Competition theory suggests that interspecific prey competition can result in changes to the dietary niche, but obtaining timeseries of data from sympatric species experiencing temporal variation in competition is challenging. Scotland is an important area for two species of seals, but over the past 20 years, populations of harbour seals Phoca vitulina vitulina in some areas have declined, and competition with stable/increasing numbers of grey seals Halichoerus grypus is a potential factor. Here, we standardised disparate seal diet datasets (based on analysis of prey hard parts in faeces) to investigate summer dietary metrics in two regions spanning a 24-year period. We estimated dietary niche breadth (Simpson's diversity, D) modelled as a function of sample size, proportional biomass diet composition corrected for digestion, and interspecific dietary niche overlap (Pianka index, O). Dietary niche overlap was high across time and space (O >0.7). In the Moray Firth, the harbour seal dietary niche was narrow when the population was high and stable (1987-1995, mean +/- SE, D = 2.9 +/- 0.3) but was significantly broader when the population depleted (2010, D = 7.2 +/- 0.5). Prey composition shifted from energy-rich sandeels and clupeids to a more diverse diet of gadids and flatfish. Conversely, the dietary niche of the stable/increasing grey seal population was consistently narrow, dominated by sandeels (e.g. 2010, D = 2.3 +/- 0.4). In southeast Scotland, where grey seal numbers were stable and harbour seals declined, there was no trend in the dietary niche of either species, though sandeels were less prevalent in later years. We propose that grey seals outcompeted harbour seals in the Moray Firth, and that the availability of sandeels to both seal species decreased in southeast Scotland. Changes in prey availability have therefore likely been a factor in regional harbour seal declines and could be preventing recovery.

Quantifying and mitigating transboundary effects of anthropogenic activity is a key challenge in environmental management, particularly for wide-ranging species such as large predators, fish and migratory birds, relying on habitats across multiple national jurisdictions. This challenge is especially complex in marine ecosystems, where the movement of species and impacts across borders is largely unobserved. Central-place foragers, such as pinnipeds and seabirds, exemplify this complexity: abundance is typically assessed on local (regional or national) scales on land, yet at-sea movements and drivers of abundance occur on broader transboundary scales. Resolving this mismatch is critical to effective conservation, especially in areas such as the Northwest European Shelf (NWES), which features globally important predator populations (including two pinniped species) alongside growing anthropogenic pressures and a mosaic of national maritime borders. We model an unprecedented GPS dataset from 236 grey (Halichoerus grypus) and 606 harbour seals (Phoca vitulina) tracked in waters of seven countries across the NWES (United Kingdom, Ireland, France, Belgium, Netherlands, Germany and Denmark). Using regional habitat association models, we generate at-sea distribution estimates for both species at 5 km resolution, scaled to haulout counts, producing country-specific and NWES-wide density maps. Analysis of the extent to which seals making foraging trips from one country occupy the Exclusive Economic Zones (EEZs) of other countries revealed substantial transboundary overlap, particularly for grey seals, and harbour seals in the southern North Sea. A case study apportioning grey seal density within three adjacent offshore marine protected areas in different EEZs revealed that, where total density in a given area is required, overlooking transboundary distribution can underrepresent numbers by an order of magnitude. Synthesis and applications. This study provides the first comprehensive, regionally scalable distribution estimates for pinnipeds across the NWES and its constituent countries. The modelling framework is adaptable to other central-place and migratory species, supporting transboundary biodiversity assessments and international conservation policy. We discuss common limitations and misconceptions of species distribution estimates, highlight priorities for future work and underscore the need for transboundary efforts to manage wide-ranging species, providing a foundation for future ecological modelling and decision-making across shared ecosystems. Quantifier et att & eacute;nuer les effets transfrontaliers des activit & eacute;s anthropiques est un d & eacute;fi majeur dans la gestion environnementale, en particulier pour les esp & egrave;ces & agrave; large r & eacute;partition telles que les grands pr & eacute;dateurs, les poissons et les oiseaux migrateurs, qui d & eacute;pendent d'habitats r & eacute;partis sur plusieurs juridictions nationales. Ce d & eacute;fi est particuli & egrave;rement complexe dans les & eacute;cosyst & egrave;mes marins, o & ugrave; les d & eacute;placements des esp & egrave;ces et les impacts transfrontaliers sont largement inobserv & eacute;s. Les esp & egrave;ces & agrave; places centrales, telles que les pinnip & egrave;des et les oiseaux marins, illustrent bien cette complexit & eacute;: leur abondance est g & eacute;n & eacute;ralement & eacute;valu & eacute;e & agrave; l'& eacute;chelle locale (r & eacute;gionale ou nationale) sur terre, alors que leurs mouvements en mer et les facteurs qui d & eacute;terminent leur abondance se produisent & agrave; une & eacute;chelle transfrontali & egrave;re plus large. Il est essentiel de r & eacute;soudre ce d & eacute;calage pour assurer une conservation efficace, en particulier dans des zones telles que le plateau nord-ouest europ & eacute;en (NWES), qui abrite des populations de pr & eacute;dateurs d'importance mondiale (dont deux esp & egrave;ces de pinnip & egrave;des) et qui est soumise & agrave; des pressions anthropiques croissantes et & agrave; une mosa & iuml;que de fronti & egrave;res maritimes nationales. Nous avons mod & eacute;lis & eacute; un jeu de donn & eacute;es GPS sans pr & eacute;c & eacute;dent provenant de 236 phoques gris (Halichoerus grypus) et 606 phoques veaux-marins (Phoca vitulina) suivis dans les eaux de sept pays du NWES (Royaume-Uni, Irlande, France, Belgique, Pays-Bas, Allemagne et Danemark). & Agrave; partir de mod & egrave;les r & eacute;gionaux d'association d'habitats, nous avons g & eacute;n & eacute;r & eacute; des estimations de r & eacute;partitions en mer des deux esp & egrave;ces avec une r & eacute;solution de 5 km, mises & agrave; l'& eacute;chelle en fonction du nombre d'animaux recens & eacute;s sur les zones de repos terrestres, afin de produire des cartes de densit & eacute; sp & eacute;cifiques & agrave; chaque pays et & agrave; l'ensemble de la r & eacute;gion NWES. L'analyse de la r & eacute;partition des zones de chasse des phoques partant d'un pays et se superposant aux Zones & Eacute;conomiques Exclusives (ZEEs) d'autres pays a r & eacute;v & eacute;l & eacute; un chevauchement transfrontalier important, en particulier pour les phoques gris et les phoques veaux-marins dans le sud de la mer du Nord. Une & eacute;tude de cas portant sur la r & eacute;partition de la densit & eacute; des phoques gris dans trois zones marines prot & eacute;g & eacute;es adjacentes situ & eacute;es dans diff & eacute;rentes ZEE a r & eacute;v & eacute;l & eacute; que, lorsqu'il est n & eacute;cessaire de d & eacute;terminer la densit & eacute; totale dans une zone donn & eacute;e, le fait de n & eacute;gliger la r & eacute;partition transfrontali & egrave;re peut entra & icirc;ner une sous-estimation des chiffres d'un ordre de grandeur. Synth & egrave;se et applications: Cette & eacute;tude fournit les premi & egrave;res estimations compl & egrave;tes et adaptables & agrave; l'& eacute;chelle r & eacute;gionale de l Le cadre de mod & eacute;lisation est adaptable & agrave; d'autres esp & egrave;ces & agrave; place centrales et migratrices, permettant les & eacute;valuations transfrontali & egrave;res de la biodiversit & eacute; et les politiques internationales de conservation. Nous discutons des limites et des id & eacute;es fausses courantes concernant les estimations de la r & eacute;partition des esp & egrave;ces, soulignons les priorit & eacute;s pour les travaux futurs et insistons sur la n & eacute;cessit & eacute; d'efforts transfrontaliers pour g & eacute;rer les esp & egrave;ces & agrave; large r & eacute;partition, fournissant ainsi une base pour la mod & eacute;lisation & eacute;cologique et la prise de d & eacute;cision futures dans les & eacute;cosyst & egrave;mes partag & eacute;s.

Reference intervals for clinical chemistry blood parameters are valuable for both individual diagnostics for animals in managed or veterinary care, and for evaluating wild population health. However, for marine mammals obtaining sufficient data from suitable groups or populations is logistically difficult. Here, we have assembled a large dataset of clinical chemistry results from free-living adult UK harbour seals (Phoca vitulina), analysed in the same commercial laboratory. We applied an open-source algorithm (available as the R package refineR, R version 4.5.2, refineR version 2.0.0) to produce robust reference intervals from these Real-World Data. This novel approach resulted in the generation of 95% reference intervals with 90% confidence bounds for 18 key chemistry parameters indicative of a range of physiological processes including, inflammation, nutritional status, kidney function and liver function. Reference intervals were also generated for triiodothyronine, the active thyroid hormone important in the regulation of metabolism. These intervals will provide critical baseline data for the assessment of harbour seal health as, to our knowledge, this is the largest dataset on which clinical chemistry reference intervals from wild-caught adult harbour seals have been based.

Inshore and offshore coastal regions are becoming increasingly occupied by anthropogenic infrastructure. This trend will continue with the drive for offshore renewable energy development to reduce carbon emissions and provide energy security. The introduction of structures to the marine environment can have direct and indirect effects on benthic and pelagic habitats, and subsequent impacts on species contributing to these ecosystems. Fish are both prey and predators and, therefore, important components to the functioning of food webs in these environments. Should their behaviour, distribution and/or populations be altered by introduced structures then it is important to understand the direction and magnitude of effects, both at local and regional seascape scales, to understand how these effects may influence ecological interactions. The migratory behaviour of some fish species also contributes temporal and spatial variability and uncertainty to observed patterns, which should be characterised to provide a fuller understanding of the consequences of introduced structures. Acoustic telemetry provides insights into the movement and behaviour of individual fish at scales from single wind turbines to regional networks of offshore wind farm developments. Here we review how acoustic telemetry has added to the understanding of fish behaviour around introduced structures and discuss how its use can be (and is being) expanded to provide a wider ecological understanding of the impacts of offshore wind farms through collaborative networks, and integrated research techniques and analyses.

Theoretical research has considered how animals should optimise foraging strategies to maximise fitness, adapting search scale to exploit different habitats and minimise competition. Empirical studies have described multi-scale area-restricted search (ARS) strategies for some species, but the physical and biological mechanisms underpinning such behaviour are rarely studied. Our objectives were to quantify the presence, prevalence, and habitat associations of scale-dependent foraging for two sympatric seal species, accounting for regional variation across the seascape. We analyse a GPS telemetry dataset of 116 grey (Halichoerus grypus) and 325 harbour seals (Phoca vitulina) tracked throughout the North Sea. We test the existence of multi-scale ARS, comparing hidden Markov models (HMMs) with two ARS states against more conventional HMMs (one ARS state). We quantify regional variation and examine the scale-dependence of foraging habitat associations using post-hoc “use-encounter” models. Both species exhibited nested broad-scale and focussed ARS. Accounting for scale resulted in increases of up to 25

Seabirds are often considered sentinels of ocean health, making dietary studies crucial not only for understanding their ecology and marine trophic webs, but also for informing conservation efforts and detecting ecosystem changes that may threaten biodiversity. However, determining the diet of wide-ranging, pelagic species is challenging, and there is a series of limitations associated with current techniques. In this study, we investigated buccal swabbing and DNA metabarcoding as a combined method to determine the diet of Manx shearwaters (Puffinus puffinus) and northern fulmars (Fulmarus glacialis) during different stages of the breeding season. We detected 14 fish taxa, with prey DNA successfully amplified in 68% of Manx shearwater samples and 28% of northern fulmar samples. We suggest that differences in amplification success between sample types are due to the time elapsed between feeding and swabbing when sampling various breeding stages. We present the first species-level dietary data for chick-provisioning Manx shearwaters and reveal a potential reliance on calorie-dense European sprat (Sprattus sprattus). In addition to identifying two fish taxa not previously documented as northern fulmar prey, our results highlight the continued importance of fishery discard species in their diet during the breeding season. This study suggests both species may be sensitive to shifts in prey availability and fishing practices and demonstrates the utility of buccal swabbing for DNA metabarcoding as a minimally invasive tool for dietary analysis in pelagic seabirds.

Seabirds are vulnerable to bycatch in longline fisheries but for most species the impacts are largely unknown. To address this knowledge gap, studies can estimate bycatch directly using observations or calculate the theoretical risk of bycatch using overlap indexes. Here we quantify the scale and risk of bycatch of northern fulmar ( Fulmarus glacialis) in the Norwegian offshore longline fishery using a ten-year time series of bycatch observations from a reference fleet programme, and large-scale datasets of fishing activity and northern fulmar distribution. We estimated an average of 0.01 (95 % CI: 0.008-0.03) northern fulmars bycaught per 1000 hooks, which results in a highly varying estimated annual bycatch of between 51 and 16242 (95 % CI) northern fulmars per year, with the largest hotspot in the Norwegian Sea during June-August. We compared these estimates with overlap indexes calculated for northern fulmars and the same fishing activity. This pinpointed the highest risk of bycatch within the breeding season, where fishing activity increased in the waters around the largest cluster of breeding colonies in the northeast Atlantic. Strong correlations between estimated bycatch and calculated overlap indexes validate overlap indexes as an indirect evaluation of risk and strengthen evidence for management decisions based on the spatial and temporal trends identified in our analyses.

Many man-made marine structures (MMS) will have to be decommissioned in the coming decades. While studies on the impacts of construction of MMS on marine mammals exist, no research has been done on the effects of their decommissioning. The complete removal of an oil and gas platform in Scotland in 2021 provided an opportunity to investigate the response of harbour porpoises to decommissioning. Arrays of broadband noise recorders and echolocation detectors were used to describe noise characteristics produced by decommissioning activities and assess porpoise behaviour. During decommissioning, sound pressure spectral density levels in the frequency range 100 Hz to 48 kHz were 30-40 dB higher than baseline, with vessel presence being the main source of noise. The study detected small-scale (< 2 km) and short-term porpoise displacement during decommissioning, with porpoise occurrence increasing immediately after this. These findings can inform the consenting process for future decommissioning projects.

Climate change affects timing of reproduction in many bird species, but few studies have investigated its influence on annual reproductive output. Here, we assess changes in the annual production of young by female breeders in 201 populations of 104 bird species (N = 745,962 clutches) covering all continents between 1970 and 2019. Overall, average offspring production has declined in recent decades, but considerable differences were found among species and populations. A total of 56.7% of populations showed a declining trend in offspring production (significant in 17.4%), whereas 43.3% exhibited an increase (significant in 10.4%). The results show that climatic changes affect offspring production through compounded effects on ecological and life history traits of species. Migratory and larger-bodied species experienced reduced offspring production with increasing temperatures during the chick-rearing period, whereas smaller-bodied, sedentary species tended to produce more offspring. Likewise, multi-brooded species showed increased breeding success with increasing temperatures, whereas rising temperatures were unrelated to reproductive success in single-brooded species. Our study suggests that rapid declines in size of bird populations reported by many studies from different parts of the world are driven only to a small degree by changes in the production of young.

Mitigation measures to disperse marine mammals prior to pile-driving include acoustic deterrent devices and piling soft starts, but their efficacy remains uncertain. We developed a self-contained portable hydrophone cluster to detect small cetacean movements from the distributions of bearings to detections. Using an array of clusters within 10 km of foundation pile installations, we tested the hypothesis that harbour porpoises (Phocoena phocoena) respond to mitigation measures at offshore windfarm sites by moving away. During baseline periods, porpoise movements were evenly distributed in all directions. By contrast, animals showed significant directional movement away from sound sources during acoustic deterrent device use and piling soft starts. We demonstrate that porpoises respond to measures aimed to mitigate the most severe impacts of construction at offshore windfarms by swimming directly away from these sound sources. Portable directional hydrophone clusters now provide opportunities to characterize responses to disturbance sources across a broad suite of habitats and contexts.

Plastic pollution is distributed patchily around the world's oceans. Likewise, marine organisms that are vulnerable to plastic ingestion or entanglement have uneven distributions. Understanding where wildlife encounters plastic is crucial for targeting research and mitigation. Oceanic seabirds, particularly petrels, frequently ingest plastic, are highly threatened, and cover vast distances during foraging and migration. However, the spatial overlap between petrels and plastics is poorly understood. Here we combine marine plastic density estimates with individual movement data for 7137 birds of 77 petrel species to estimate relative exposure risk. We identify high exposure risk areas in the Mediterranean and Black seas, and the northeast Pacific, northwest Pacific, South Atlantic and southwest Indian oceans. Plastic exposure risk varies greatly among species and populations, and between breeding and non-breeding seasons. Exposure risk is disproportionately high for Threatened species. Outside the Mediterranean and Black seas, exposure risk is highest in the high seas and Exclusive Economic Zones (EEZs) of the USA, Japan, and the UK. Birds generally had higher plastic exposure risk outside the EEZ of the country where they breed. We identify conservation and research priorities, and highlight that international collaboration is key to addressing the impacts of marine plastic on wide-ranging species.

Abstract After decades of research, development and demonstration (RD&D), mature concepts for the geological disposal of long-lived and high-level radioactive waste exist and some are close to being implemented. Underground research laboratories (URLs) have made an essential contribution to this progress. They enable in situ characterization and testing of host rocks and the demonstration of technologies and component performances at representative scales and under realistic geological conditions. They also offer a tool for training personnel and show aspects of the geological disposal concepts to stakeholders, including the public. In this paper we will present the different types and roles of URLs and we will discuss how the RD&D role of URLs has evolved and how it is likely to evolve in the near future.

The potential for long distance movements in common bottlenose dolphins (Tursiops truncatus) from six UK and Irish study sites was examined using photographs of natural markings. Here we provide the first evidence for long-term re-sightings between the Moray Firth, Inner Hebrides and across international borders to the Republic of Ireland as determined for eight individuals over a ten year period from 2001 to 2010. Minimum dispersal distances of up to 1,277km were resolved providing a new distance record for the species in European waters. Although none of the sightings were made within protected areas, several were made in waters used by animals from a Special Area of Conservation (SAC) revealing some evidence for connectivity between areas previously regarded as discrete. Our findings highlight the need to mitigate broader-scale anthropogenic impacts affecting these dolphins across multiple sites throughout their coastal range. Accordingly, we underline the importance of developing wider conservation measures for this species in UK and Irish waters, but particularly in prospective corridor areas potentially linking designated SACs in the Moray Firth, Cardigan Bay and Shannon Estuary.

Area-restricted search (ARS) behaviour is commonly used to characterize spatio-temporal variation in foraging activity of predators, but evidence of the drivers underlying this behaviour in marine systems is sparse. Advances in underwater sound recording techniques and automated processing of acoustic data now provide opportunities to investigate these questions where species use different vocalizations when encountering prey. Here, we used passive acoustics to investigate drivers of ARS behaviour in a population of dolphins and determined if residency in key foraging areas increased following encounters with prey. Analyses were based on two independent proxies of foraging: echolocation buzzes (widely used as foraging proxies) and bray calls (vocalizations linked to salmon predation attempts). Echolocation buzzes were extracted from echolocation data loggers and bray calls from broadband recordings by a convolutional neural network. We found a strong positive relationship between the duration of encounters and the frequency of both foraging proxies, supporting the theory that bottlenose dolphins engage in ARS behaviour in response to higher prey encounter rates. This study provides empirical evidence for one driver of ARS behaviour and demonstrates the potential for applying passive acoustic monitoring in combination with deep learning-based techniques to investigate the behaviour of vocal animals.

The biodiversity of marine and coastal habitats is experiencing unprecedented change. While there are well-known drivers of these changes, such as overexploitation, climate change and pollution, there are also relatively unknown emerging issues that are poorly understood or recognized that have potentially positive or negative impacts on marine and coastal ecosystems. In this inaugural Marine and Coastal Horizon Scan, we brought together 30 scientists, policymakers and practitioners with transdisciplinary expertise in marine and coastal systems to identify new issues that are likely to have a significant impact on the functioning and conservation of marine and coastal biodiversity over the next 5–10 years. Based on a modified Delphi voting process, the final 15 issues presented were distilled from a list of 75 submitted by participants at the start of the process. These issues are grouped into three categories: ecosystem impacts, for example the impact of wildfires and the effect of poleward migration on equatorial biodiversity; resource exploitation, including an increase in the trade of fish swim bladders and increased exploitation of marine collagens; and new technologies, such as soft robotics and new biodegradable products. Our early identification of these issues and their potential impacts on marine and coastal biodiversity will support scientists, conservationists, resource managers and policymakers to address the challenges facing marine ecosystems. A panel of scientists, policymakers and practitioners have used an iterative voting process to collate a list of 15 priority emerging issues likely to affect marine and coastal biodiversity over the next 5–10 years.

Abstract Data on sex ratios, age classes, reproductive success and health status are key metrics to manage populations, yet can be difficult to collect in wild cetacean populations. Long‐term individual‐based studies provide a unique opportunity to apply unoccupied aerial system (UAS) photogrammetry to non‐invasively measure body morphometrics of individuals with known life history information. The aims of this study were (1) to compare length measurements from UAS photogrammetry with laser photogrammetry and (2) to explore whether UAS measurements of body width could be used to remotely determine pregnancy status, sex or age class in a well‐studied bottlenose dolphin population in Scotland. We carried out five boat‐based surveys in July and August 2017, with concurrent photo‐identification, UAS and laser photogrammetry. Photographs were measured using bespoke programmes, MorphMetriX for UAS photos and a Zooniverse project for laser photos. In total 64 dolphins were identified using photo‐ID, 54 of which had concurrent UAS body length and 47 with laser body length measurements. We also measured body widths at 10% increments from 10% to 90% of body length for 48 individuals of known sex, age class and/or pregnancy status. There was no significant difference in the length of individuals measured with UAS and laser photogrammetry. Discriminant analyses of the body width–length (WL) ratios expected to change during pregnancy, correctly assigned pregnancy status for 14 of the 15 females of known pregnancy status. Only one pregnant female was incorrectly assigned as not pregnant. However, our results showed that length and body width cannot accurately allocate these bottlenose dolphins to sex or age class using photogrammetry techniques alone. The present study illustrates that UAS and laser photogrammetry measurements are comparable for small cetaceans and demonstrates that UAS measurements of body WL ratio can accurately assign pregnancy status in bottlenose dolphins.

Understanding spatiotemporally varying animal distributions can inform ecological understanding of species' behavior (e.g., foraging and predator/prey interactions) and support development of management and conservation measures. Data from an array of echolocation-click detectors (C-PODs) were analyzed using Bayesian spatiotemporal modeling to investigate spatial and temporal variation in occurrence and foraging activity of harbor porpoises (Phocoena phocoena) and how this variation was influenced by daylight and presence of bottlenose dolphins (Tursiops truncatus). The probability of occurrence of porpoises was highest on an offshore sandbank, where the proportion of detections with foraging clicks was relatively low. The porpoises' overall distribution shifted throughout the summer and autumn, likely influenced by seasonal prey availability. Probability of porpoise occurrence was lowest in areas close to the coast, where dolphin detections were highest and declined prior to dolphin detection, leading potentially to avoidance of spatiotemporal overlap between porpoises and dolphins. Increased understanding of porpoises' seasonal distribution, key foraging areas, and their relationship with competitors can shed light on management options and potential interactions with offshore industries.

Given the patchiness and long-term predictability of marine resources, memory of high-quality foraging grounds is expected to provide fitness advantages for central place foragers. However, it remains challenging to characterize how marine predators integrate memory with recent prey encounters to adjust fine-scale movement and use of foraging patches. Here, we used two months of movement data from harbour seals (Phoca vitulina) to quantify the repeatability in foraging patches as a proxy for memory. We then integrated these data into analyses of fine-scale movement and underwater behaviour to test how both spatial memory and prey encounter rates influenced the seals' area-restricted search (ARS) behaviour. Specifically, we used one month's GPS data from 29 individuals to build spatial memory maps of searched areas and archived accelerometery data from a subset of five individuals to detect prey catch attempts, a proxy for prey encounters. Individuals were highly consistent in the areas they visited over two consecutive months. Hidden Markov models showed that both spatial memory and prey encounters increased the probability of seals initiating ARS. These results provide evidence that predators use memory to adjust their fine-scale movement, and this ability should be accounted for in movement models.

Marine predator populations are crucial to the structure and functioning of ecosystems. Like many predator taxa, pinnipeds face an increasingly complex array of natural and anthropogenic threats. Understanding the relationship between at-sea processes and trends in abundance at land-based monitoring sites requires robust estimates of at-sea distribution, often on multi-region scales. Such an understanding is critical for effective conservation management, but estimates are often limited in spatial extent by spatial coverage of animal-borne tracking data. Grey ( Halichoerus grypus ) and harbour seals ( Phoca vitulina ) are sympatric predators in North Atlantic shelf seas. The United Kingdom (UK) and Ireland represents an important population centre for both species, and Special Areas of Conservation (SACs) are designated for their monitoring and protection. Here we use an extensive high-resolution GPS tracking dataset, unprecedented in both size (114 grey and 239 harbour seals) and spatial coverage, to model habitat preference and generate at-sea distribution estimates for the entire UK and Ireland populations of both species. We found regional differences in environmental drivers of distribution for both species which likely relate to regional variation in diet and population trends. Moreover, we provide SAC-specific estimates of at-sea distribution for use in marine spatial planning, demonstrating that hotspots of at-sea density in UK and Ireland-wide maps cannot always be apportioned to the nearest SAC. We show that for grey seals, colonial capital breeders, there is a mismatch between SACs (where impacts are likely to be detected) and areas where impacts are most likely to occur (at sea). We highlight an urgent need for further research to elucidate the links between at-sea distribution during the foraging season and population trends observed in SACs. More generally, we highlight that the potential for such a disconnect needs to be considered when designating and managing protected sites, particularly for species that aggregate to breed and exhibit partial migration (e.g. grey seals), or spatial variation in migration strategies. We demonstrate the use of strategic tracking efforts to predict distribution across multiple regions, but caution that such efforts should be mindful of the potential for differences in species-environment relationships despite similar accessible habitats.