Subsea power cables used to transport offshore wind-generated energy emit electromagnetic fields (EMFs) into the marine environment. These anthropogenic fields overlap with naturally occurring biotic and abiotic EMFs that elasmobranchs can detect and use for navigation, prey detection and predator avoidance. When egg-laying sites overlap with subsea cables, embryos may be chronically exposed to EMFs throughout development, yet the effects remain largely unknown.We compared the morphology and behaviour of thornback skate (Raja clavata) and small-spotted catshark (Scyliorhinus canicula) juveniles exposed to alternating current EMFs (1.8 and 4.6 μT) throughout embryogenesis, with an unexposed control group. In skates, tail length was significantly shorter (5.3%) in exposed individuals at both one and three months post-hatching, while in exposed sharks, the pectoral fins were 11.2% larger at birth, a transient effect that disappeared after one month. These results may indicate subtle changes in growth of EMF exposure. As tail/fin-to-body ratios were unchanged, the extremities could be proxy for overall differences in length, possibly obscured by postural variation.Activity levels and ventilation rates did not differ between treatments, nor were startle or avoidance responses observed following acute EMF exposure. However, sharks showed a significant difference in ventilation rate between 0.1 μT (background) and 10 μT, indicating EMF detection.We conclude that chronic exposure during embryogenesis causes subtle, species-specific growth effects without behavioural impairment. Although ecological relevance of these findings is unclear, it highlights the need for caution when extrapolating EMF sensitivity across elasmobranch species.

Globally, the tope shark Galeorhinus galeus, also known as the school or soupfin shark, is classified by the IUCN as Critically Endangered. It is known to undertake broad-scale movements throughout continental and pelagic European waters, where it encounters threats posed by incidental fisheries capture and environmental change. The identification of pupping and nursery habitats is essential for the conservation and management of elasmobranch populations, including highly migratory shark species. For the Northeast Atlantic region, published evidence on the location of pupping areas remains limited. Catch records and survey observations of neonates in July 2022 support the existence of a pupping area for tope sharks in the Dutch Wadden Sea and adjacent coastal North Sea waters. Given its designation as a UNESCO World Heritage Site, we discuss the role of the Dutch Wadden Sea area as a potential pupping ground for the Critically Endangered tope shark.

In the European Union, the European Water Framework Directive (WFD) has prompted water authorities to implement restoration measures and monitor biological indicators such as fish and macroinvertebrates. In this study, we examined the effects of longitudinal connectivity and morphology restoration together with water quality improvement on the long-term development (2002-2024) of fish and macroinvertebrate assemblages in the lowland Dommel river catchment in the Netherlands (1500 km2). Despite the gradual improvement in connectivity, morphology, and water quality, contrasting responses were observed between the two indicator groups examined. The proportion of sites with high or good ecological status based on period-average EQR scores for macroinvertebrates increased from 6% in 2002-2009 to 25% in 2017-2024. Positive indicator taxa such as Gammarus and Calopteryx were associated with well-connected, re-meandered or non-channelised reaches and higher oxygen concentrations. In contrast, fish ecological quality ratios declined with the proportion of sites with High or Good status dropping from 9% in 2002-2009 to just 2% in 2017-2024. In addition, GAMMs showed fish density declines throughout the catchment after years with dry summers, indicating that the overarching effects of droughts can mask potential positive effects of restoration. These results demonstrate that biological indicators might reflect changes at different spatial and temporal scales, likely due to contrasting dispersal abilities, life-cycle durations, and sensitivities to fragmentation and drought between the indicator groups. Moreover, this study highlights that achieving both WFD targets and long-term ecological recovery requires addressing broad-scale stressors such as climate change.

Anthropogenic activities are degrading coral reef ecosystems and driving global declines in reef-associated sharks and rays, yet the role of non-extractive human disturbance such as tourism and coastal development remains poorly understood. We quantified how environmental conditions and human presence shape reef shark and ray distributions across seven Caribbean reef systems using 995 Baited Remote Underwater Video (BRUV) deployments conducted between 2012 and 2017 across six inhabited Dutch Caribbean islands and the remote Saba Bank. Habitat characteristics and proxies of human pressure were quantified for each site, including coastal building density and SCUBA diving activity derived from geotagged social media data. We recorded 455 elasmobranchs, dominated by Caribbean reef sharks (Carcharhinus perezi), southern stingrays (Hypanus americanus) and nurse sharks (Ginglymostoma cirratum). Reef shark abundance declined sharply with increasing coastal development and diving intensity on populated reefs despite otherwise suitable habitat conditions, whereas nurse sharks and southern stingrays were less sensitive to human activity and more strongly associated with habitat features. In contrast, on the minimally disturbed Saba Bank, elasmobranch distributions were primarily explained by environmental gradients such as depth and habitat complexity. Synthesis and applications: Species-specific sensitivities mean that non-extractive human presence can strongly reshape reef predator distributions even where fishing pressure is limited. Integrating tourism intensity and coastal development into marine spatial planning is therefore essential for reef shark conservation, and combining ecological surveys with open-source social and spatial datasets provides a scalable, low-cost approach for assessing human disturbance in data-limited reef systems.

Passage behaviour at barriers of migratory fish is well studied in rivers, whereas little is known about fish movement at estuarine barriers. In response to climate change, more estuarine barriers are likely to be erected in the future, further compromising connectivity for diadromous fish. As these fish species are already severely endangered, there is an urgent need to better understand passage behaviour at barriers to facilitate connectivity. These fish face challenges at estuarine sluice gates, where suitable passage windows occur only infrequently. To enhance fish migration, sluice management at the Haringvliet barrier, located at the mouth of the Rhine and Meuse rivers, was recently adjusted to allow controlled inflow from the sea (“Kier management”). We assessed upstream passage windows for large anadromous fish prior to this adjustment, using long-term (1996–2018) telemetry (NEDAP Trail System®) and environmental data. Of 1,751 tagged Atlantic salmon (Salmo salar), sea trout (Salmo trutta), and sea lamprey (Petromyzon marinus), 516 (29

Offshore wind energy plays a crucial role in the transition to low-carbon power generation. The expansion of offshore wind farms (OWFs) introduces anthropogenic electromagnetic fields (EMFs) from subsea power cables. Elasmobranchs, including sharks and rays, are sensitive to natural electric and magnetic fields used for navigation and prey detection, making them potentially vulnerable to anthropogenic EMFs as they may alter orientation, foraging or habitat use. Coastal shelf seas, where most OWFs are constructed, often overlap with essential elasmobranch habitats, including complex biogenic structures such as mussel beds that provide foraging and refuge opportunities. We aimed to determine whether exposure to an AC-EMF representative of subsea power cables and/or complex mussel substrate over non-complex substrate influences behaviour and substrate preferences in juvenile small-spotted catsharks (Scyliorhinus canicula). This study investigated the behavioural responses of 14 juvenile S. canicula exposed to a gradient of alternating current (AC) EMF (max. RMS 2 μT) and complex biogenic mussel substrate in a 22 factorial design in a circular preference chamber. Across 28 trials, time spent and distance travelled in each quadrant did not differ with EMF exposure and/or the presence of complex biogenic substrate. Hidden Markov models revealed subtle but significant changes in behavioural states (inactive, local activity, transit), where sharks were more inactive (+6%pt.) and less locally active (-5%pt.) in the EMF + mussel quadrant compared to the bare and EMF-only quadrants, and increased transiting behaviour (+6%pt.) in the mussel-only quadrant relative to the EMF + mussel quadrant. These findings show that S. canicula juveniles show no preference in substrate or alternating-current EMF exposure, but they did subtly change their behaviour. Future work should examine direct-current systems and field-based tracking and assess the extent to which these observations generalise to field settings and other elasmobranchs.

Abstract The very beginning of the ICES Journal of Marine Science in 1903 featured an article by C.G.J. Petersen on European plaice. At that time, Petersen had already published his landmark report in which he described a large-scale mark-recapture study of plaice in the North Sea using simple bone buttons as tags. Petersen would become famous for lending his name to disc-shaped fish tags and the method for estimating population size from mark-recapture experiments. His pioneering work not only laid the foundation for the field of marine tagging studies but also set the tone for the journal as a venue for quantitative, hypothesis-driven science with direct relevance for fisheries management. Since then, tagging has advanced from using basic external markers to include an extraordinary suite of tools, including acoustic tags, electronic archival tags, satellite telemetry, chemical markers, and genetic methods. Our ability to interpret outcomes of mark-recapture experiments has steadily followed suit. These advances in tagging tools and models have transformed our ability to study fish movement, behaviour, connectivity, and population dynamics. This centennial reflection celebrates Petersen’s enduring influence on marine science, highlighting the progression of tagging techniques and the ICES Journal of Marine Science itself.

Subsea power cables, required for offshore generated wind power transport, emit electromagnetic fields (EMFs) into the marine environment. EMFs also occur naturally, resulting from biotic (animals) and abiotic (geomagnetic field) sources. Skate and oviparous shark embryos in the egg can sense EMF from predators and respond by reducing their normal movement ('freezing response') to prevent detection and subsequent predation. When nursery areas overlap with power cables, embryos will be exposed to varying levels of anthropogenic EMFs and effects thereof on embryonic development is currently understudied. Here, we present behavioral responses of thornback ray (Raja clavata) embryos to varying field-related EMF levels (1.8-4.6 μT) generated by alternating current throughout embryogenesis (∼20 weeks). Chronically exposed individuals were overall more active, including 33 % more tail undulations and 150 % increased body movements, compared to non-exposed individuals. This increased activity suggests that eggs exposed to EMFs generated by subsea power cables might be at risk of increased predation. We found no indications of reduced health or survival after hatching, or changes in development time or biometry. Effects on subsequent life stages cannot be excluded, follow-up studies should observe hatchling development. We did not observe an increase in freezing response resulting from EMF change as described by other researchers who used different types and intensities of EMF cues. We recommend that different species, along with DC exposure, should be studied to gain a more complete insight into the potential effects of EMF exposure during embryogenesis of these EMF-sensitive species.

Niche partitioning enables ecologically similar species with overlapping resource requirements to coexist. This study examines dietary niche partitioning in three sympatric skate species Raja brachyura, R. clavata and R. montagui in the North Sea and English Channel. Stomach content analysis (SCA) and stable isotope analysis (SIA) were used to compare diets between species, sex, and length class. Stomachs from 111 R. brachyura, 234 R. clavata, and 99 R. montagui were analysed using the prey-specific index of relative importance (%PSIRI). SCA showed small individuals (<35 cm LT) of all species consumed diets dominated by Caridea species (>63.9 % PSIRI). As size increased, an ontogenetic diet shift was observed until 55 cm LT for all species, regardless of maturity stage or maximum attainable length. Larger individuals above 55 LT showed significant differences in diet among species. R. brachyura, consumed large amounts of teleost species (>70.6 %PSIRI), R. clavata predominantly consumed brachyuran crabs (>57.2 %PSIRI), and R. montagui consumed a mix of brachyuran crabs (33.0 %PSIRI) and teleost species (43.2 %PSIRI). SIA showed interspecific differences in the isotopic niche space between the three skate species, but no intraspecific differences across length classes. The results from this study are similar to patterns found in diet studies of other sympatric skate species where small individuals primarily consume Caridea species, and niche partitioning is observed for larger individuals. This knowledge is essential for ecosystem-based management approaches, especially in areas such as the North Sea, where these species share habitat areas with fishing operations.

In the context of river connectivity restoration, fishways play a crucial role in facilitating the migration of fish past barriers, but their form and functionality are often determined by various sociomaterial complexities. This study uses the case of fishway development in the Waterschap Brabantse Delta management area of the Netherlands to explore such complexities. Taking a network approach, we investigated the implementation and management of fishways as a process of assembling heterogeneous networks that involve both human and nonhuman actors. Using data from interviews, field observations and document analysis, the research revealed fishways to be networks of actors that included fish, engineers and maintenance personnel. We further demonstrate that fishways are embedded as actors, or 'nodes', within broader networks that exert a reciprocal influence on their functioning. By following fishways across different phases of their development trajectory and tracing the participation or withdrawal of actors, we explore changes in the networks and their subsequent impact on fishway design and performance.

Grey mullets (family Mugilidae) are widespread across coastal, brackish, and freshwater habitats, and have supported fisheries for millennia. Despite their global distribution and commercial value, little is known about their movement ecology and its role in the co-existence of sympatric mullet species. Gaps in knowledge about migratory behaviour, seasonal occurrence, and movement scales have also impeded effective management, highlighting the need for further research. This study aimed to identify key habitats and timing of grey mullet presence across the Dutch Wadden Sea, North Sea, and freshwater areas, and to explore potential behavioral differences between two grey mullet species: thicklip mullet (Chelon labrosus) and thinlip mullet (Chelon ramada). Using acoustic telemetry, we tracked 86 tagged grey mullet over three years (thicklip mullet, N = 74; thinlip mullet, N = 12), combining data from 100 local acoustic receivers and the European Tracking Network. Both species were detected in the Wadden Sea from April to November, however, thinlip mullet arrived in the Wadden Sea earlier than thicklip mullet (median date = May 16 vs. June 7). Individual residency in the Wadden Sea lasted a median 97 days for thicklip mullet and 94 days for thinlip mullet. Thinlip mullet were also detected by more receivers and over a larger area than thicklip mullet, indicating differences in movement behaviour. Both species showed an affinity for receivers near major harbours, with thinlip mullet more often detected near fresh water outflows. Seasonal migrations between coastal and offshore waters were also observed, with one thinlip mullet returning to freshwater across consecutive years. North Sea detections spanned ten months, with a gap during the presumed spawning period (Jan–Feb). Our data suggest that thinlip mullet show a preference for deeper gullies while thicklip mullet may spend more time in shallow areas and flooded tidal flats. These findings highlight the importance of the Wadden Sea as a seasonal foraging ground and provide insights into the migratory patterns of grey mullets.

The development of offshore wind farms (OWFs) in coastal seas presents both risks and opportunities for threatened elasmobranch populations, but their actual influence on elasmobranch presence and habitat use remains unclear. As more OWFs are planned, the lack of available space puts pressure on stakeholders to create multi-use areas as demands from fisheries, conservation and the energy sector become increasingly overlapping. Insight into interactions between these demands is needed to support policymakers in marine spatial planning and management decisions. One of the important questions is to what extent OWFs influence elasmobranch presence and habitat use. Here, we (i) ascertain elasmobranch occurrence in OWFs, (ii) determine whether elasmobranch presence varies between sampling locations, and (iii) investigate the influence of seasonality on elasmobranch presence. We collected 436 seawater samples within four OWFs and the presence of 5 different elasmobranch species (2 sharks, 3 skates) was confirmed. The overall detection probability of elasmobranchs in the four OWFs was 8.5 %. A quarterly sampling campaign over 2 years demonstrated the seasonality of Mustelus asterias on the subsea power cables, which corresponded with known migratory movements of this species. Our findings confirm that sharks and skates are present in OWFs. We advise caution when introducing multi-use activities that could counteract the benefits of bottom trawling fisheries exclusion. Investigating how these threatened species use OWFs would aid policymakers in determining the (multi-use) function of OWFs in terms of EU legislative acts including the Habitat Directive, MSFD, Biodiversity Strategy 2030 and the Nature Restoration Law.

Subsea power cables are expanding in number and capacity due to increasing demand to transport offshore generated energy. Energy transported through a cable creates an electromagnetic field (EMF). Elasmobranchs are dependent on their perception of the earth's magnetic field and biologically induced electric fields, for orientation, navigation, locating conspecifics and detecting prey. EMF levels from subsea power cables will add to natural signals potentially disrupting elasmobranch perception, but the effects are not fully understood. Reported behavioural responses include attraction, disturbance, and indifference, varying with exposure type, level and experimental set-up. In this study, the effects of EMF on swimming behaviour of 14 individual small-spotted catshark Scyliorhinus canicula were studied. All sharks were exposed to field-relevant EMF gradients cables in three trials: 15.0 μT AC, 19.6 μT DC, and a control treatment. Sharks showed no startle response to EMF onset, did not alter movement towards or away from the cable, and crossed it as frequently as in control trials. Hidden Markov Models showed that behavioural states were best explained by EMF treatment, trial order and sex. Sharks showed 25 % less time transiting during DC trials when compared to AC and control trials. These findings indicate reason for further refined studies to better determine behavioural effects from direct current subsea power cables with S. canicula, for example using tagging studies. In addition, exploring effects on other species will help obtain a broader understanding of the potential impacts of EMF on benthic elasmobranchs.

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River networks worldwide are highly fragmented and this is particularly prevalent in the human-dominated lowland landscape of the Netherlands. Fishways function as measures to facilitate passage alongside barriers for fish communities and have been evaluated mostly in single case studies. We compared fish monitoring data of 82 fishways with data of fish observed in adjacent water stretches, conducting the first nationwide study on passage for the full spectrum of fish species present. In total, 35 out of 38 (92%) native species recorded in fishways' surroundings used fishways, while per fishway the median was 59% related to a variety of factors (fish behaviour, fishway type, monitoring design). The species using fishways most frequently were perch Perca fluviatilis (71/78 of fishways present), roach Rutilus rutilus (70/79) and gudgeon Gobio gobio (68/77). Logistic regression models showed the effect of monitoring duration and timing in detecting specific fish species ascending fishways. This large-scale analysis highlights the need to consider all native fish species during the design and monitoring of fishways. The obtained information from this study can be used by water managers for improving monitoring schemes and river connectivity which is an essential component for achieving the European Water Framework Directive goals.

Migratory fish populations have experienced great declines, and considerable effort have been put into reducing stressors, such as chemical pollution and physical barriers. However, the importance of natural sounds as an information source and potential problems caused by noise pollution remain largely unexplored. The spatial distribution of sound sources and variation in propagation characteristics could provide migratory fish with acoustic cues about habitat suitability, predator presence, food availability and conspecific presence. We here investigated the relationship between natural soundscapes and local river conditions and we explored the presence of human-related sounds in these natural soundscapes. We found that 1a) natural river sound profiles vary with river scale and cross-sectional position, and that 1b) depth, width, water velocity, and distance from shore were all significant factors in explaining local soundscape variation. We also found 2a) audible human activities in almost all our underwater recordings and urban and suburban river parts had elevated sound levels relative to rural river parts. Furthermore, 2b) daytime levels were louder than night time sound levels, and bridges and nearby road traffic were much more prominent with diurnal and weekly patterns of anthropogenic noise in the river systems. We believe our data show high potential for natural soundscapes of low-land river habitat to serve as important environmental cues to migratory fish. However, anthropogenic noise may be particularly problematic due to the omnipresence, and relatively loud levels relative to the modest dynamic range of the natural sound sources, in these slow-flowing freshwater systems.

Diadromous fish such as the European eel ( Anguilla anguilla L.) are hampered by a high density of barriers in estuaries and freshwater systems. Modified and fragmented waterbodies lack tidal flows, and habitat may be less accessible and underutilized compared to free-flowing rivers and estuaries. With rising sea levels and increased occurrence of droughts, the number of barriers may further increase, implying that the need to study migration in such areas may even become more urgent worldwide. To study glass eel migration and behaviour in such highly modified water systems, a mark-recapture study was carried out in the North Sea Canal (NSC) basin, which drains into the North Sea via a large sluice complex. In total, eight uniquely tagged groups (3,797 glass eels) were released near the sluice complex, and 11 groups (2,663 glass eels) were released at inland barriers upstream over a 28 km long stretch in the NSC in spring 2018. The sluice complex attracted 10.3 million glass eel and did not block or delay their immigration. The large and diurnally intensively used coastal ship locks and allowings some saltwater intrusion, efficiently facilitated glass eel migration. Once in the NSC, water outlets from adjacent polders attracted glass eels relative proportional to the discharge of pumping stations. In the NSC, average migration speeds of 0.7 km/day (max. 1.8 km/day) were measured, and this increased with higher temperatures. Redistribution of glass eel from accumulations at inland barriers to other outlet locations was observed in both upstream and downstream directions in the NSC. Passage success and residence time (‘delays’ of 4.1–13.7 days) varied between the different inland barriers. Most of the glass eel, however, appears to settle in the easily accessible habitats within the brackish NSC catchment. This study combined an integral assessment of successive bottlenecks in a modified inland water system.

Freshwater species biodiversity is under threat. The average global decline for migratory fish species is estimated to be more than 75% since 1970. Atlantic salmon is one of these species with a steep decline in north-western Europe and it even went extinct in the river Rhine in the 1950s. The causes for this decline have been posted to habitat loss, pollution, climate change and overfishing. Annual stocking in the Rhine since the late 1980s resulted in an initial increase in the Atlantic salmon numbers after which numbers collapsed again. In this paper, we lay out the recent decline, estimate losses of smolts and adults at different sections in the freshwater habitat and elaborate on potential causes of the recent decline and these losses. We found that the salmon population of the river Rhine has declined rapidly over the past two decades, with a current estimated spawning population of only similar to 350-800 individuals. The percentage of salmon smolts returning as adults to spawning grounds is estimated at 0.5%-0.6%, well below the 3% supposedly needed to maintain a self-sustaining population. Many individuals disappear during their migrations, with the highest percentage of smolts disappearing in the German tributaries (44%) and the Dutch lower Rhine (71%), while the percentage of disappearing adults is highest in both the Dutch (74%) and the German (78%) Rhine. Causes for the losses per river section remain unclear and possible threats, some specific to the river Rhine, are being discussed. The large losses of smolts and adults in inland waters, compared with open sea losses, indicate that restocking the river Rhine might only result in a self-sustaining population with more ecological restoration than carried out so far along the intensively shipped and highly regulated river course and associated high levels of predation, and might be increasingly limited by future climate change.