Skate Species Research Articles

Communities of metazoan parasites in seven sympatric skate species (Elasmobranchii, Rajidae) from the English Channel and Celtic Sea differing in conservation status

AbstractElasmobranch populations are in steep decline mainly due to overfishing bycatch, but parasites may accelerate the collapse of vulnerable and/or highly parasitized species. We therefore studied metazoan parasites of Rajidae from the northeast Atlantic: vulnerable Leucoraja fullonica, near‐threatened Raja brachyura, Raja clavata, Raja microocellata and Raja undulata, and least‐concerned Raja montagui and Leucoraja naevus. Overall prevalence varied from 19% for R. montagui to 100% for L. fullonica. Parasite communities differed between skate species, and prevalence and abundance were higher for L. fullonica, R. microocellata, and R. undulata. We recorded 11 parasite taxa in the study: three nematodes, six cestodes, one monogenean, and one myxosporean. Whatever the skate species, the parasite component community comprised at least two nematode taxa among Phocanema spp., Proleptus sp. and Anisakis simplex. DNA‐sequencing revealed that Phocanema azarasi and Phocanema krabbei both occurred in R. microocellata and R. undulata. Phocanema spp. was first recorded in L. fullonica, L. naevus, R. microocellata, R. montagui, and R. undulata, as Proleptus sp. in L. fullonica, and A. simplex in L. fullonica and R. clavata, Rockacestus sp. and Nybelinia sp. in R. undulata, and gill‐myxosporeans on L. fullonica, L. naevus, R. microocellata, and R. undulata. The occurrence of 16 new host–parasite associations suggests potential environmental changes. Information provided by trophically transmitted helminths confirmed an opportunistic skate diet based on crustaceans and fish. We discuss results in terms of host fitness loss, bioindicator role of parasites, and anisakiasis risk. We recommend incorporating parasitology in research to improve elasmobranch conservation.

Description of a Remarkable New Skate Species of Leucoraja Malm, 1877 (Rajiformes, Rajidae) from the Southwestern Indian Ocean: Introducing 3D Modeling as an Innovative Tool for the Visualization of Clasper Characters.

A remarkable new deep-water skate, Leucoraja longirostris n. sp., is described based on eight specimens caught during different expeditions to the southern Madagascar Ridge in the southwestern Indian Ocean. The new species differs from all congeners by its remarkably long and acutely angled snout (horizontal preorbital length 17.2-22.6% TL vs. 8.5-11.9% TL and 4.2-6.1 vs. 1.7-3.5 times orbit length, snout angle 65-85° vs. 90-150°). Furthermore, it is apparently endemic to the Madagascar Ridge, distant from the known distribution areas of all congeners. In addition to L. fullonica and L. pristispina, L. longirostris n. sp. is also the only species with plain dorsal coloration. Furthermore, the new species is the only Leucoraja species with an external clasper component dike and, besides L. wallacei, the only one with four dorsal terminal (dt) cartilages. The shape of the accessory terminal 1 (at1) cartilage with four tips is also unique within the genus. A new approach for the visualization of the clasper characters is introduced based on 3D models of all skeletal and external features. This enables a much easier and much more precise interpretation of every single clasper component, of the entire structure, and, in particular, the relationship between external features and skeletal cartilages. A new English translation of the first diagnosis of Leucoraja is provided, along with a revised generic diagnosis and a key to the species of Leucoraja in the Indian Ocean.

An assessment of post-capture condition and survival of Rajidae caught in fixed nets

Skates (Rajiformes: Rajidae) are commercially and ecologically-important elasmobranchs captured in targeted and mixed fisheries globally. Many species are of conservation concern and are subject to high levels of discarding. They are currently exempt from the European Union’s landing obligation due to evidence of high discard survival, although the bulk of this evidence comes from mobile gear fisheries. Improved evidence on survival after discarding from fixed gears is necessary. We assessed immediate survival rates and post-capture condition (injury, reflex impairment, and vitality) of four skate species commonly discarded from seasonal Dover sole, Solea solea and plaice, Pleuronectes platessa gillnet fisheries in the Irish Sea and Bristol Channel, UK. We also modelled the relationship between a suite of environmental and operational fishing variables to the condition of thornback skate, Raja clavata. This is the dominant skate captured as bycatch and is classified as near threatened by the IUCN due to declining populations trends. At-vessel survival on gear hauling was high for all species (R. clavata = 99.43%, R. montagui = 100%, R. microocellata = 100%, R. brachyura = 100%). Post-capture condition of R. clavata was size-dependent, with larger skates demonstrating higher levels of injury and reflex impairment and poorer vitality. Higher vitality in smaller individuals and strong negative effects of aerial exposure on skate vitality highlight the likely benefits of prioritising discarding of unwanted catch upon gear hauling. This may maximise survival and future reproductive potential and facilitate population recovery.

The distribution of the juvenile stages and eggcases of skates (Rajidae) around the British Isles

Abstract Skates (Rajidae) include species of conservation and commercial importance, but there is limited information regarding their ecologically important habitats, including spawning and nursery areas, in northern European seas. Data on the distribution, relative abundance and length composition of skates were collated from scientific trawl surveys, and based on length‐frequency information, length‐at‐hatching and length‐at‐maturity, skates were allocated to length‐based categories representing early‐juvenile stages (EJS) and mid‐juvenile stages (MJS). Citizen science data were used to inform on the occurrence of eggcases. The Great Eggcase Hunt (starting in 2003) citizen science recording project collects georeferenced data for elasmobranch eggcases, primarily from around the British Isles. This provided verified species‐specific data for a total of 227,407 specimens from 11 skate species. Purse Search Ireland (starting in 2005) provided a further 2491 records of 10 skate species from around Ireland. The most frequently observed skate eggcases were from the genus Raja and occurred around much of the British Isles. The EJS and MJS of such species were observed most frequently in inshore waters. Data for EJS were more limited for Raja microocellata and Raja undulata, indicating that the EJS of these species occur in shallower (<20 m) waters. Juveniles of the genus Leucoraja occurred further offshore. Only the eggcases of Leucoraja naevus were recorded regularly, with the eggcase of Leucoraja fullonica described here. The data collated here provide an overview of skate spawning and nursery areas, thus informing fisheries management and marine spatial planning, including the identification and delineation of Important Shark and Ray Areas (ISRAs).

A tale of two species: Disaggregating mixed historical catches of two most common skates in the Northeast Pacific Ocean

Historical catch data represent a key source of information for fisheries stock assessment models. Historically, commercial fishery catch statistics were estimated from the portion of catch landed in ports. However, many species with relatively low economic value have not been recorded on a species basis but instead as a part of an aggregate category. Reconstructing component species catch from an aggregate category is a common challenge for fisheries stock assessment efforts around the world. Skates are one group of species with low economic value for which landed catch has not been commonly reported by species. In this paper, we present a novel approach to disaggregate the historical catch of the two most abundant skate species on the West Coast of the United States, longnose skate (Caliraja rhina) and big skate (Beringraja binoculata), landed within the aggregated skate category, in ports of Washington State. We used a combination of fishery-dependent and fishery-independent data sources to account for changes in the spatial extent of the fishery over time, and differences in the depth distribution of these two skate species. While developed to disentangle aggregate catch of longnose and big skates, the approach is not limited to skates on the West Coast of the United States, but can be adapted for any species which landings have been reported within an aggregated category elsewhere.

A Faunistic Revision of Longnose Skates of the Genus Dipturus (Rajiformes: Rajidae) from the Southern Southwestern Atlantic Ocean, Based on Morphological and Molecular Evidence

The identity of longnose skates (Dipturus-Zearaja-like skates) in the southern cone of the Americas has been a topic of extensive debate. This study employs a comprehensive analysis encompassing morphometrics, claspers, and the examination of COI and NADH2 sequence data to conclusively demonstrate the existence of only two longnose skate species in the southwestern Atlantic Ocean, extending south of 35 °S. Notably, Dipturus argentinensis Díaz de Astarloa, Mabragaña, Hanner and Figueroa, 2008 is revealed as a junior synonym of D. trachyderma (Krefft and Stehmann, 1975). Dipturus leptocaudus (Krefft and Stehmann, 1975) remains a northern valid species, but the specimen from the Falkland Islands (Malvinas) is recognized as a misidentification of D. trachyderma. Zearaja flavirostris (Philippi, 1892) and Dipturus lamillai Concha, Caira, Ebert and Pompert, 2019 are confirmed as junior synonyms of Zearaja brevicaudata (Marini, 1933). These findings contradict the previous report of six species within the same region over the last decade and underscore the presence of D. trachyderma and Z. brevicaudata south of 35 °S in the southwestern Atlantic. Additionally, this study notes the occurrence of only one specimen of Z. chilensis (Guichenot, 1848) in the Falkland Islands (Malvinas), suggesting an unusual frequency of this eastern Pacific skate in the southern Southwest Atlantic. Given that clasper morphology serves as the key distinguishing trait between Dipturus and Zearaja species, we provided a detailed analysis of the clasper characteristics of the Atlantic D. trachyderma, unequivocally situating it within Dipturus. The diagnostic characteristics include: (i) the presence of cartilage with the distal portion referred to as the sentinel, a feature absent in Zearaja; (ii) longer ventral terminal cartilage with the distal end referred to as the funnel, compared to Zearaja; and (iii) a non-spatulate distal lobe, a distinctive trait specific to Dipturus.

Survivorship of discarded cuckoo ray in bottom trawl fisheries in the northern Bay of Biscay, Southern Celtic and Irish Seas

Estimating the survivorship of discarded fish is often crucial for stock assessment and resultant management of fisheries resources. In order to assess the survivorship of cuckoo ray (Leucoraja naevus) caught in commercial bottom trawl fisheries, experiments involving French and Irish fisheries were conducted. These experiments were particularly challenging considering the fisheries are offshore. Data from these experiments were analysed jointly in this study. Results from monitoring of individuals held in captivity in onshore facilities and observations of vitality status were combined to produce estimates of long-term survival rates across fishing trips in all four seasons. These rates varied greatly between fishing trips and ranged between 3.6% and 26%. Based on indications that the observed mortality may be at least partly attributable to the conditions of captivity, we propose an approach to reduce the underestimation bias when estimating discard survival. This approach produced higher bounds for the estimated discard survival rates. The estimated survival rates are lower than those of other skate species in the North-East Atlantic, suggesting the cuckoo ray is less resilient to trawl-and-release, although previous studies were conducted in shallower water and with shorter haul duration.

New records and range extension of two deep-sea skate genera,AmblyrajaandNotoraja(Elasmobranchii: Rajiformes), in the southeastern Pacific Ocean

AbstractTwo species of deep-sea skates,Amblyraja frerichsiandNotoraja martinezi, are reported for the first time in Peruvian waters, representing the first record of their respective genera from the country. Earlier,A.frerichsiwas known from both the southwestern Atlantic (off Brazil, Uruguay, and Argentina) and southeastern Pacific (off Chile), whileN.martineziwas known from Costa Rica to Ecuador in the eastern-central and southeastern Pacific. This study contributes to the limited knowledge of deep-sea fish diversity along the South American west coast, and stresses the need of further taxonomic research in the region, particularly in light of the ecological importance of this group of fishes in marine ecosystems and its conservation state worldwide.

Population ecology and juvenile density hotspots of thornback ray (Raja clavata) around the Shetland Islands, Scotland.

Elasmobranchs are facing global decline, and so there is a pressing need for research into their populations to inform effective conservation and management strategies. Little information exists on the population ecology of skate species around the British Isles, presenting an important knowledge gap that this study aimed to reduce. The population ecology of thornback ray (Raja clavata) around the Shetland Islands, Scotland, was investigated in two habitats: inshore (50-150 m deep) and shallow coastal (20-50 m deep), from 2011 to 2022, and 2017 to 2022, respectively. Using trawl survey data from the annual Shetland Inshore Fish Survey, the size composition of R. clavata catches was compared between shallow and inshore habitats across 157 trawl sets, and 885 individuals, over the years 2017-2022. Catch per unit effort (CPUE) of R. clavata was significantly higher in shallow than that in inshore areas (ANOVA, F = 72.52, df = 1, 5, p < 0.001). Size composition also significantly differed between the two habitats (analysis of similarities, R = 0.96, p = 0.002), with R. clavata being smaller in shallow areas and juveniles (<60 cm) occurring more frequently. Spatial distribution maps confirmed density hotspots of juveniles in shallow habitats, with repeated use of certain locations consistent over time. The results of this study provide the first evidence for R. clavata using shallow areas for potential nurseries in Shetland, which can inform the IUCN's Important Shark and Ray Area process. Furthermore, this study provides important new population ecology information for R. clavata around Shetland, which may have important conservation implications and be valuable for informing species and fisheries stock assessments in this region.

Effects of chondroitin sulfate oligosaccharides on osteoclast differentiation of RAW264 cells, and myotube differentiation of C2C12 cells.

Chondroitin sulfate (CS) is a glycosaminoglycan, and CS derived from various animal species is used in drugs and food supplements to alleviate arthralgia. The CS is a high molecular weight compound, and hydrolysis of CS by intestinal microbiota is thought to be required for absorption in mammalians. Chondroitin sulfate oligosaccharides (Oligo-CS) are produced by hydrolysis with subcritical water from CS isolated from a species of skate, Raja pulchra for the improvement of bioavailability. The present study conducted in vitro experiments using murine cell lines, to compare the biological activities of Oligo-CS and high molecular weight CS composed with the similar disaccharide isomer units of D-glucuronic acid and N-acetyl-D-glucosamine (CS-C). The results show that Oligo-CS inhibits osteoclast differentiation of RAW264 cells significantly at lower concentrations than in CS. The cell viability of a myoblast cell line, C2C12 cells, was increased when the cells were grown in a differentiated medium for myotubes with Oligo-CS, where there were no effects on the cell viability in CS. These results suggest that in vitro Oligo-CS exhibits stronger bioactivity than high-molecular weight CS.

A ray of hope? The re-appearance of Irish Sea skate decades after local extinction.

A 2019 photograph of a blue skate Dipturus batis caught in Welsh waters is the first species-specific evidence of the "common skate" complex occurring in its former range in the main body of the Irish Sea over four decades after it was considered to have been extirpated there. This possible precursor to recolonisation of former range adds to growing evidence for the recovery of skate species in the North Atlantic and highlights the valuable role of anglers and social media as a complement to important, yet costly, scientific surveys when monitoring rare fishes.

Blondes do it better? A comparative study on the morphometry and life-history traits of commercially important skates blonde ray Raja brachyura, thornback ray Raja clavata, and spotted ray Raja montagui, with management implications

Due to data limitation and challenges with species-specific reporting, the management of European skate fisheries currently lacks measures tailored to individual species. Given these caveats, detailed information about skate stocks at the species level is warranted. The morphometry and life-history parameters were determined for samples of 170 individuals (58 Raja brachyura, 92 R. clavata, 20 R. montagui) collected by observers in commercial fisheries, and two bottom trawl surveys (December 2020 – August 2021) in the Southern North Sea and Eastern English Channel. A combination of three morphometric measurements were considered and proven successful (3.846% misclassification, N = 78) in species discrimination for easily confusable species R. brachyura and R. montagui. Through the application of these morphometric descriptors in electronic catch monitoring via Artificial Intelligence (AI), the misidentification of easily confusable species may be reduced in future catch reporting from surveys, fisheries-dependent data, and market sampling. Additionally, the morphometric differences between species may have implications for species’ catchability, and gear modification to avoid bycatch of skates under the European Landing Obligation (LO). The length-weight relationships indicated a faster rate of increase in body mass per unit of size for R. montagui. Life-history trait parameter estimates were obtained for R. clavata and R. brachyura. Age was determined through digital processing of images from visually enhanced whole vertebral centra (0.01% crystal violet staining). Age reading precision was low (CV > 45%, PA < 60%). The von Bertalanffy and logistic growth models provided more accurate fits for the length-at-age data than the Gompertz model. Macroscopic maturity assessment revealed that male R. montagui matured at the smallest size (Lm = 57.5 cm), while female R. brachyura matured at the largest size (Lm = 87.6 cm). Maturity ogives were fitted for males (L50 = 65.4 cm) and females (L50 = 84.1) of R. clavata and females of R. brachyura (L50 = 82.7 cm), with statistically significant intersexual differences for R. clavata. The Legal Minimum Landing Size (LMLS) measures of smaller sizes may not offer adequate protection (to immature specimens) of the skate species in the present study, that mature at larger sizes. The life-history trait parameters for growth and maturity are discussed in relation to published results of earlier studies, and no major changes in the life-history parameter traits were observed in recent years. The interspecific differences in the morphometries and life-history traits of R. brachyura, R. clavata, and R. montagui highlight the need for species-specific data collection and management considerations of skate stocks in the Northeast Atlantic Region at the local and EU level.

Gradients of Variation in the At-Vessel Mortality Rate between Twelve Species of Sharks and Skates Sampled through a Fishery-Independent Trawl Survey in the Asinara Gulf (NW Mediterranean Sea)

Elasmobranchs are priority species for conservation due to their rapid decline determined by the unbalanced struggle between a fragile bio-ecology and strong anthropogenic impacts, such as bycatch from professional fishing. In this context, measuring species resistance to catch of poorly selective gear is of paramount importance. During June-October 2022, five experimental fishing campaigns were carried out in the Asinara Gulf (northern Sardinia) through 35 geographically and bathymetrically representative hauls of an area between 30 and 600 m in depth. Skates prevailed over sharks in the number of species, with seven and five species, respectively. We first evaluated the status of each individual with respect to stress due to the trawl's catch using a three-graded scale. We also recorded individual biometrics (total and disk length, weight and sex, and maturity for males) on board by implementing the best practices in manipulating individuals for physiological recovery and release at sea. After capture, skates resulted in generally better conditions than sharks, although deepwater species of both groups exhibited a worse state than coastal species. The estimated vitality rates also depended on the size of the individuals. This work provides standardized data on the intermingled effect of size, species type, and inhabited depth on the resistance response of some elasmobranch species against capture by trawl fishery activities.

Demographic analyses reveal differential biological vulnerability in four Southwestern Atlantic skates

More than 33 skate species have been registered in the Southwestern Atlantic Ocean (SAO) 34°S, where hotspots of the endemic threatened marine chondrichthyans with high scientific and conservation priorities have been detected. The lack of species-specific landing data highlights the importance of biological data-based methods to determine the vulnerability and productivity of skate species. In this work, we estimated the demographic parameters for the skates Atlantoraja castelnaui, Rioraja agassizi, Sympterygia bonapartii, and Zearaja brevicaudata in SAO, by implementing Leslie matrix models. The finite rate of population growth ( ʎ) for the four skate's species ranged from 1.036 ( Z. brevicaudata) to 1.196 ( R. agassizi). Elasticity analyses and the performance of different harvest strategies show that reducing juvenile catches appears to be one of the most effective management actions for these species since they result in higher survivorship of age classes with more influence in λ. Differences in the biological productivity of the skates suggest that, to achieve sustainable exploitation of skates, the commitment of all stakeholders to improve species-level information is necessary.

Embryonic development timeline in skates (Chondrichthyes: Rajiformes): Sympterygia acuta as the first case study in the family Arhynchobatidae

Oviparous elasmobranch embryos (Chondrichthyes) have been the focus of several embryological studies; they are useful models for studying early ontogeny in vertebrates, as can help explore the existence of common developmental patterns among species. Skates (Rajiformes) are the most speciose order of oviparous elasmobranchs, however, few studies are focused on embryo development and only based on one skate family: Rajidae. Here, we extended the study of embryo development to other skate family, Arhynchobatidae, which represent about 1/3 of all skate species. Three adult female bignose fanskates (Sympterygia acuta) were held in captivity in order to provide the first complete embryonic development timeline for any species within the Arhynchobatidae family. Our results allowed further comparisons at the embryonic scale of different oviparous elasmobranch families, providing an updated cross-species overview of the early ontogeny. Incubation in S. acuta lasted 97 ± 1.4 days at 11–21.7 °C, and hatching size was 93.2 ± 0.2 mm in total length and 49.2 ± 0.3 mm in disc width. Early embryos of S. acuta were anatomically similar to other oviparous elasmobranch embryos, with several structures appearing at the same time, but late embryonic development was comparatively delayed. The late resorption of both the external yolk sac and the external gill filaments, and also the delay in the slit opening could indicate a low metabolic demand in S. acuta, which would probably be coupled with its seasonal reproductive cycle. Some structures such as external gill filaments and claspers appeared at a similar time in some species of Rajidae and also in Arhynchobatidae, but at different times in species of the same family, showing an inconsistency also found within shark families. Although the sequential scheme remained relatively constant, small heterochronies would be present within skates, within sharks, and also between skates and sharks.

A long-term view on recent changes in abundance of common skate complex in the North Sea

Following decades of declines, populations of large fish recently started to increase in the North Sea, presumably due to reduced fishing pressure. However, population recovery may be too readily claimed, since standardised sampling of fish stocks commenced only in the 1970s, well after many species had already collapsed. A true recovery must be seen from a long-term perspective. The critically endangered common skate (Dipturus batis, Rajidae) species complex is an example of a large-bodied fish that mostly disappeared before standardised monitoring took place. Here, we put the recent increase in population size into a 120-year perspective, throughout three geographical divisions in the North Sea. We analysed a large range of mostly undisclosed historical data and contemporary sources. A reconstruction of Dutch commercial landings data confirms that the species used to be very abundant between 1901 and 1920, and shows how it steadily declined from 1920 onwards until it got extirpated around 1970. Based on a quantitative analysis of standardised catch numbers from fishery-independent surveys time, we conclude that the current abundance of the species is still below historical baselines and represents a local recovery at most. We further demonstrate a prominent and consistent pattern in size distribution, with larger (mature) individuals only occurring in the northern North Sea. A large dataset on historical stomach contents from the central North Sea confirmed the diet of young common skate, which consisted predominantly of shrimps. Our review exemplifies the importance of marine historical ecology to deduce the natural richness of the North Sea.

Spatio‐temporal dynamics of the common skate species complex: Evidence of increasing abundance

AbstractAimThe critically endangered common skate species complex is a large‐bodied and long‐lived batoid, which has experienced local extirpations and population declines over the past century mainly due to overfishing. Due to its decline, fisheries management measures were introduced to prevent further decline and fragmentation of populations. For example, in 2009, a landings prohibition was introduced in the European Union, which banned the retention of common skate onboard commercial fishing vessels with captured individuals to be discarded. We aimed to explore the spatial and temporal population dynamics of the common skate species complex, against the backdrop of changes in fisheries management measures.LocationNortheast Atlantic Ocean.MethodsWe used publicly available fishery‐independent trawl survey data from several regions of the Northeast Atlantic shelf to examine trends in incidence and abundance for the common skate species complex. We also constructed a species distribution model to identify changes in the spatio‐temporal distribution of the common skate.ResultsA sustained increase in the common skate species complex was evident in several areas of its distribution. An increase was observed in five separate trawl surveys encompassing distinct regions of its distribution. Despite the observed increase, little evidence of recolonizing previously extirpated areas was evident.Main ConclusionsThe findings demonstrate the effectiveness of fisheries management measures in contributing to an increase in the common skate species complex. Such measures may also be effective if applied to numerous other batoid species currently threatened with extinction.

Diversity of the egg tertiary membranes in some elasmobranch species and comments on the actual yolk-sac viviparous reproductive mode

After fertilization, elasmobranch eggs (mostly megalecithal) are encapsulated in a tertiary membrane of different complexity, in the oviducal gland. The gross morphological characteristics of this membrane in elasmobranchs are highly variable, attaining it most sophisticates’ forms within the oviparous species. The purpose of this study was to contribute to the knowledge of the elasmobranch tertiary membranes, in both viviparous and oviparous species, and to discuss the relation with their role in protecting the embryos their development. In addition, the limitations of yolk-sac viviparity as a single reproductive mode category are commented, suggesting a new arrangement of taxa within that reproductive mode and new terminology. Data on tertiary membranes in elasmobranchs come from a large number of sources ranging from 1977 to date. Specimens were captured on board of commercial and scientific research cruises, from the following some major geographic regions: South Western Atlantic, South Western Pacific, and different areas of the Mediterranean Sea. In all, our records of tertiary membranes belong to ten species of sharks of five orders; Hexanchiformes, Squatiniformes, Heterodontiformes, Lamniformes, Carcharhiniformes, and eight species of rays and skates, of four orders: Rajiformes, Rhinopristiformes, Torpediniformes and Myliobatiformes. The reproductive modes reported for all this species were both lecithotrophic (yolk-sac viviparity, short single oviparity) and matrotrophic (ovotrophy, placentotrophy, limited and lipidic histotrophy). Six morphological and structural major patterns were detected within the yolk-sac viviparous species (type I to VI); a thin, diaphanous, shapeless membrane; candle capsules; oviparous-like tough egg capsule, and cases where no tertiary membrane was present.

Parasites as tags for stock identification of a highly exploited vulnerable skate Dipturus brevicaudatus (Chondrichthyes: Rajidae) in the south‐western Atlantic Ocean, a complementary tool for its conservation

Abstract Populations of chondrichthyans are particularly sensitive to elevated fishing mortality due to their low reproductive potential and very low population growth. Dipturus brevicaudatus is a common and endemic skate from waters of Argentina and is frequently caught as by‐catch and is therefore extremely vulnerable to commercial fishing. There are signs of overfishing in both the northern and southern Argentine Sea and the species has been classified as ‘Vulnerable’ by the International Union for Conservation of Nature. A parasitological examination of 96 skates revealed 1,913 individual macroparasites belonging to 18 species and one myxozoan. Three parasite species were long‐lived and were suitable biological tags, resulting in statistically significant differences between North Patagonic and both Buenos Aires and South Patagonic regions and therefore can be used to identify stocks of D. brevicaudatus. The inclusion of short‐lived parasites in the analyses suggested the existence of three stocks. Parasite assemblages from the northern Argentine Sea (Buenos Aires, Argentine biogeographical province) and from Patagonian waters (North and South Patagonic, Magellanic biogeographical province) varied in terms of parasite infracommunity species richness and the relative abundance of parasite guilds. Using parasites to infer the existence of three stocks of D. brevicaudatus provides the basis to help establishing management and recovery strategies to preserve the diversity of this vulnerable skate in the south‐west Atlantic Ocean. Due to the low host specificity of the long‐lived parasite tags, there is potential to extend this methodology to other skate species that are commercially exploited in the region, whose population structures remain unknown.

Interspecific differences in the flow regimes and drag of North Pacific skate egg cases.

Skates are a diverse group of dorso-ventrally compressed cartilaginous fishes found primarily in high-latitude seas. These slow-growing oviparous fishes deposit their fertilized eggs into cases, which then rest on the seafloor. Developing skates remain in their cases for 1-4 years after they are deposited, meaning the abiotic characteristics of the deposition sites, such as current and substrate type, must interact with the capsule in a way to promote long residency. Egg cases are morphologically variable and can be identified to species. Both the gross morphology and the microstructures of the egg case interact with substrate to determine how well a case stays in place on a current-swept seafloor. Our study investigated the egg case hydrodynamics of eight North Pacific skate species to understand how their morphology affects their ability to stay in place. We used a flume to measure maximum current velocity, or "break-away velocity," each egg case could withstand before being swept off the substrate and a tilt table to measure the coefficient of static friction between each case and the substrate. We also used the programming software R to calculate theoretical drag on the egg cases of each species. For all flume trials, we found the morphology of egg cases and their orientation to flow to be significantly correlated with break-away velocity. In certain species, the morphology of the egg case was correlated with flow rate required to dislodge a case from the substrate in addition to the drag experienced in both the theoretical and flume experiments. These results effectively measure how well the egg cases of different species remain stationary in a similar habitat. Parsing out attachment biases and discrepancies in flow regimes of egg cases allows us to identify where we are likely to find other elusive species nursery sites. These results will aid predictive models for locating new nursery habitats and protective policies for avoiding the destruction of these nursery sites.