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Climate change will exacerbate the negative effects associated with the introduction of non-indigenous species in marine ecosystems. Predicting the spread of invasive species in relation to environmental warming is therefore a fundamental task in ecology and conservation. The Baltic Sea is currently threatened by several local stressors and the highest increase in sea surface temperature of the world’s large marine ecosystems. These new thermal conditions can further favour the spreading of the invasive round goby (Neogobius melanostomus), a fish of Ponto-Caspian origin, currently well established in the southern and central parts of the Baltic Sea. This study aims to assess the thermal habitat suitability of the round goby in the Baltic Sea considering the past and future conditions. The study combines sightings records with known physiological models of aerobic performance and sea surface temperatures. Physiological models read these temperatures, at sighting times and locations, to determine their effects on the aerobic metabolic scope (AMS) of the fish, a measure of its energetic potential in relation to environmental conditions. The geographical mapping of the AMS was used to describe the changes in habitat suitability during the past 3 decades and for climatic predictions (until 2100) showing that the favourable thermal habitat in the Baltic Sea has increased during the past 32 years and will continue to do so in all the applied climate model predictions. Particularly, the predicted new thermal conditions do not cause any reduction in the AMS of round goby populations, while the wintertime cold ranges are likely expected to preserve substantial areas from invasion. The results of this research can guide future monitoring programs increasing the chance to detect this invader in novel areas.

Understanding the characteristics and conditions that make non-indigenous species (NIS) successful at establishing in recipient communities is a key in determining their potential impacts on native species, as well as to improve management actions such as prevention of future invasions. The round goby (Neogobius melanostomus) is one of the most widespread non-indigenous fish species in the Northern Hemisphere, including the coastal zones of the Baltic Sea. The impacts of round goby in the Baltic Sea are pronounced and multifaceted, yet our knowledge regarding the underlying assembly processes determining its establishment is limited. To overcome this knowledge gap, we applied a trait-based approach to assess the degree of niche overlap and functional (trait) similarity between round goby and native fish species in coastal areas from the Baltic Sea, based on the functional distinctiveness metric. Our results show that round goby is generally quite similar (or not dissimilar) to the native fish of the regional species pool, at least in terms of its overall trait composition. Conversely, round goby demonstrates pronounced differences compared to the native community in its display of parental care and territorial behaviour. Such differences in individual traits could play an important role in round goby’s invasion success in the Baltic Sea, including its interactions with native species (e.g. competition). Our results and their potential implications may be highly relevant for conservation and management if integrated within existing risk assessment tools for biological invasions in order to prioritise and enhance the effectiveness of preventative actions towards the expansion of round goby.

The present report describes results of the project ”Future perspectives for cod in the Eastern Baltic Sea–stock development, fisheries and management challenges in a changing ecosystem (FREMTOR)”, funded by European Maritime and Fisheries Fund and the Danish Fisheries Agency. The project provides new knowledge on some key aspects of the status of the stock and associated drivers, as well as synthesizes existing information, altogether providing state-of-the art basis for evaluating future perspectives for this stock. Improved understanding of the factors responsible for the low productivity of the stock is crucially important for being able to assess its recovery potential. Furthermore, realistic targets for stock recovery in the current state of the ecosystem need to take account of changes in biological parameters, including growth and recruitment as well as other indicators of stock health, such as body condition and parasite load. In this project, time series of various biological parameters of the Eastern Baltic cod (incl. condition, size at maturity, parasite load, length structure) were extended back to the 1940s, i.e. the beginning of intensive exploitation of this stock. These data show that the cod stock is presently in unprecedented poor state. Cod body condition and infection load of C. osculatum were in similar adverse states in the 1940s–1950s, however, the relatively more favorable states of other biological parameters back then indicate that the stock was overall in a better shape than is presently the case. The long–term data reveal concurrent trends in seal abundance and C. osculatum infection, the latter being inversely related to cod body condition, which provides correlation based evidence regarding potential associations between these variables. Spatial contrasts in the present state of cod are apparent in cod from the Åland Sea being in a substantially better condition and larger in size compared to the average specimen

The Eastern Baltic cod (Gadus morhua) stock is currently in a very poor state, with low biomass and adverse trends in several life history and demographic parameters. This raises concern over whether and to what level recovery is possible. Here, we look for new insights from a historical perspective, extending the time series of various stock health indicators back to the 1940s, i.e. to the beginning of intensive exploitation of the Eastern Baltic cod. The historical data confirm that the stock deterioration in recent years is unprecedented, as all indicators are presently in their worst states on record. Cod body condition and energy reserves were equally low in the 1940s-1950s, accompanied by high parasitic liver worm infection, comparable to that measured in recent years. However, other stock parameters (size structure, size at maturity, stock distribution) are currently in their worst states over the past 80 years. In contrast, the state of cod in the 1970s to early 1990s that is often perceived as a desirable target, was exceptional, with the most favorable indicator levels in the time series. Long-term observation data reveal concurrent or asynchronous trends in different indicators of stock health and to what extent these have coincided with changes in possible external drivers. In this way, the extended time series contribute to ongoing research on understanding the collapse of the cod and its recovery potential.

Infection levels with the parasitic nematode Contracaecum osculatum in Eastern Baltic cod have increased in the last decades. Eastern Baltic cod is transport host for this parasite that has a high affinity for the liver of the fish. The liver is a highly vital organ and damage to the liver tissue can result in reduced functionality of the organ. Previous studies have revealed that cod with high infections loads reveal impaired physiological performance, reduced nutritional condition and show signs of having a liver disease. Yet, little is known about the pathological changes and inflammatory reactions of the cod liver related to the infections. In this study, we performed histological examinations on 30 Baltic cod livers caught in the eastern part of the Baltic Sea (length; 38 ± 0.9 cm, weight; 454 ± 34.8 gram) and three Sound cod livers (length; 63 ± 2.9 cm, weight; 3396 ± 300.2 gram) to categorize the degree of inflammation and its relation to pathological changes in infected cod livers. We further investigated how C. osculatum infection levels varied with intensity of inflammation and co-infections. We found that high infection loads with C. osculatum caused severe inflammation in the liver tissue of cod and reduced fat content of the hepatocytes. Conspicuous amounts of glycogen were found in the muscle and intestinal epithelial cells of the nematodes and parasitic co-infections occurred more frequently in the most heavily infected livers.

Changes in physiological processes can reveal how individuals respond to environmental stressors. It can be difficult to link physiological responses to changes in vital rates such as growth, reproduction and survival. Here, bioenergetics modelling can aid in understanding non-intuitive outcomes from stressor combinations. Building on an established bioenergetics model, we examine the potential effects of parasite infection on growth rate and body condition. Parasites represent an overlooked biotic factor, despite their known effects on the physiology of the host organism. As a case study, we use the host–parasite system of Eastern Baltic cod (Gadus morhua) infected with the parasitic nematode Contraceacum osculatum. Eastern Baltic cod have during the past decade experienced increasing infection loads with C. osculatum that have been shown to lead to physiological changes. We hypothesized that infection with parasites affects cod growth negatively as previous studies reveal that the infections lead to reduced energy turnover, severe liver disease and reduced nutritional condition. To test this, we implemented new variables into the bioenergetics model representing the physiological changes in infected fish and parameterized these based on previous experimental data. We found that growth rate and body condition decreased with increased infection load. Highly infected cod reach a point of no return where their energy intake cannot maintain a surplus energy balance, which may eventually lead to induced mortality. In conclusion, parasite infections cannot be ignored when assessing drivers of fish stock dynamics.

During the 2010s, Atlantic cod Gadus morhua L. in the eastern Baltic Sea experienced increasing infection loads of the parasitic nematode Contracaecum osculatum (Rudolphi) in their livers. Starting in 2021, a mandatory part of the routine sampling protocol on Baltic monitoring surveys is to assign a liver category to individual cod livers, based on the number of nematodes visible on the liver surface, to follow spatiotemporal changes in nematode infection loads. The validity of the liver category method has never been evaluated. Based on data from 642 cod livers, the method was verified and found to be a good predictor of the total number of nematodes. Moreover, the probability of cod being in a critical condition increased with the parasite load. In addition to their direct applicability to Baltic cod, the present findings may inspire others working with disease in fish stocks to include parasite monitoring.

The goal of the ICES Working Group on the Introduction and Transfers of Marine Organisms (WGITMO) is to contribute to the ICES vision and mission by addressing specific science objectives related to the introduction, spread and impacts of non-indigenous marine species (NIS). The expert working group provides information and advice on the impacts of human activities (e.g. pressures caused by invasive species, litter, aquaculture, biofouling) on marine ecosystems including the Arctic. The Terms of Reference include annual national reporting on NIS detections and research, climate change impact on the spread of NIS, the risk and impacts of NIS, vectors of spread of NIS including biofouling, marine debris and aquaculture. This report summarizes the key findings and outcomes from the 2020–2022 term. A joint meeting in (2020) with WGBOSV [Ballast Water and Other Ship Vectors] and WGHABD [Harmful Algal Bloom Dynamics] was held to discuss scientific issues of joint interest including climate change in the Arctic environment, and early detection of and response to species through the use of innovative molecular tools. The discussion on molecular tools was used by the groups to develop the framework for a targeted Molecular Tools workshop in conjunction with at the International Conference for Marine Bioinvasions (ICMB) in May 2023. National activities were presented and discussed from over 20 countries each year. Common themes included, new species introduction and monitoring, recent activities on traditional and innovative methods for detection and surveillance (including molecular tools). Science highlights include the submission of the ICES biofouling viewpoint to the International Maritime Organization (IMO) regarding biofouling guidelines, development and publication of a global scale NIS screening tool, and publication of a global review of NIS trends over 50 years. The Information system on aquatic non-indigenous and cryptogenic species AquaNIS continu

The ecosystem approach to fisheries management is a key element in the Common Fisheries Policy (CFP). In light of the importance of the North Sea to the Danish fisheries, Denmark needs to take a leading role in the development of best practice and methods for the implementation of the ecosystem approach to fisheries management. The ECOMAN project consisted of five separate work packages. WP1: Sampling designs should ensure the provision of accurate data on biological parameters and stock information to be used as input to single-species assessments and multi-species models. Via this project, we were able to engage in discussions and thereby influence the development of data collection approaches. WP2: The productivity of fish stocks in the North Sea has undergone substantial changes in recent decades, with some species increasing in numbers, while other species have shown prolonged recruitment failure with a resulting decline in stock size. Here we investigated multiple aspects of stock productivity in order to identify important drivers, such as density dependent regulation and spawner quality; all of which are ecological processes not accounted for conventional stock-by-stock fisheries management. Also methods for estimating biomass reference points were scrutinized and changes over time in the environmental conditions were visited. The latter results provided a basis for improving how reference points are defined and which processes needs further attention (i.e. in management strategy evaluations). WP3: Natural mortality is a key component of estimation of historical stock development and the setting of reference points such as Bescapement, MSY Btrigger, FMSY and Fcap. The natural mortality varies considerably over time, and particularly the mortality introduced by grey seal and mackerel has increased for North Sea fish stocks in the most recent decades. The present project was used to built an application that allow scientists and stak

The aim of the Workshop on Stickleback and Round Goby in the Baltic Sea (WKSTARGATE) was to update, summarize and synthesize the current knowledge of both species with respect to their ecology and potential use in fisheries. Ongoing projects were presented in the mornings of the first two days to inform participants about current research in the respective countries (Annex 5). The group discussed ecological aspects that are relevant to fisheries, such as distribution and abundances, feeding ecology and behaviour. So far, no dedicated monitoring exists for either of the species. However, the Baltic International Acoustic Survey (BIAS), coordinated by ICES, has been used to estimate abundances and biomasses of pelagic stickleback in offshore areas across the Baltic, while the availability and quality of coastal data differs between countries, but is generally scarce. Even less current abundance information is available for round goby. It is only available for areas where it is already commercially exploited like Latvia. Most studies on the ecological impact of these species focus on trophic interactions, and its resulting competition. It became clear, that these ecological impacts depend on the abundance of the species and that the effects therefore vary on a temporal and spatial scale. Estimating the ecological impact of a stickleback or round goby fishery is therefore very speculative without reliable abundance data, why the group decided to describe the known impact of the species on the ecosystem. Landings of round goby increased significantly in Latvia during the last years, while stickleback landings decreased over the last five decades but most recently show a slight increase. A targeted round goby fishery exists in Latvian waters, while the stickleback is solely taken as bycatch in all Baltic countries. Both fisheries are currently unmanaged and, with the exception of round goby fishery in Latvia, unregulated. In Latvia the round goby fishery

During the 2010s, Atlantic cod Gadus morhua L. in the eastern Baltic Sea experienced increasing infection loads of the parasitic nematode Contracaecum osculatum (Rudolphi) in their livers. Starting in 2021, a mandatory part of the routine sampling protocol on Baltic monitoring surveys is to assign a liver category to individual cod livers, based on the number of nematodes visible on the liver surface, to follow spatiotemporal changes in nematode infection loads. The validity of the liver category method has never been evaluated. Based on data from 642 cod livers, the method was verified and found to be a good predictor of the total number of nematodes. Moreover, the probability of cod being in a critical condition increased with the parasite load. In addition to their direct applicability to Baltic cod, the present findings may inspire others working with disease in fish stocks to include parasite monitoring.

This study examines the impact of boldness on foraging competition of the highly invasive round goby Neogobius melanostomus Pallas 1815. Individual risk tolerance, or boldness, was measured as the time to resume movement after a simulated predation strike. Fish that resumed movement faster were categorized as "bold," fish that took more time to resume movement were categorized as "shy" and those that fell in between these two categories were determined to have "intermediate" boldness. Competitive impacts of boldness in N. melanostomus were determined in a laboratory foraging experiment in which interspecific (juvenile Atlantic cod Gadus morhua Linnaeus 1758) and intraspecific (intermediate N. melanostomus) individuals were exposed to either bold or shy N. melanostomus competitors. G. morhua consumed fewer prey when competing with bold N. melanostomus than when competing with shy N. melanostomus, whereas intermediately bold N. melanostomus foraging was not affected by competitor boldness. Bold and shy N. melanostomus consumed similar amounts of prey, and the number of interactions between paired fish did not vary depending on the personality of N. melanostomus individuals. Therefore, intraspecific foraging competition was not found to be personality dependent. This study provides evidence that individual differences in boldness can mediate competitive interactions in N. melanostomus; nonetheless, results also show that competition is also governed by other mechanisms that require further study.