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Aim The current biodiversity crisis calls for conservation measures that limit or reduce the negative human impact on key habitats and vulnerable wild populations. To effectively protect biodiversity at all levels, including intra‐specific diversity, conservation measures should, ideally, be aligned with the connectivity and genetic structure of wild populations. In this review, we synthesise the scientific literature on connectivity and population structure of marine species in a marginal sea. Location The study focuses on the Skagerrak—a marginal sea in the northeast Atlantic Ocean. Methods We reviewed a total of 172 scientific publications assessing connectivity or population structure in 48 species. From this material, we summarised the main patterns of connectivity and population structure across species, as well as the taxonomic and geographic representation of the scientific literature within this field. Results Our review shows that contemporary connectivity with adjacent seas is high, but asymmetric, for most species. Simultaneously, most species have multiple distinct populations in the Skagerrak, separate from those in adjacent seas. Within the Skagerrak, population structure is common both among coastal populations and between coastal and offshore populations, but less frequent among offshore populations. In many mobile species, multiple populations temporarily overlap in certain areas, but retain their genetic divergence through homing or other barriers to gene flow. Main Conclusions Even in one of the most intensively studied marine regions within the field of connectivity and population structure, there are still large knowledge gaps limiting both our understanding of connectivity and its application in management decisions. Nevertheless, it is evident that the presence of population structure despite high connectivity, and temporal variability in population assemblages, poses a challenge for area‐based protection measures. This underscores the need for adaptive management that monitors and manages intra‐specific diversity on multiple temporal and spatial scales.

The age information of commercially important species is crucial in fisheries management. Age of various fish and molluscan species has routinely been determined by counting annual growth bands deposited within the hard structures. In crustaceans such structures were previously believed to be lost and replaced due to molting. However, a technique was recently developed to use growth bands deposited in hard structure retained through molting as an age indicator. In the present study, the applicability of the novel technique is investigated for four crustacean species collected from Northern Atlantic for the first time: European lobster, Homarus gammarus (Linnaeus, 1758); Norway lobster, Nephrops norvegicus (Linnaeus, 1758); Atlantic rock crab, Cancer irroratus Say, 1817; and northern shrimp, Pandalus borealis (Krøyer, 1838). The gastric mill ossicles in the first three species were processed to show the growth bands while the eyestalk was used in the shrimp species. Four growth bands were visible in European lobster hatched in a Norwegian hatchery and maintained alive for four years before prior processing. Band counts in the other three species were identical to size-at-age interpretation determined from length-frequency analysis. Validation of the periodicity of annual deposition of growth bands is essential before applying the technique on a wider scale.

The present study attempts to elucidate possible microevolutionary adaptations of life-history traits of high-latitude populations of the holarctic, littoral oribatid mite Ameronothrus lineatus by comparing arctic and temperate populations. Additionally, the paper provides an overview of the limited research on general ecology and population biology of arctic populations. In the Arctic the larviparous A. lineatus has a 5-year life cycle (larva-to-larva), and adults survive a further 2-3 years. High survival to maturity is consistent with a low lifetime reproductive output of ca. 20 larvae. The life history can be regarded as an extreme version of the typical oribatid life history. However, several life-history features suggest specific adaptations of arctic populations. In particular, the pre-moult resting stage is synchronized with the warmest part of the arctic summer, which shortens this vulnerable part of development. High reproductive investment by females at relatively low temperatures may represent a physiological adaptation to the cool arctic summer. Finally, prolonged cold exposure positively affects reproduction and survival the following summer, suggesting adaptation of the species to the highly seasonal arctic environment. On the other hand, the ability of all life-cycle stages to overwinter, and a flexible life history with the species being able to take advantage of favourable climatic conditions to accelerate development and larviposition, seem to be ancestral features. Thus, the success of A. lineatus in arctic habitats is probably attributable to a combination of derived and ancestral life-history traits. Studies of conspecific temperate populations are required to elucidate further local adaptations of arctic populations.

The fatty acid (FA) content of northern shrimp Pandalus borealis sampled on commercial shrimp fishing grounds in 2 fjords in northern Norway was analyzed to elucidate if the shrimp feed on aquaculture waste from salmon aquaculture farms. Shrimp were sampled in February and June 2021 in 1 fjord without farms (Balsfjord) and 1 fjord with farms (Kvænangen), at varying distances from the farms (1.5 to 13.3 km). A laboratory experiment conducted as part of our study showed that the FA profile of shrimp feeding on salmon feed pellets changed toward the FA profile of the pellets. The terrestrial FAs 18:2n-6 and 18:3n-3 changed the most and were chosen as suitable FA trophic markers (FATMs) for identifying feeding on aquaculture waste by shrimp in the fjords. The terrestrial FATMs were not detected at elevated levels in the lipids of the shrimp from Kvænangen, indicating that none of the sampled shrimp had been feeding on aquaculture waste. FA analysis has the potential to inform diet studies, and we found differences in the shrimp diet between and within the fjords as well as seasonal differences. Calanus finmarchicus was a more abundant prey in outer Kvænangen compared with inner Kvænangen and Balsfjord, while shrimp at the latter 2 sites had a higher content of benthic FATMs. At all sites, C. finmarchicus constituted a more important food item in winter than in summer.

No reliable measures of age currently exist in the Antarctic krill, Euphausia superba (Dana, 1852). The eyestalks from 51 individuals were dissected, cut in longitudinal sections and studied for identifying growth zones. The krill was collected at the South Orkney Islands during January and February 2015, and varied between 30 and 53 mm in total body length. Up to six growth zones were identified, each zone consisting of one light and one dark section. The width of the longitudinal sections increased with increasing body length, although there were differences between sexes. Females tended to have narrower growth zones from the third zone and onwards compared with males. Data show that male subadult stages (MIIA1, MIIA2 and MIIA3) had 2.2 ± 0.8 (average ± SD) zones and adult male stages had 3.8 ± 0.8 zones. The female juvenile stage (FIIB) had 1.7 ± 0.5 zones and adult females (FIIIA-E) had 3.7 ± 1.0 zones. There were positive relationships between the number of zones and the maturity stage, and between the number of zones and body length. Further knowledge about molting process in the Antarctic krill and a verification of the ageing procedure from krill with a known age is needed before the number of growth zones can be definitely established as an indicator of age. The detection of growth zones in the Antarctic krill will be an important contribution to the understanding of the biology of the species if the zones actually represent annual growth.

We developed and characterized 20 microsatellite primer loci for the northern shrimp Pandalus borealis . All 20 loci were polymorphic with number of alleles ranging from 3 to 36 and with observed heterozygosity between 0.04 and 0.93. In addition, we tested the utility of these markers in three related shrimp species, P . montagui , Atlantopandalus propinqvus and Dichelopandalus bonnieri . These new markers will prove useful in the identification of stock structure and hence, assessment of the commercially important species P. borealis .

Survival and reproduction of an arctic population of Ameronothrus lineatus were studied at four constant temperatures (5, 10, 15, and 21 °C). By simulating winters in the laboratory, an adult population was followed through 3 \"years\". Increasing temperatures reduced adult longevity. Females survived longer than males. A temperature of 15 °C was the most favourable for reproduction, with highest larviposition rate and reproductive output. Lifetime reproductive output was also high at 10 °C, while lower numbers of larvae at 21 °C indicated the beginning of heat stress. Comparison with field data showed that the reproductive performance at 15 °C corresponded to reproduction in a natural population experiencing a mean temperature of 8–9 °C, suggesting a positive effect of daily temperature fluctuations. A simulated winter with freezing temperatures increased male survival and positively affected all aspects of reproduction the following laboratory summer.

Ameronothrus lineatus was studied in the northernmost part of its distribution range in the high Arctic. Survey sampling was carried out on two sites on West Spitsbergen: a Carex saltmarsh and a smooth layer of cyanobacteria, where the species was the only oribatid mite collected. On the latter substrate all mites were visible and could be sampled by hand, making it ideal for demographic studies. An experimental population was established in microcosms based on the mite-cyanobacteria system, and all three populations were sampled regularly during summer 1998. Our results suggested spatial variation in reproductive patterns between local populations. Influence of climatic variation on demography was illustrated by comparing reproduction in 1997 and 1998. Synchronous larviposition was observed in early summer, but recruitment also occurred throughout the season, and gravid females were found on all sampling dates. Both the saltmarsh and the cyanobacteria populations were female biased. A highly synchronous moulting involving at least 80% of the juveniles was observed in July, and it seems that juveniles moult once per season on average. The synchronous moult is suggested to be a phenological strategy to exploit the short arctic summer by timing moulting with the warmest part of the season. A generation time of 5 years was estimated. Juvenile stages of one year, continuous recruitment, and high adult longevity (2–3 years) give stable stage structures and stable population dynamics.

The current biodiversity crisis calls for conservation measures that limit the negative human impact on important habitats and sensitive wild populations. To effectively protect biodiversity at all levels, including intraspecific diversity, conservation measures should be aligned with the connectivity and genetic structure of wild populations. In this review, we synthesise scientific literature on the connectivity and population structure of marine species in the Skagerrak - a marginal sea in the northeast Atlantic Ocean. We discuss the results in relation to the current management practices in the region, as well as the general transferability of our findings. The Skagerrak is one of the most intensively studied regions within this research field, and our findings show that the overall connectivity with adjacent seas is high, but asymmetric, for most species. Simultaneously, most species have populations in the Skagerrak distinct from both each other, and those in adjacent seas. Most of this population structure is associated with the convoluted Skagerrak coastline - population structure is common both among coastal populations and between coastal and offshore populations. In many mobile species, multiple populations are temporally sympatric in certain areas, but retain their genetic divergence through natal homing or other barriers to gene flow. The presence of population structure despite high connectivity is a challenge for area-based protection measures, and calls for temporally flexible management that also monitors intraspecific genetic diversity on multiple timescales.Competing Interest StatementThe authors have declared no competing interest.