Over the past three decades, many seal populations in the Northern Hemisphere have recovered, resulting in various unintended effects on fisheries. In the Baltic Sea, the growing grey seal (Halichoerus grypus) population is causing serious problems in coastal fisheries by damaging fishing gear and catches. Largely due to these losses, coastal fishing has declined dramatically during the last decades. The policies in the European Union (EU) strongly support the protection of seal populations. The EU trade ban on seal products, introduced in 2009, is part of this legislation. As a result of this all-inclusive trade ban, legally hunted seals are often dumbed in the sea, or they end up as waste in landfills. This conflicts with traditional hunting practices, where the utilization of prey is an important element of the hunt. In effect, the trade ban has markedly reduced small-scale seal hunting and has contributed to the deepening of the seal-fishery conflict in the Baltic Sea region. Besides, handcrafting skills of seal products as subsidiary income in coastal communities has been lost. We provide evidence that small-scale commercial utilization of legally hunted seals should be considered, rather than wasting such a valuable resource. This would nurture cultural traditions and support favorable development in coastal areas. Seal hunting in the EU is well regulated and monitored, and the current hunting pressure does not endanger the growing and healthy seal populations. We fully agree that thriving marine mammal populations are a valuable element of healthy ecosystems.

Breeding colonies of the continental subspecies of the great cormorant (Phalacrocorax carbo sinensis) have rapidly expanded in Europe during the past half century. To assess the impact of cormorant breeding colonies on bird biodiversity, we used annual monitoring datasets of breeding bird communities from 261 islands in the northeastern Baltic Sea along the Estonian coast, covering the period 1960-2022. We found that Shannon's diversity index values for breeding birds were significantly negatively related to the establishment of cormorant breeding colonies but started to recover on islands after cormorants abandoned the colonies. This indicates considerable competition between great cormorant and other species for breeding habitat on these islands and stresses the impact that highly abundant colonial bird species can have on the biodiversity of small marine islands.

Commercial small-scale fisheries along the Baltic Sea coasts have declined over the years although these fisheries are viewed as important for coastal development and food security at the local, national, and EU levels. The viability and future of small-scale fisheries are severely challenged by problems caused by grey seals. The conflict, occurring between Baltic Sea coastal fisheries and conservation of the grey seals, has been severe since the mid-1990s and continues despite attempts to find a more balanced situation. Resting on reviews of multiple material, this paper explores the state-of-the-art opportunities for mitigating the seal-fisheries conflict and asks how these are related to social struggles and social justice. Our paper concludes that co-existence of coastal fisheries and the grey seal is possible but necessitates political will and co-designed seal management plans that help implement context-specific measures. Seal deterrents, for instance, give hope as a supplementary conflict mitigation measure – along with seal-proof fishing gear – but provide only partial relief. From the fisheries sector’s position, influencing the size of the seal population is a logical solution. The lifting of the EU trade ban of seal products as a regional derogation would allow sustainable management of seal populations so that they be used as renewable natural resource. Monitoring changes in the seal population is crucial for maintaining a balanced population. Reaching co-existence is timely, because – unlike the seal – the diverse Baltic coastal fishing culture is increasingly endangered.

The capture of the nonindigenous tubenose goby Proterorhinus semipellucidus in the Pada River (Baltic Sea basin) is reported.Previous records about the occurrence of this species in this region are restricted to the brackish coastal surfzone area.The finding implies that in the Baltic Sea region P. semipellucidus has started to invade from brackish coastal waters to freshwater tributaries of the Gulf of Finland.

The unintended consequences of marine mammal recoveries have created complex issues for resource managers to solve. In the Baltic Sea, the grey seal (Halichoerus grypus) population has increased rapidly during recent decades, and the conflict between seal conservation and fishery has escalated. Although the magnitude of economic losses varies depending on the type of fisheries, there is strong evidence that the grey seal population negatively impacts coastal fisheries and indirectly threatens the cultural heritage connected to it. The current management paradigm is biased towards the preservation of seal populations and it is failing to adequately consider socio-economic impacts of seal population. There is a need to strike a balance between seal conservation and the viability of coastal fisheries, taking into consideration local circumstances. This paper contributes to resolving this problem by assessing the existing governance arrangement. We conclude that the inconsistencies between and within different regulatory frameworks in HELCOM recommendations and European Union law are a structural constraint to tackling the problem. Further to that, some of the existing management criteria applicable to Baltic grey seal population need to be revisited by giving more consideration to regional conditions within the Baltic Sea. For instance, if the data shows that the Baltic grey seal population in its core distribution area has reached a sustainable status and is no longer at risk, then the use of peripheral areas as an indication of inadequate state of the entire Baltic Sea grey seal population is questionable.

Mixed-stock fishery of sea trout (Salmo trutta L.) in the Baltic Sea targets wild and stocked individuals, which could lead to over-exploitation of wild stocks. Studying the contribution of different spawning streams and hatcheries to the fishery would aid in more effective management and conservation of S. trutta. The aim of the present study was to explore whether and to what extent parr from different S. trutta spawning streams and hatcheries can be differentiated, using water chemistry and otolith microchemistry. Water chemistry samples demonstrated significant spatial variation between different study regions (Estonia and Finland); Sr, Mg, Fe, Zn, Ca and Cu contributed most to the separation. Parr collected from different sites showed a significant difference in otolith element:Ca and Sr-87:Sr-86 ratios on a smaller scale, including only Estonian samples (n = 361), and on a larger scale incorporating parr from Estonia, Latvia and Finland (n = 564). The reclassification rate of S. trutta parr was on average 73%, being highest in Finnish streams and hatcheries, followed by Latvia and Estonia. Otolith Sr-87:Sr-86 and Sr:Ca ratios were the most important in discriminating fish from different sampling sites. The present study demonstrates the potential of separating S. trutta on various spatial scales including different countries and neighbouring streams, making this the most extensive otolith microchemistry study to date in terms of streams and regions studied. The established reclassification rules form a basis for future work investigating the natal origin of adult S. trutta caught in the mixed-stock fishery in the sea.

Direct abundance estimates obtained during the acoustic surveys are essential in stock assessment and management process of pelagic fish stocks. The conventional methodology of surveys includes direct hydroacoustic measurements of fish abundance along the pre-defined transects, combined with biological data, obtained from trawl hauls. Considerable amount of survey time and resource are currently allocated to steaming along transects. High operational costs of the research vessel are the major issue in performing such surveys. However, the advances in modern technologies potentially allow to overcome this problem by deploying the autonomous surface vehicles (ASV) for the routine acoustic measurements. A hybrid solution of reduced survey vessel usage by allocating suitable tasks to ASV, would be the most cost-effective solution, but currently not fully compatible with the requirements of maintaining the long -term time series. We argue that autonomous vessels should be considered as an economical alternative and tested through pilot studies.

The Northern pike (Esox lucius) is an important predator in freshwater and brackish water environments. In the Baltic Sea, Northern pike follows isolation by distance pattern across wide geographical scale. However, relatively little is known about the fine-scale population divergence and temporal genetic variation of Baltic pike, especially for populations inhabiting the eastern part of the Baltic Sea. Here, we characterize the population genetic structure of Northern pike and its temporal stability in six freshwater and one brackish water locations on the Island of Saaremaa (Estonia). To accomplish this, we collected 274 young-of-the-year (YOY) of two consecutive cohorts and 218 older specimens that were genotyped using ten microsatellite loci. Our results indicate a temporally stable population differentiation between geographically close locations and reduced diversity of the north-western Saaremaa populations. Furthermore, the correspondence analysis and a discriminant analysis of principal components showed a clear differentiation between north-western and the Gulf of Riga spawning grounds. Our findings support the idea that natal homing behavior, limited dispersal and facultative anadromy in Baltic pike likely promotes the existence of temporally stable genetic divergence among different populations at small geographical scale, representing an essential prerequisite for development of local adaptations. Thus, conservation, management and restoration efforts of coastal Baltic Sea pike should preserve the existing genetic differences between different populations and avoid actions, which potentially increase genetic homogenization.

The Gulf of Riga stock of Baltic herring (Clupea harengus membras L.) has been maintained through several management tools. One such tool is the restriction of vessels’ main engine power (<221 kW). This restriction was implemented in the early 1990s and is based on the vessel types available in the area and on the assumption that the gear size used in trawl fishery depends on the vessel size (power). In the current study, we compared vessels with different engine powers using the same gears currently allowed in the gulf, to identify whether vessel power had any relation to catch structure. The results showed that engine power did not explain the differences in catch structure, which were more dependent on season and depth of water. Easing the power restriction of the trawl vessels in the Gulf of Riga will most likely not have a major negative impact on the sustainable management of the herring population. However, vessels with higher engine power should not use larger trawl gear than is currently used in the gulf.

Ecosystems are simultaneously regulated by bottom-up (resources) and top-down forces (predators). However, because predator-prey interactions operate on spatial scales beyond the reach of manipulative experimentation, the actual roles of predators remain poorly understood. In ecosystems where predators are naturally absent, biological invasions provide a unique experiment to shed light on the large-scale and long-term effects of predators in the recipient ecosystem. We combined data from long-term benthic monitoring, environmental conditions, and a census of round goby population to identify changes in the dominant benthic bivalve population (Mytilus trossulus and Macoma balthica) following round goby (Neogobius melanostomus) invasion. This study demonstrates a substantial change in the food web, with earlier primarily bottom-up regulated communities facing strong top-down control effects. The introduction of a novel predator to a species-poor ecosystem substantially reduced dominant invertebrate populations in a large part of the study area, sometimes with a time lag of 1−3 years. In general, the effects were less in high productivity areas. In some cases, community recovery was observed but rarely to pre-invasion levels, raising the question of the ability of bivalve communities to act as a natural filter. This study demonstrates long-term trends in benthic communities and by excluding possible effects due to the changing environment provides more evidence that invasive species transform coastal ecosystems over time and space.

During the last decades the number of grey seals Halichoerus grypus has increased more than tenfold in the Baltic Sea, having reached 38 000 in 2019, which has created increasing problems to the coastal fisheries.In the current study, 151 Estonian coastal fishermen were interviewed to sum all direct and indirect monetary losses.The total cost of the damage caused by seals to the Estonian fisheries was around 0.9 million euros in 2009.With the aim of mitigating seal impact, Acoustic Harassment Devices (AHDs, Lofitech) were tested.This study presents quantitative data on seal damage in Estonian trap-net fisheries and analyses the results of the attempts made to mitigate the damage by means of AHDs.It can be concluded that AHDs provided sufficient protection against the seal attacks.

For the first time, an overlooked aspect of partial migration was quantified using otolith microchemistry and brown trout, Salmo trutta, as a model species. Relative contributions of freshwater resident and anadromous female brown trout to mixed-stock sea trout populations in the Baltic Sea were estimated. Out of 236 confirmed wild sea trout sampled around the coast of Estonia 88% were of anadromous maternal origin and 12% were of resident maternal origin. This novel finding underscores the importance of the resident contingent in maintaining the persistence and resilience of the migratory contingent.

To stop the decline of the European eel population, one of the measures taken is translocating eels for restocking, despite its conservational value being largely unknown. We aimed to contribute to this knowledge gap by (i) investigating the origin of eels caught in coastal waters of Estonia and Finland using otolith microchemistry and (ii) directly estimating restocked eel escapement from Narva River Basin District (NRBD), which is part of the primary Eel Management Unit in Estonia. In Estonia, 74% of the sampled eels (n = 140) were natural recruits and 26% were restocked. In Finland, 27% of the sampled eels (n = 235) were natural recruits and 73% were restocked. Only 1% of all the coastally collected eels were originally restocked to NRBD. These new data together with the reported commercial landings from the escapement route conflict with the current silver eel escapement estimation for NRBD and question the accuracy and value of such indirect calculations compiled for most Eel Management Units throughout the European Union. It is concluded that restocking eels to freshwaters may be futile as a conservation measure in some situations, and better escapement is likely achieved in restocking eels to coastal waters or undammed freshwater systems with a direct connection to the sea.

The growing grey seal (Halichoerus grypus) population in the Baltic Sea has created conflicts with local fisheries, comparable to similar emerging problems worldwide. Adequate information on the foraging habits is a requirement for responsible management of the seal population. We investigated the applicability of available dietary assessment methods by comparing morphological analysis and DNA metabarcoding of gut contents (short-term diet; n = 129/125 seals, respectively), and tissue chemical markers i.e. fatty acid (FA) profiles of blubber and stable isotopes (SIs) of liver and muscle (mid- or long-term diet; n = 108 seals for the FA and SI markers). The methods provided complementary information. Short-term methods indicated prey species and revealed dietary differences between age groups and areas but for limited time period. In the central Baltic, herring was the main prey, while in the Gulf of Finland percid and cyprinid species together comprised the largest part of the diet. Perch was also an important prey in the western Baltic Proper. The DNA analysis provided firm identification of many prey species, which were neglected or identified only at species group level by morphological analysis. Liver SIs distinguished spatial foraging patterns and identified potentially migrated individuals, whereas blubber FAs distinguished individuals frequently utilizing certain types of prey. Tissue chemical markers of adult males suggested specialized feeding to certain areas and prey, which suggest that these individuals are especially prone to cause economic losses for fisheries. We recommend combined analyses of gut contents and tissue chemical markers as dietary monitoring methodology of aquatic top predators to support an optimal ecosystem-based management.

Seals and fish-eating birds have increased in the Baltic Sea and there is concern that they compete with fisheries. Using data from around year 2010, we compare consumption of different fish species by seals and birds to the catch in the commercial and recreational fishery. When applicable this is done at the geographical resolution of ICES subdivisions. Predation by birds and mammals likely has limited impact on the populations of the commercially most important species (herring, sprat, and cod). In the central and southern Baltic, seals and birds consume about as much flatfish as is caught by the fishery and competition is possible. Birds and seals consume 2-3 times as much coastal fish as is caught in the fishery. Many of these species are important to the fishery (e. g. perch and whitefish) and competition between wildlife and the fishery is likely, at least locally. Estimated wildlife consumption of pike, sea trout and pikeperch varies among ICES subdivisions and the degree of competition for these species may differ among areas. Competition between wildlife and fisheries need to be addressed in basic ecosystem research, management and conservation. This requires improved quantitative data on wildlife diets, abundances and fish production.

Dynamics of coastal fish assemblages in the Baltic Sea are still rather poorly understood. In particular, little information is available on migrations and movements of the small-bodied littoral fish species like Eurasian minnow (Phoxinus phoxinus L.). Minnow is considered typically as a river and lake species, but it also inhabits brackish coastal waters of the Baltic Sea. In this study, we investigated movement patterns of brackish water inhabiting minnows using otolith microchemistry. Fish were collected from four different sites (from two bays and two stream mouths) around Saaremaa Island. The results indicated that at least three distinct migration patterns exist: (i) seawater residency, (ii) fast springtime migrations to fresh water or to bays with lower salinities (areas near stream mouths) and (iii) prolonged migrations to fresh water (some cases overwintering in streams). Migration patterns listed above were not evenly distributed among sites, and some individuals did not migrate to fresh water in every year. None of the analysed fish were freshwater residents. Additionally, potentially ontogenetic effects on Mn and Sr concentrations were observed in the otolith core regions, which may have important implications for the interpretation of otolith chemistry data. The overall findings of this study demonstrate that much more complex migration patterns exists in brackish water inhabiting minnow populations than previously thought and at least some individuals do enter freshwater to reproduce.

As discussions and debates are crucial to science, we appreciate the comments by Heikinheimo et al. (in press) on our article on competition for Baltic Sea fish resources between fishery and wildlife. We cannot see that the comments by Heikinheimo et al. changes the general conclusion derived in our original article-that there are cases of competition between wildlife and fisheries in the Baltic Sea, although not for all species and not to the same extent everywhere. Our responses are structured in the same order as the comments by Heikinheimo et al.

Many coastal and offshore fish species are highly dependent on specific habitat types for population maintenance. In the Baltic Sea, shallow productive habitats in the coastal zone such as wetlands, vegetated flads/lagoons and sheltered bays as well as more exposed rocky and sandy areas are utilized by fish across many life history stages including spawning, juvenile development, feeding and migration. Although there is general consensus about the critical importance of these essential fish habitats (EFH) for fish production along the coast, direct quantitative evidence for their specific roles in population growth and maintenance is still scarce. Nevertheless, for some coastal species, indirect evidence exists, and in many cases, sufficient data are also available to carry out further quantitative analyses. As coastal EFH in the Baltic Sea are often found in areas that are highly utilized and valued by humans, they are subjected to many different pressures. While cumulative pressures, such as eutrophication, coastal construction and development, climate change, invasive species and fisheries, impact fish in coastal areas, the conservation coverage for EFH in these areas remains poor. This is mainly due to the fact that historically, fisheries management and nature conservation are not integrated neither in research nor in management in Baltic Sea countries. Setting joint objectives for fisheries management and nature conservation would hence be pivotal for improved protection of EFH in the Baltic Sea. To properly inform management, improvements in the development of monitoring strategies and mapping methodology for EFH are also needed. Stronger international cooperation between Baltic Sea states will facilitate improved management outcomes across ecologically arbitrary boundaries. This is especially important for successful implementation of international agreements and legislative directives such as the Baltic Sea Action Plan, the Marine Strategy Framework Directive, the Habitats Directive, and the Maritime Spatial Planning Directive, but also for improving the communication of information related to coastal EFH among researchers, stakeholders, managers and decision makers. In this paper, efforts are made to characterize coastal EFH in the Baltic Sea, their importance and the threats/pressures they face, as well as their current conservation status, while highlighting knowledge gaps and outlining perspectives for future work in an ecosystem-based management framework.