Many commercially exploited fish stocks have declined over the last few decades. It is therefore essential to identify natural populations and understand local adaptation for sustainable management. Salinity is a key environmental factor shaping local adaptation, and adaptive trait divergence often occurs at the egg and larval stages. The strong salinity gradient in the brackish Baltic Sea has driven rapid adaptation in multiple taxa. The Eastern Baltic cod (Gadus morhua) has adapted to low salinity with buoyant and tolerant eggs and larvae, but the stock has declined both in abundance and geographical range during the last decades. The main reproduction area of this stock is in the Bornholm Basin in the southern Baltic Proper. Cod in this area, however, exhibit stunted growth and small body sizes. In contrast, large and healthy cod in reproductive condition are found in the Åland Sea in the northern Baltic Proper, more than 400 km away, where environmental salinity is too low for successful reproduction. This raises the question of whether these fish represent a locally adapted population capable of successful reproduction in the lower salinities (5-10 psu in the northern Baltic Proper). Here, we experimentally assessed egg and yolk-sac larvae survival across salinities, egg size, egg and larval neutral buoyancy and egg survival on sediment, to test whether northern (Åland) cod show adaptation to low salinity at early life stages as compared to southern cod. Mortality of larvae increased with decreasing salinity in cod from both areas, with the lowest survival at 7 psu. At 9 psu, more than 50% of northern cod larvae survived, suggesting that development could occur in the northern region. Egg size and buoyancy were similar between northern and southern cod, and eggs and larvae were negatively buoyant, sinking under local salinity conditions. Nevertheless, the eggs survived and hatched well on sediment, indicating potential for demersal spawning. Our findings show no strong evidence of adaptive divergence to lower salinity in northern cod; however, their ability to tolerate sediment contact at early life stages suggests that Eastern Baltic cod may reproduce outside their historical spawning grounds.

Coastal marine ecosystems are increasingly affected by rising temperatures, declining oxygen and other human-driven stressors, which can alter predator-prey interactions. In the Baltic Sea, native fish and invasive comb jellies interact within a changing physical environment, with outcomes likely impacted by their different hypoxia tolerances. We investigated whether the abundant, native three-spined stickleback (Gasterosteus aculeatus) feeds on larvae (similar to 400 mu m) of the invasive comb jelly Mnemiopsis leidyi. Experiments were conducted under normoxic (100% O-2) and moderate hypoxic (30% O-2) conditions. We here present the first evidence of fish predation on M. leidyi larvae. Oxygen conditions had a significant impact on feeding rates (P < 0.001) and hypoxia led to a 68% and 69% reduction in G. aculeatus feeding on M. leidyi and the copepod Acartia tonsa, respectively. Consequently, while sticklebacks can ingest M. leidyi larvae, even moderate hypoxia will reduce its predation pressure on M. leidyi larvae and moreover reduce its potential to compete with larger M. leidyi for the common copepod prey A. tonsa.

Invasive species often cause ecological damage, including competition with native species for space, food and survival. One of these invasive species is the round goby (Neogobius melanostomus), a fish introduced into fresh and brackish waters around the world. To understand how round gobies perform if they continue spreading into higher-salinity oceans, we investigated how individuals differ in physiological and behavioural (co)sensitivity to salinity changes. We measured growth, metabolic rates, activity, dispersal and exploration of 59 individuals in the lab after acclimation to salinities ranging from freshwater to seawater (0, 10, 19, 24 and 28 ppt), and quantified trait variation and sensitivity to salinity as reaction norm intercepts and slopes. The fastest-growing individuals at 10 ppt-the salinity the fish were caught at-were also the most active and explorative at this salinity and thus more likely to spread. However, above 15 ppt, these individuals lost their growth advantage and experienced the most severe reductions in aerobic capacity (maximum metabolic rate and aerobic scope). As growth and aerobic capacity are linked to survival and performance in nature, this cross-over at 15 ppt is likely to determine the phenotypic composition of round goby populations across salinities and their continued invasion success and ecological impact.

Teleost fish rely on multiple sensory systems to navigate in the aquatic environment, yet these mechanisms are often scarcely known during their early life history stages. Salinity is an important environmental parameter affecting the performance of fish larvae, and recent studies have shown that some larval fish use it as an environmental cue to position, orientate and navigate. Further, fish larvae are often visual feeders, and light has been suggested to be a crucial environmental cue for visual orientation and feeding. In this study, we investigate whether first-feeding larvae of eastern Baltic cod (Gadus morhua callarias) are driven by light and salinity. We examine this in the context of their natural vertical distribution and movement; from deeper, darker and more saline (13-17 psu) water layers towards the more light-exposed and less saline (7 psu) surface water. Further, we investigate the relationship between this movement and the neutral buoyancy measured both in active and passive larvae. We found that the larvae remain in a saline environment close to the salinity which supports their neutral buoyancy. They show negative phototaxis, suggesting an active avoidance of the lighter and less saline upper water layers. These results improve our understanding of the ecophysiology and behaviour of larval fish during the first-feeding stage and serve as an important input for restocking efforts of eastern Baltic cod and potential future production of hatchery rearing of this population.

The aerobic metabolic scope (AMS) of marine ectotherms, a measure of their energetic potential in relation to environmental conditions, can be adopted as a metric to support the prediction of their habitat distribution through time and space. Considering that climate change will exacerbate the negative effects associated with the introduction of non-indigenous species in marine ecosystems, the prediction of their thermal habitat suitability is a central task in ecology and conservation. In this context we proposed a methodology to (i) infer the suboptimal AMS of an invader and (ii) to map its suitable thermal habitat during the past decades and in a global warming scenario. Specifically, we combined sightings records, known physiological models of aerobic performance and sea surface temperatures to determining their effects on the AMS of the fish. The methodology has been tested in the Baltic Sea that is currently displaying the highest increase in sea surface temperature of the world’s large marine ecosystems and that hosts the invasive round goby (Neogobius melanostomus), a fish of Ponto-Caspian origin. As the most widely dispersed invasive fish in the world this species has produced vast ecological impacts in colonized environments including the alteration of benthic invertebrate composition through predation and competition with native fish for food, shelter, and spawning grounds. Round goby, introduced to the Baltic Sea about 30 years ago, is well established in the southern and central parts of the basin but is still absent, aside from sporadic observations, from the north and coldest areas of this region. Its distribution is likely to expand beyond the current cold boundaries due to predicted climate change. However, the proportion of the currently uninvaded areas of the Baltic Sea that will become suitable thermal habitat for round goby, and how fast it can be expected to occur, remain uncertain. Via AMS mapping, we described changes in the round goby thermal 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 considering the current climate models predictions. Notably, although the predicted new thermal conditions do not cause any reduction in the AMS of round goby populations, the wintertime cold ranges are likely expected to preserve substantial areas from invasion. The results of this research are intended to guide future monitoring programs, increasing the chance to detect this invader in novel areas, and to enhance the reliability of the projected changes in ecological models that incorporate the thermal safety margins of native and non-indigenous species.

Non-indigenous species (NIS) are on a rise globally. They can pose strong impacts on ecosystems in their non-native range and can therefore be a serious threat to biodiversity. Here, we compile the existing information available regarding the extent to which commercially and recreationally used fish stocks (exploited fish species) are affected by NIS. To do so, we conducted a literature review to summarize the known and presumed impacts of four case study NIS with already known strong effects on the Baltic Sea ecosystem: round goby, Neogobius melanostomus, sea walnut, Mnemiopsis leidyi, mud crab, Rhithropanopeus harrisii, and fishhook water flea, Cercopagis pengoi. We found that round goby, mud crab and fishhook water flea are documented to serve as a new food source for native fish species, while sea walnut and fishhook water flea are supposedly affecting planktivorous fish through resource competition. Round goby is very likely a strong competitor for the benthivore fish community. There are also indications that it feeds on juvenile fish and fish eggs. Generally, our results show that large knowledge gaps exist, while the published impacts on exploited fish species are often solely based on correlations (e.g. decreasing abundances of native species with cooccurring increasing abundances of NIS), regionally restricted studies or expert judgements. In addition, many studies are older and the current population size of the NIS, which is obviously associated with their impact, is unknown. Thus, the majority of described impacts of NIS on commercially and recreationally used fish stocks seems to stem from assumptions. Therefore, more field observations and experimental studies are needed to be able to scientifically evaluate the impact of NIS. Nevertheless, in this review, the available information was summarized, even if they are speculative, and specific knowledge gaps were identified. Moreover, we outline further investigations that are needed to advance our mechanistic understanding of the interactions between NIS and exploited fish species in the Baltic Sea. This knowledge is essential for the sustainable management of aquatic resources and management of NIS of the Baltic Sea.

Predatory fish in the wild and in particular Eastern Baltic cod Gadus morhua are facing varying and unpredictable feeding opportunities. This implies a high assimilative capacity including nutritional state-related hyperphagia to maintain long-term positive energy balance. Heavy infection of cod livers by the parasitic nematode Contracaecum osculatum is suspected to jeopardize the assimilative capacity and thus the vital rates, growth, reproduction and mortality of the cod population. We examine the effect of C. osculatum on growth, condition and liver weight of cod in laboratory experiments with ad libitum feeding. We find that highly infected cod displays reduced hepatosomatic index and reduced or negative growth and change in condition factor. We suggest that in the wild, cod also with lower infection levels may be constrained by an impaired liver capacity as they here also face feeding constraints. Measurements of growth hormone expression together with knowledge about the endocrine regulation of appetite and growth are used to explain the data. We find strong indications that for cod with high infection level this plays a significant role for all the three vital population rates, and that these effects should be considered in the management of the cod stock in the eastern Baltic Sea.

The skin microbiome contributes to host health by facilitating pathogen exclusion, priming the immune system modulation and maintaining skin homeostasis. We examined how the composition of the skin microbiota is associated with body size and condition in two populations of eastern Baltic cod (Gadus morhua callarias), at Bornholm basin (ICES subdivision 25) and in the Aland Sea (subdivision 29). These locations are characterized by differences in body size and experience distinct hydrographic conditions, including salinity, oxygen and temperature. We used 16s rRNA gene metabarcoding to compare skin microbiota diversity and composition in these populations. We found significant differences between the populations in both alpha and beta diversity, with higher microbial diversity observed in the Aland cod characterized by larger body size. Moreover, we identified a greater number of significantly enriched Gene Ontology (GO) metabolic pathways in Aland cod as compared to those from the Bornholm Basin, with many pathways absent in Bornholm. However, other GOs, such as those associated with the nervous system, were uniquely detected in the Bornholm cod. Differences in microbial communities can be influenced by external environmental factors, determined by the habitats where individual cod reside. These results indicate a potential relationship between the skin microbiome and host health; however, direct evidence for a causal impact remains limited, highlighting the importance of incorporating host skin microbiota interactions in studies of aquatic populations. ### Competing Interest Statement The authors have declared no competing interest. Swedish Research Council, 2021–01701 BALTICWATERS2030

An animal's behavioural traits can influence the outcomes of ecological interactions within their food web, including what they eat, their vulnerability to predation and who they compete with. Despite this, few studies have directly measured links between among-individual behavioural and trophic variation. Invasive species like the round goby, Neogobius melanostomus, are often found to have consistent among-individual differences in behaviour within and between populations across their invasion front. Therefore, an individualized approach to invasive populations and their ecological interactions may be valuable to understanding their impacts on recipient ecosystems. Using nonlethal methods to measure trophic variation (i.e. stable isotope analysis via fin clips) and passive individual tagging, we analysed behavioural trait/personality variation and trophic variation to explore links between the two. Focusing on an established population of round gobies in the Guldborgsund strait in the southwest Baltic Sea, we found significant among-individual variation in bold–exploratory traits in novel environment and refuge emergence assays. We also found strong intraspecific trophic variation, with particularly high variation in carbon-12 – carbon-13 (δ13C), suggesting that individual round gobies differ in what they are feeding on and/or where they forage. Diet reconstruction results support previous studies showing that gastropods and bivalves are major contributors to their diet, but the large differences in isotope values suggest that individual variation influences how they interact with prey communities. There were few links between behavioural and trophic variation; nevertheless, this study shows that measuring behavioural–trophic links is a viable approach for exploring whether and how behavioural traits may influence individual level ecological variation.

This study examines the sediment preferences of small sandeel (Ammodytes tobianus), an elongated forage fish common in marine and brackish environments of northern Europe. Sandeel have a high fidelity for sandy habitats and spend much of their lives buried, as an important part of both their diel behavioural cycles and overwintering behaviour. A series of independent choice laboratory assays were conducted using wild-caught schools of small sandeel, to: (1) determine their preferred substrate composition for burial; (2) identify the upper limits of sediment grain-sizes that may be utilised for burial, and; (3) investigate the effects of light intensity on burial behaviour. Initial experiments found a clear preference for burial within sediments composed of coarse sand and fine gravel (0.5-4.0 mm) but showed at least some utilisation of both coarser and finer sediment compositions. In further trials, burial was found to be almost entirely eliminated in sediments that contained significant gravel components >4.0 mm. Light manipulation experiments also showed that light intensity was an important factor that influences their choice of burial area. This study is the first to experimentally investigate the sediment preferences and grain-size tolerance limits of small sandeel. These results specifically highlight how subtle differences in both sediment composition and light may influence the habitat usage of small sandeel, which may be valuable for understanding their distribution in the wild and to inform future management and conservation for the species.

The effects of pelagic trawling on the health and welfare of Atlantic herring (Clupea harengus L.) were investigated on a refrigerated seawater vessel operating in the North Sea. A total of 495 Atlantic herring (Clupea harengus L.) were sampled during five hauls from two fishing trips in September 2021 and 2022. For assessments of consciousness and mortality, a Reflex Action Mortality Predictor test (i.e. RAMP-test) was used on herring collected following trawling and pumping. Inspections for external and internal damage or wounds were performed via morphological welfare indicators and analyses of photos and radiographs. In addition, blood samples were taken and analysed for haematological indicators of stress. Following trawling and pumping, only 5% of the investigated herring showed signs of external wounds associated with the morphological indicators of welfare, and no internal damage was observed in the radiographic inspections. However, 96% of the assessed herring scored 0 on all three reflexes included in the RAMP-test and were therefore judged dead. On average, herring lost 95% of their scales, while 95% of herring had a very high degree of ruptured red blood cells (i.e. haemolysis). Extensive scale loss results in a deterioration of the skin's protective barrier function, which in turn impairs the osmoregulatory capacity of the herring. This was evident by elevated levels of plasma osmolality and circulating chloride concentrations, which could also likely explain the high occurrence of haemolysis in captured herring. Extended trawling time and larger catch size proved to be two important factors to consider, as the former led to increased plasma levels of osmolality, whereas the latter was associated with elevated plasma levels of lactate and cortisol. In conclusion, the high mortality appears to be influenced by a combination of factors such as severe stress, loss of osmoregulatory ability, crowding density within the trawl, and extended trawling times. This study provides important information on the welfare of wild Atlantic herring caught using pelagic trawls and highlights areas where improvements can be made to safeguard the welfare of fish captured in pelagic fisheries in the future.

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.

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.

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.

A quantity of studies have documented how greater condition and older age of individual fish enhances reproductive success via a variety of processes from egg production to larval survival. If these relations translate into relationships between stock demography and recruitment, maintaining a "healthy" age composition may be a key step towards ensuring stock productivity. We examined the correlation between indicators of the age and size composition and the residual-variation from the stock-recruitment relationship (i.e. the variation not explained by the spawning stock biomass) in eleven commercially exploited fish stocks in the North Sea. We also analyzed to what extent spawner age and size composition responded to variations in fishing mortality. Before detrending to accommodate any temporal patterns, recruitment residuals correlated negatively with the relative age composition in all eleven stocks (plaice, whiting, and saithe being significant). However, after detrending the recruitment residuals, the correlations weakened and only nine out of eleven stocks correlated negatively. Lastly, we also conducted a delay difference analysis, which yielded a more or less even distribution between negative and positive correlations, and in addition two stocks (cod and Norway pout) came out showing significant positive correlations between recruitment residuals and age composition. In contrast, eight of the eleven stocks showed positive correlations between spawner weight and recruitment residuals and correlations were significant for plaice, cod, and saithe. After detrending the data, the significant correlations disappeared, but the overall tendency towards positive correlations between spawner weight and recruitment residuals prevailed. Overall, these results suggest that fish condition is more likely to show positive relationships with recruitment success than age composition. Lastly, it was found that the majority of stocks showed a strong link between fishing mortality and the two indicators of age composition, and when looking across stocks, the age composition appears to have returned to the same level as in the 1960 s. Conversely, spawner weight at age have not recovered, and considering the indications of a positive link between spawner weight and recruitment, recruitment success of the North Sea stocks may remain low in spite of the apparently successful rebuilding of the age composition.

The round goby (Neogobius melanostomus) is a fish native to the Ponto-Caspian region that is highly invasive through freshwater and brackish habitats in northern Europe and North America. Individual behavioural variation appears to be an important factor in their spread, for example a round goby's personality traits can influence their dispersal tendency, which may also produce variation in the behavioral composition of populations at different points along their invasion fronts. To further analyze the drivers of behavioral variation within invasive round goby populations, we focused on two populations along the Baltic Sea invasion front with closely comparable physical and community characteristics. Specifically, this study measured personality within a novel environment and predator response context (i.e., boldness), and directly analyzed links between individuals' personality traits and their physiological characteristics and stress responses (i.e., blood cortisol and lactate, brain neurotransmitters). In contrast to previous findings, the more recently established population had similar activity levels but were less bold in response to a predator cue than the older population, which suggests that behavioral compositions within our study populations may be more driven by local environmental conditions rather than being a result of personality-biased dispersal. Furthermore, we found that both populations showed similar physiological stress responses, and there also appeared to be no detectable relationship between physiological parameters and behavioral responses to predator cues. Instead, body size and body condition were important factors influencing individual behavioral responses. Overall, our results reinforce the importance of boldness traits as a form of phenotypic variation in round goby populations in the Baltic Sea. We also highlight the importance of these traits for future studies specifically testing for effects of invasion processes on phenotypic variation in the species. Nonetheless, our results also highlight that the physiological mechanisms underpinning behavioural variation in these populations remain unclear.

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.

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.

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.

For externally fertilising organisms in the aquatic environment, the abiotic fertilisation medium can be a strong selecting force. Among bony fishes, sperm are adapted to function in a narrow salinity range. A notable exception is the family Gobiidae, where several species reproduce across a wide salinity range. The family also contains several wide-spread invasive species. To better understand how these fishes tolerate such varying conditions, we measured sperm performance in relation to salinity from a freshwater and a brackish population within their ancestral Ponto-Caspian region of the round goby, Neogobius melanostomus. These two ancestral populations were then compared to nine additional invaded sites across northern Europe, both in terms of their sperm traits and by using genomic SNP markers. Our results show clear patterns of ancestral adaptations to freshwater and brackish salinities in their sperm performance. Population genomic analyses show that the ancestral ecotypes have generally established themselves in environments that fit their sperm adaptations. Sites close to ports with intense shipping show that both outbreeding and admixture can affect the sperm performance of a population in a given salinity. Rapid adaptation to local conditions is also supported at some sites. Historical and contemporary evolution in the traits of the round goby sperm cells is tightly linked to the population and seascape genomics as well as biogeographic processes in these invasive fishes. Since the risk of a population establishing in an area is related to the genotype by environment match, port connectivity and the ancestry of the round goby population can likely be useful for predicting the species spread.