Changes in environmental conditions resulting from climate change are linked to shifting species' distribution ranges. While water temperature is a well-established determinant of fish biogeography, the role of salinity has received comparatively less attention, while there is ample evidence that salinity tolerance is temperature-specific. Grey mullets (Mugilidae family) are considered climate change indicator species due to their strong temperature dependence. Although mullets are euryhaline, salinity gradients have been shown to spatially structure grey mullet communities, potentially influencing their distribution and interactions with thermal niches. To improve predictions of how mullets may respond to climate change, we examined how salinity influences temperature preference in juvenile thinlip mullet (Chelon ramada)-a euryhaline species common to temperate estuaries. Using a circular water temperature preference chamber, we found that juvenile C. ramada consistently preferred a temperature of 26.2 +/- 0.1 degrees C, independent of salinity treatments (5 and 25 g/kg). This was contrary to osmoregulatory theory, which predicts that salinity may influence thermal preference due to the energetic cost of maintaining homeostasis. Instead, our findings indicate that, within the tested range, salinity does not modulate thermal preference in juveniles-suggesting that temperature alone may be a sufficient predictor of habitat selection under future climate scenarios. These results provide novel, species-and size-specific insights that can directly inform model predictions of thinlip mullet distribution in warming estuarine and coastal systems.

This study examines how southern wintering areas may contribute to organochlorine (OCs) loads in arctic seabirds during breeding. Light-sensitive geolocators (GLS loggers) were deployed on Arctic skuas (Stercorarius parasiticus) in one high arctic and two subarctic colonies. Hexcahlorobenzene (HCB), chlordanes, mirex, p,p'-dichlorodiphenyldichloro-ethylene (p,p'-DDE), and polychlorinated biphenyls (PCBs) were measured in the blood of breeding adults at the nest (58 individuals, a total of 128 samples) in northern Norway and Svalbard between 2009 and 2015. We compared OC concentrations and OC profiles among nesting skuas wintering in five Atlantic regions, determined by the GLS loggers: the coast of Argentina, the Caribbean, off West Africa, off the coast of southern Africa, and the Mediterranean Sea. As predicted, HCB, which is semi-volatile and has high long-range transport potential, showed high prevalence in birds wintering in all regions except the Mediterranean. Mirex showed the highest prevalence in birds wintering off the coasts of Argentina and southern Africa, in accordance with high background levels previously documented in the Southern Ocean. Chlordanes were particularly prevalent in skuas wintering off southern Africa, whereas p,p'-DDE seemed relatively evenly distributed among wintering areas. As predicted, the prevalence of PCBs was much higher in birds wintering in the Mediterranean Sea than in birds from other regions. This study thus suggests that the Mediterranean Sea and the mid- and southern Atlantic are essential sources of different OCs in the blood of Arctic skuas breeding in the European Arctic.

Most fish species require different habitats to complete their life cycle, with coastal areas often playing a crucial role as nursery areas for juveniles. The Wadden Sea is an important nursery area for juvenile fish in the North Sea ecoregion. Despite extensive research on the nursery function of the Wadden Sea for demersal species, its role in the life cycle of small pelagic fish (SPF) remains largely unknown. This limits our understanding of the Wadden Sea ecosystem, as SPF are the dominant component of the overall fish biomass and serve as important food source for higher trophic levels. We studied the role of the Wadden Sea in the life cycle of Atlantic herring (Clupea harengus), European sprat (Sprattus sprattus), European smelt (Osmerus eperlanus) and European anchovy (Engraulis encrasicolus) through monthly stow net and seasonal trawl surveys. Our study showed that SPF use the Dutch Wadden Sea primarily as juvenile habitat, with herring being the dominant marine juvenile representative. Length frequency distributions and genetic analysis revealed that the juvenile herring originated predominantly from southwestern waters, such as the English Channel. Additionally, the Wadden Sea still provides spawning grounds for herring and anchovy, with no substantial spawning observed for sprat. While smelt can complete nearly its entire life cycle in the Wadden Sea, it depends on connectivity to nearby freshwater for spawning. In summary, the Wadden Sea functions as a nursery for juveniles, and to a lesser extent as spawning grounds for adults. A thorough understanding of these functions is crucial for identifying bottlenecks and implementing effective conservation and management strategies.

Small fish, including species with small adult sizes and juveniles of larger species, play a central role in marine food webs as prey for top predators, such as seabirds, marine mammals, and piscivorous fish. However, reliable data on small fish are lacking as conventional fisheries surveys primarily focus on larger, commercially important species and underestimate small fish. Consequently, little is known about absolute biomasses, and fine-scale distribution patterns of this important trophic group. Based on 1307 quantitative Triple-D samples from the Dutch Exclusive Economic Zone and the UK sector of the Dogger Bank, biomass densities were estimated for pooled small demersal fish and for the most abundant species individually. Our estimates suggest that small demersal fish biomass is at least twice as high as reported in trawl-based studies. Uniformly distributed species such as dab (Limanda limanda) and plaice (Pleuronectes platessa) contributed most to the overall small fish biomass, while sandeels (Ammodytidae) showed particularly high local densities. These new prey biomass estimates might change our understanding of the North Sea ecosystem's carrying capacity, and establishes a baseline for monitoring changes in small fish communities driven by climate change and human impacts.

Salt marsh conditions and morphology influence the ecological function of marsh habitats. However, little is known about how the interplay between these factors impacts fish in the highly modified marshes typical of north-western Europe. We conducted a two-year survey at two salt marsh areas in the Dutch Wadden Sea to test the influence of marsh creek morphology on in-creek abiotic conditions and the abundance of four numerically dominating fish species representing the different life-history strategies of fish in the marshes. We measured fish abundance and several abiotic factors seasonally, and quantified the morphological attributes of each creek (such as volume), and of the creek basin (such as drainage density and bathymetry). Modelling results revealed a salinity gradient and/or a greater capacity for water retention (creek volume or drainage density) were associated with higher abundances of Atlantic herring (Clupea harengus), European eel (Anguilla), gobies (Pomatoschistus sp.) and the three-spined stickleback (Gasterosteus aculeatus) across most seasons. During recruitment months sticklebacks and gobies were more abundant in brackish water conditions with a higher capacity for water retention. Eels were more abundant in creeks with larger volumes and lower salinities. Larval and juvenile herring were more numerous in brackish water than in water with higher salinity, but an effect of creek morphology on herring abundance was not detected. While recruitment processes were important determinants in the seasonal abundance of most species, creek morphology and abiotic conditions affected the use of salt marsh habitat by the fish species studied. Intertidal marsh creeks that retain water during low tide and have a salinity gradient hold more fish than marshes lacking these characteristics. These findings highlight the effects of local salt marsh conditions on fish assemblages. Taking the specific marsh conditions and their effects on fish into account in salt marsh management can improve outcomes for fish.

Small pelagic fish (SPF) play a vital role in marine ecosystems, transferring energy from plankton to higher trophic levels. However, SPF dynamics in shallow coastal areas are often under-studied, despite their importance as crucial prey near breeding sites of fish-eating birds such as common terns (Sterna hirundo). This study had two connected objectives: to map the pelagic fish food landscape in the inlets of the Dutch Wadden Sea and to describe its use by common terns. Two hydro-acoustic fish surveys were conducted in May and October 2022, while tern distribution was assessed using aerial counts and GPS tracking. Prey selection was analyzed through camera traps and DNA analysis of faeces. Herring and sprat were the predominant species in the SPF community, with sandeel also abundant in May. Smelt was most common in freshwater-influenced areas. Most fish were below 12 cm, fitting the preferred size range for common terns. SPF energy density ranged from 20.1 to 22.1 kJ/g dry weight, with sprat having the highest values. SPF biomass showed significant spatiotemporal variability across transects, inlets, and seasons. Common terns foraged frequently around the gullies, feeding mostly on herring. A significant correlation was found between fish biomass and common tern density, indicating food landscape may be a good predictor of tern distribution and vice versa. This study presents the first integrated biomass estimates of SPF in the subtidal areas of Dutch Wadden Sea for both spring and autumn, providing essential data for future energy budget models and management strategies.

Few experimental studies have been performed on the impact of shrimp fisheries on benthic life. One recent study observed an increase in densities of the Atlantic jackknife clam in hydraulic dredge samples at heavily fished plots and hypothesized that individuals may have been attracted by and immigrated into the disturbed areas (Tulp et al., 2020). We analysed additional box core data taken in the same experiment, and the results could not support this hypothesis. An alternative hypothesis that high densities of clams were coincidentally present in the heavily fished areas already before fishing, but were then too small to be included in the dredge, could neither be confirmed. Additional experiments are required to shed more light on the possible impact of shrimp fisheries on clam abundance and distribution.

Coastal marshes are an important habitat for many juvenile and small fishes, providing refuge and feeding opportunities. Understanding their diets can reveal more about the food web and the underlying factors impacting fish in these threatened habitats. We compared the diets of common goby (Pomatoschistus microps), three-spined stickleback (Gasterosteus aculeatus), Atlantic herring (Clupea harengus), European flounder (Platichthys flesus), European seabass (Dicentrarchus labrax), and European smelt (Osmerus eperlanus). These fish were collected seasonally from Dutch Wadden Sea salt marshes using fyke nets set at fixed stations from three locations with varying degrees of habitat modification. Comparing visual identification and DNA metabarcoding (CO1 region) of stomach contents revealed that DNA (relative read abundance) could be used semi-quantitatively for the dominant prey classes, but failed to detect some prey groups identified visually. The dominant prey of most fish species were the harpacticoid copepod Tachidius discipes, the amphipod Corophium volutator, and other crustaceans including Crangon crangon and Neomysis integer. Diets were affected by season, abiotic conditions, predator length, sampling location, and predator species. Gobies and stickleback shared similar diets, while the four other species displayed distinct diets. Flounder diets were characterised by benthic prey, herring diets consisted predominantly of copepods, smelt were generalists, while seabass diets predominantly contained Malacostraca. Fish diets at the least modified sampling location exhibited the greatest prey diversity. Our findings indicate that salt marshes provide a feeding habitat for the resident and migrant fish species studied, where the degree of marsh habitat modification may affect the marine food web.

Background Migratory birds generally have tightly scheduled annual cycles, in which delays can have carry-over effects on the timing of later events, ultimately impacting reproductive output. Whether temporal carry-over effects are more pronounced among migrations over larger distances, with tighter schedules, is a largely unexplored question. Methods We tracked individual Arctic Skuas Stercorarius parasiticus , a long-distance migratory seabird, from eight breeding populations between Greenland and Siberia using light-level geolocators. We tested whether migration schedules among breeding populations differ as a function of their use of seven widely divergent wintering areas across the Atlantic Ocean, Mediterranean Sea and Indian Ocean. Results Breeding at higher latitudes led not only to later reproduction and migration, but also faster spring migration and shorter time between return to the breeding area and clutch initiation. Wintering area was consistent within individuals among years; and more distant areas were associated with more time spent on migration and less time in the wintering areas. Skuas adjusted the period spent in the wintering area, regardless of migration distance, which buffered the variation in timing of autumn migration. Choice of wintering area had only minor effects on timing of return at the breeding area and timing of breeding and these effects were not consistent between breeding populations. Conclusion The lack of a consistent effect of wintering area on timing of return between breeding areas indicates that individuals synchronize their arrival with others in their population despite extensive individual differences in migration strategies.

Seasonally abundant arthropods are a crucial food source for many migratory birds that breed in the Arctic. In cold environments, the growth and emergence of arthropods are particularly tied to temperature. Thus, the phenology of arthropods is anticipated to undergo a rapid change in response to a warming climate, potentially leading to a trophic mismatch between migratory insectivorous birds and their prey. Using data from 19 sites spanning a wide temperature gradient from the Subarctic to the High Arctic, we investigated the effects of temperature on the phenology and biomass of arthropods available to shorebirds during their short breeding season at high latitudes. We hypothesized that prolonged exposure to warmer summer temperatures would generate earlier peaks in arthropod biomass, as well as higher peak and seasonal biomass. Across the temperature gradient encompassed by our study sites (>10°C in average summer temperatures), we found a 3-day shift in average peak date for every increment of 80 cumulative thawing degree-days. Interestingly, we found a linear relationship between temperature and arthropod biomass only below temperature thresholds. Higher temperatures were associated with higher peak and seasonal biomass below 106 and 177 cumulative thawing degree-days, respectively, between June 5 and July 15. Beyond these thresholds, no relationship was observed between temperature and arthropod biomass. Our results suggest that prolonged exposure to elevated temperatures can positively influence prey availability for some arctic birds. This positive effect could, in part, stem from changes in arthropod assemblages and may reduce the risk of trophic mismatch.

Changes in the environment can alter the suitability of habitats for organisms. In marine systems, fish species have their own specific requirements in terms of temperature and other habitat features. Behavioral responses such as thermoregulatory behavior in ectothermic species allow mobile organisms to respond to detrimental changes and search for more suitable habitats. However, for many species, limited information exists on the ecological requirements to help explain species abundance in a changing habitat. An example of a quickly changing habitat is the Wadden Sea, where five bearded rockling (Ciliata mustela) abundance has increased, unlike other Wadden Sea species. The increasing abundance of rockling has coincided with increasing average sea water temperatures and the recovery of mussel and Pacific oyster beds. Warming waters and increased structural habitat may have provided rockling with a more desirable habitat. Therefore, to better understand why rockling abundance is increasing within a changing Wadden Sea, a water temperature preference chamber was used to determine rockling's preferred temperature range. In addition, rockling's affinity for structural habitat and the trade-off between preferred temperature was examined by following their response to the systematic removal of artificial physical structures within the preferred temperature conditions. The preferred temperature range of rockling was found to be 10.4-15.7 degrees C. Following structure removals, rockling repeatedly moved away from their chosen temperatures to adjacent compartments with different temperatures but containing physical structure, indicating that the presence of physical structure was more important than preferred temperature until 18.6 degrees C. These novel findings provide insight and experimental support for the hypothesis explaining rockling's increase in the Wadden Sea: 1) mean annual temperatures have been steadily increasing towards rockling's preferred thermal range and 2) increasing mussel and Pacific oyster beds are plausibly providing structural habitat, an important habitat requirement for rockling. When fish display a strong association with physical structure it is necessary to link physiological and habitat preferences to better understand climate change related responses.

Bivalves play a key role in coastal ecosystems by supporting food web, modifying habitats, and their economic value for fisheries. Many bivalve species are under pressure, showing large variations in population sizes and distributions, with climate change and human activities considered as important drivers. The Dutch North Sea hosts high densities of bivalve species, dominated by the cut trough shell Spisula subtruncata, with strong interannual variations and a patchy distribution. To explore the causes of this variation, data of an extensive long-term spatial benthic monitoring program (1995-2021) was analysed using a Bayesian spatio-temporal hurdle model. We considered indicators related to human activities, biological processes, climate change and habitat preference as explanatory variables for the observed long-term temporal and spatial variations. Results revealed that medium sediment grain size was key determinant of S. subtruncata occurrence and density. Increasing sea water temperatures during winter and the post-settlement phase positively affected annual population densities, while strong north-westerly winds led to lower densities. These climate change related factors had an overall positive effect on this species in the region. Human activities like shellfish dredging and sand nourishment had no measurable impact. However, shrimp and flatfish beam trawling overlapped with S. subtruncata occurrence and were negatively related to densities, suggesting higher beam trawling intensity in these areas may negatively impacts densities. Overall, the effects were stronger at medium to finer sediments where the highest densities occurred, indicating a strong habitat-dependent effect. Despite identifying multiple drivers, unexplained annual variation suggests other not included factors like predation pressure, also play a role. More detailed studies on the combined effects of climate change-driven environmental stressors and human activities are needed to fully understand the population dynamics. This knowledge is essential for developing more adequate fisheries and coastal management strategies to sustain biodiversity.

Anthropogenic change is impacting ecosystems globally, causing declines in biodiversity. Long-distance migrants are particularly susceptible, as they depend on conditions over large geographical scales and are likely to experience a greater range of pressures. One long-distance migrant that has experienced substantial declines across the North-East Atlantic is the Arctic skua Stercorarius parasiticus. However, little is known about their migratory routes or strategies. We tracked 131 Arctic skuas from Scotland, the Faroe Islands, Norway and Svalbard between 2009 and 2019 using geolocators. To investigate migration strategies, we applied a hidden Markov model, using saltwater immersion data to infer stopovers and transit flights. Skuas used several discrete staging areas during migration, with an area of high marine productivity in the mid-North Atlantic being of high importance. Individuals from the different breeding populations overlapped extensively in staging areas, resulting in weak spatial connectivity between breeding and staging areas during southbound (r(M) = 0.25, 95% CI = 0.09-0.42; 0 = weak connectivity, 1 = strong connectivity) and northbound (r(M) = 0.16, 95% CI = -0.02 to 0.33) migration. Variation in migration strategies was driven by individuals from Svalbard, which belong to a population that is declining less than the other populations tracked. The relative location of wintering areas also influenced migration strategies. Individuals migrating further spent a smaller proportion of their migration at stopovers than those wintering closer. Identifying the non-breeding distribution, migration strategies and weak migratory connectivity of Arctic skuas provides a vital step towards linking conditions during migration to population dynamics and prioritising future research and conservation actions.

Shallow coastal areas are important nurseries for larvae and juveniles of many fish species. However, empirical data on small pelagic fish (SPF) in such regions are lacking, and temporal variability in abundance and habitat use by SPF are unknown. Given the critical role of SPF as a trophic link, their commercial value and their sensitivity to climate variability, there is a need for a quantitative and high-resolution monitoring method. We deployed a bottom-mounted echosounder, combined with a water current profiler, to examine the temporal variation in density, vertical distribution and movement behaviour of SPF in the Marsdiep, a dynamic tidal inlet between the North Sea and the Wadden Sea. The acoustic setup provided year-round records of fish density distribution in the water column every 90 min. Monthly fish samples were collected to help interpret acoustic observations. Our analyses indicated that the Marsdiep is a migration gateway for SPF from the North Sea into the Wadden Sea, particularly for juvenile herring and sprat. We observed clear seasonality with high fish densities from mid-spring to early autumn. Fish typically used the upper half of the water column. Swimming behaviour was primarily driven by currents. Yet, during outgoing tide, SPF resisted the outflowing current, suggesting efforts to remain in the Wadden Sea, supporting the hypothesis that the Wadden Sea serves as a preferred nursery area for SPF. Our high frequency, multi-seasonal and vertically resolved observations provide unique insight into the use of the Marsdiep by SPF. Furthermore, potential applications of autonomous echosounder moorings are discussed.

Small pelagic fish (SPF) are crucial in marine food webs, transferring energy from plankton to higher trophic levels. This study focuses on herring (Clupea harengus) and sprat (Sprattus sprattus), addressing knowledge gaps in their feeding ecology in a nursery area, the Dutch Wadden Sea. We conducted a year-long, monthly survey, and used DNA metabarcoding to analyse zooplankton samples and stomach contents of two size classes of herring and sprat. Intra-, interspecific, and seasonal variations in fish condition, stomach fullness, and diet composition, along with selective feeding, were studied. Our study showed that condition and diet composition of herring and sprat, along with zooplankton density, exhibited a clear seasonal pattern. Juvenile herring and sprat displayed opportunistic feeding behaviour, rather than showing distinct prey selection. Besides copepods, we regularly observed (larvae of) benthic invertebrates in their diet. This emphasizes the crucial role of SPF as energy transfer agents, not solely between trophic levels, but also from benthic to pelagic habitats. Furthermore, fish post-larvae were part of the diet of larger herring (10–15 cm). Because of its unprecedented temporal and taxonomical detail, this study advances the understanding of seasonal dynamics of dominant components at the base of the Wadden Sea food web.