sea with the use of a statistical model (MESAW) for source apportionment of riverine loads of pollutants. Our results show that the MESAW model was able to estimate the N load at the river mouth of 88 Baltic Sea rivers, for which we had observed data, with a sufficient degree of precision and accuracy. The estimated retention parameters were also statistically significant. Our results show that around 380 000 t of N are annually retained in surface waters draining to the Baltic Sea. The total annual riverine load from the 117 basins to the Baltic Sea was estimated at 570 000 t of N, giving a total surface water N retention of around 40%. In terms of absolute retention values, three major river basins account for 50% of the total retention in the 117 basins; i.e. around 104 000 t of N are retained in Neva, 55 000 t in Vistula and 32 000 t in Oder. The largest retention was found in river basins with a high percentage of lakes as indicated by a strong relationship between N retention (%) and share of lake area in the river drainage areas. For example in Göta älv, we estimated a total N retention of 72%, whereof 67% of the retention occurred in the lakes of that drainage area (Lake Vänern primarily). The obtained results will hopefully enable the Helsinki Commission (HELCOM) to refine the nutrient load targets in the Baltic Sea Action Plan (BSAP), as well as to better identify cost-efficient measures to reduce nutrient loadings to the Baltic Sea.

Differences between wild and reared Salmo salar stocks of two northern Baltic Sea rivers

The phenotypic divergence between natural salmon stocks and the corresponding hatchery-reared stocks was investigated in two Atlantic salmon (Salmo salar) populations from the Tornionjoki and Simojoki Rivers, where hatchery reared smolts of local origin were released during the second half of the last century. The reared salmon were larger as smolts and also heavier at sea than wild salmon in both stocks. The recapture rate was highest for wild Tornionjoki salmon (mean 2.8%) and lowest for reared Simojoki salmon (mean 1.2%). During 1999–2008, survival was higher in wild than reared fish, while the overall survival of tagged smolts declined. Survival was close to similar in Tornionjoki and Simojoki salmon when the year, background (wild or reared) and smolt length were taken into account. Migration distances differed between wild and reared stocks. In the second winter at sea, the majority of wild salmon (94–96%) were captured in the southern part of the Baltic Sea, while the reared salmon, especially those of the Simojoki stock, remained closer to their native river in the northern parts of the Baltic. Rearing also decreased the duration of the sea migration before the first maturation in Simojoki salmon, but not in Tornionjoki salmon. Of the wild Simojoki salmon, 64% returned to their home river as multi-sea-winter (MSW) fish compared to 40% of the reared fish. The population and rearing background, but not smolt length, explained the probability of maturing as one-sea-winter instead of MSW fish. Rearing produced 1.60 times more grilse than occurred among wild salmon. The reared salmon showed different life-history patterns from wild fish, and especially in the Simojoki River, the salmon stock as a whole presently deviates from previously documented wild rivers. Even though population sizes appear to be recovering, it may take time before the stocks regain their adaptive potential following the period of collapse and supportive releases.

Weight-of-evidence approach in assessment of ecotoxicological risks of acid sulphate soils in the Baltic Sea river estuaries

Acidity and leaching of metals from acid sulphate soils (ASSs) impair the water quality of receiving surface waters. The largest ASS areas in Europe are found in the coasts of the northern Baltic Sea. We used weight-of-evidence (WoE) approach to assess potential risks in 14 estuary sites affected by ASS in the Gulf of Finland, northern Baltic Sea. The assessment was based on exposure and effect profiles utilizing sediment and water metal concentrations and concurrent pH variation, sediment toxicity tests using the luminescent bacterium Vibrio fischeri and the midge Chironomus riparius, and the ecological status of benthic macroinvertebrate communities. Sediment metal concentrations were compared to national sediment quality criteria/guidelines, and water metal concentrations to environmental quality standards (EQSs). Hazard quotients (HQs) were established for maximum aluminium, cadmium and zinc concentrations at low pH based on applicable US EPA toxicity database. Sediment metal concentrations were clearly elevated in most of the studied estuaries. The EQS of cadmium (0.1μg/l) was exceeded in 3 estuaries out of 14. The pH-minima were below the national threshold value (5.5) between good and satisfactory water quality in 10 estuaries. V. fischeri bioluminescence indicated toxicity of the sediments but toxic response was not observed in the C. riparius emergence test. Benthic invertebrate communities were deteriorated in 6 out of 14 sites based on the benthic invertebrate quality index. The overall ecotoxicological risk was assessed as low in five, moderate in three and high in five of the estuary sites. The risk assessment utilizing the WoE approach indicated that harmful effects of ASSs are likely to occur in the Baltic Sea river estuaries located at the ASS hotspot area.

Saving the Baltic Sea, the inland waters of its drainage basin, or both? spatial perspectives on reducing P-loads in eastern Sweden.

Nutrient loads from inland sources to the Baltic Sea and adjacent inland waters need to be reduced in order to prevent eutrophication and meet requirements of the European Water Framework Directive (WFD) and the Baltic Sea Action Plan (BSAP). We here investigate the spatial implications of using different possible criteria for reducing water-borne phosphorous (P) loads in the Northern Baltic Sea River Basin District (NBS-RBD) in Sweden. Results show that most catchments that have a high degree of internal eutrophication do not express high export of P from their outlets. Furthermore, due to lake retention, lake catchments with high P-loads per agricultural area (which is potentially of concern for the WFD) did not considerably contribute to the P-loading of the Baltic Sea. Spatially uniform water quality goals may, therefore, not be effective in NBS-RBD, emphasizing more generally the need for regional adaptation of WFD and BSAP-related goals.

Importance of saprotrophic freshwater fungi for pollen degradation.

Fungi and bacteria are the major organic matter (OM) decomposers in aquatic ecosystems. While bacteria are regarded as primary mineralizers in the pelagic zone of lakes and oceans, fungi dominate OM decomposition in streams and wetlands. Recent findings indicate that fungal communities are also active in lakes, but little is known about their diversity and interactions with bacteria. Therefore, the decomposer niche overlap of saprotrophic fungi and bacteria was studied on pollen (as a seasonally recurring source of fine particulate OM) by performing microcosm experiments with three different lake types. Special emphasis was placed on analysis of fungal community composition and diversity. We hypothesized that (I) pollen select for small saprotrophic fungi and at the same time for typical particle-associated bacteria; (II) fungal communities form specific free-living and attached sub-communities in each lake type; (III) the ratio between fungi or bacteria on pollen is controlled by the lake's chemistry. Bacteria-to-fungi ratios were determined by quantitative PCR (qPCR), and bacterial and fungal diversity were studied by clone libraries and denaturing gradient gel electrophoresis (DGGE) fingerprints. A protease assay was used to identify functional differences between treatments. For generalization, systematic differences in bacteria-to-fungi ratios were analyzed with a dataset from the nearby Baltic Sea rivers. High abundances of Chytridiomycota as well as occurrences of Cryptomycota and yeast-like fungi confirm the decomposer niche overlap of saprotrophic fungi and bacteria on pollen. As hypothesized, microbial communities consistently differed between the lake types and exhibited functional differences. Bacteria-to-fungi ratios correlated well with parameters such as organic carbon and pH. The importance of dissolved organic carbon and nitrogen for bacteria-to-fungi ratios was supported by the Baltic Sea river dataset. Our findings highlight the fact that carbon-to-nitrogen ratios may also control fungal contributions to OM decomposition in aquatic ecosystems.

Coastal migrations, temporary use of neighbouring rivers, and growth of sea trout (Salmo trutta) from nine northern Baltic Sea rivers

Abstract Degerman, E., Leonardsson, K., and Lundqvist, H. 2012. Coastal migrations, temporary use of neighbouring rivers, and growth of Sea trout (Salmo trutta) from nine northern Baltic Sea rivers – ICES Journal of Marine Science, 69: 971–980. The wild migratory trout (Salmo trutta) stocks in the northern Baltic Sea are threatened, and the fishery is thought to play a significant role in the population decline. Therefore, knowledge about the migration patterns of these stocks is needed to develop appropriate management plans. For this reason, we analysed the movement pattern from recaptures of mandatory releases of tagged hatchery-reared trout smolt from nine rivers in the region (1998–2007). The median time from release until recapture was 366 days, and the median migration distance at recapture was 27 km, with a dominating southward direction for northern stocks. Most of the recaptured fish were immature (65%), and recaptures in rivers (55%) dominated over recaptures along the coast (44%). Riverine recaptures were most frequent during autumn–spring. A total of 16% of all recaptures in rivers occurred in non-natal rivers. Straying was about twice as frequent among fish from small rivers as from large rivers. The results from the river Gideälven indicate that using broodstock from other rivers in stocking programmes may lead to a high proportion of strayers. In addition, temporary use of neighbouring rivers was more frequent in large rivers during winter. Growth, in terms of length increment, was high, especially in fish from southern parts of the Baltic, and higher than in neighbouring warmer and more saline sea areas.

Effects of muscle lipid concentration on wild and hatchery brown trout (Salmo trutta) smolt migration

Annually, hatchery programs are releasing millions of salmonid smolts into the Baltic Sea. Recent estimations indicate a decline in smolt sea survival, questioning the ecological and socioeconomic values of these programs. Concurrently, hatchery smolts have increased in lipid concentration. Salmonids display partial migration, and it is suggested that the ratio of migrants/residents is affected by individual smolt energetic status. To test whether the increased energetic status of hatchery smolts could explain the noted decrease in survival, we released wild trout smolts, conventional hatchery smolts, and hatchery smolts of low energetic status into a Baltic Sea river. Using telemetry, we obtained data on the number of successful migrants, their swimming speed, and diel migratory behaviour. A much lower proportion of conventional smolts (30%) successfully migrated to the coast. No difference was found between wild (74%) and hatchery smolts of low energetic status (64%). Furthermore, conventional smolts migrated slower and showed no diel migratory pattern. The results are of high relevance for hatchery programs producing partially migrating fish.

Impact of the European Water Framework Directive on local-level water management: Case study Oxunda Catchment, Sweden

The Water Framework Directive (WFD) of the European Union provides a common framework for water policy that focuses on holistic and integrated water management in river basins. In many member states, implementation of the WFD has shifted the main responsibility for local water issues from the municipal level to the regional or supra-regional levels. In this study, we investigated how the implementation of the WFD has influenced local-level water management including the interpretation of the new environmental quality standards. Specifically, we considered Sweden, which has traditionally had relatively strong governance at the municipal level. Because a sufficient amount of time has now passed for evaluation of WFD-related effects on operational water handling, we interviewed individuals directly involved in water planning and land use planning at the municipal level in one sub catchment in the Northern Baltic Sea River Basin District of Sweden, as well as representatives for superior levels and associations. Despite divergent views regarding the priority of water issues in physical planning among the local-level planners interviewed, they had all participated in successful inter-municipal pre-WFD collaboration projects. Although such collaborations could help increase the understanding and acceptance of WFD-related goals and costs, as well as facilitate conflict solving, as shown in the Oxunda Catchment, they have not gained much attention in the WFD implementation process. Additionally, physical planners have generally been reluctant to accept new environmental quality standards resulting from WFD implementation, in part because they lack precise definitions, but also because they could challenge the municipal routine of weighing various objectives against each other. Furthermore, despite WFD-related increases in ambition levels, lack of resource improvements at the municipal level were identified as potential problems by local environmental planners.

Governance of Water Resources in the Phase of Change: A Case Study of the Implementation of the EU Water Framework Directive in Sweden

In this article, focusing on the ongoing implementation of the EU Water Framework Directive, we analyze some of the opportunities and challenges for a sustainable governance of water resources from an ecosystem management perspective. In the face of uncertainty and change, the ecosystem approach as a holistic and integrated management framework is increasingly recognized. The ongoing implementation of the Water Framework Directive (WFD) could be viewed as a reorganization phase in the process of change in institutional arrangements and ecosystems. In this case study from the Northern Baltic Sea River Basin District, Sweden, we focus in particular on data and information management from a multi-level governance perspective from the local stakeholder to the River Basin level. We apply a document analysis, hydrological mapping, and GIS models to analyze some of the institutional framework created for the implementation of the WFD. The study underlines the importance of institutional arrangements that can handle variability of local situations and trade-offs between solutions and priorities on different hierarchical levels.

Migration performance of wild and hatchery sea trout ( Salmo trutta L.) smolts—Implications for compensatory hatchery programs

Migration success of hatchery-reared and wild sea trout smolts through the lower stretches and the estuary of a Baltic Sea river were studied. During 3 years, wild and hatchery trout smolts were implanted with acoustic transmitters and released 14 km upstream from the river mouth. In order to monitor their out-migration pattern, acoustic receivers were deployed along the migratory route. Data on number of fish detected and date and time of detections were analysed and the migratory performance of wild and hatchery-reared fish was compared. A significantly higher proportion of wild fish (80%) successfully migrated to the coast compared to fish of hatchery origin (27.5%) and migration was faster in wild smolts. Hatchery fish were larger and had a higher condition factor and lipid concentrations, which are proposed as possible reasons for the poorer migratory performance of the hatchery-reared fish.

Genetic assessment of Atlantic salmon Salmo salar and sea trout Salmo trutta stocking in a Baltic Sea river

Microsatellite DNA variation was used to assess the outcome of stocking Atlantic salmon Salmo salar and migratory trout Salmo trutta in River Sävarå, N Sweden. No information on pre‐stocking genetic composition of S. salar and S. trutta in River Sävarå was available. In 2 year‐classes of S. salar smolt, microsatellite data indicated that post‐stocking genetic composition differed markedly (FST= 0·048) from the main donor strain, Byskeälven S. salar, and from other Gulf of Bothnia S. salar stocks (FST 0·047 and 0·132). The STRUCTURE programme failed to detect any substructuring within Sävarå salmon. It was concluded that only minor introgression estimated to a proportion of 0·11 (95% CI 0·07–0·16) has occurred in S. salar. Salmo trutta showed overall low differentiation among populations with maximum FST of 0·03 making analysis more cumbersome than in S. salar. Still, the SävaråS. trutta deviated significantly from potential donor populations, and STRUCTURE software supported that majority of trout in Sävarå formed a distinct genetic population. Admixture was more extensive in S. trutta and estimated to 0·17 (95% CI 0·10–0·25).

Estimation of Atlantic salmon smolt carrying capacity of rivers using expert knowledge

Abstract A mixed salmon fishery with both natural wild salmon stocks and reared salmon exists in the Baltic Sea. The agreed-upon goal of management is to safeguard the wild stocks, and the practical management objectives have been agreed to attain at least a 50% maximum salmon production capacity in each river. This natural production capacity is, however, largely unknown. Here, a new approach has been used to estimate the salmon maximum production capacity of northern Baltic Sea rivers. A probabilistic salmon production capacity model was built entirely upon expert knowledge. The model describes the external physical and biological factors of the rivers and the juvenile salmon stocks' response to these factors. We found that the experts estimated the carrying capacity to be considerably higher than former estimates. A very high uncertainty was, however, connected with these estimates. We also found considerable disagreement over the general carrying capacity level among the experts; the major uncertainty emerged from the conflicting views of the experts. The result implies that perhaps operational management objectives other than those based on maximal smolt production levels should be considered to decrease the uncertainty connected with evaluation of management success.