Rostroraja Alba Research Articles

Updating knowledge on the Critically Endangered white skate Rostroraja alba (Lacepède, 1803) through the first record in North-East Sardinian coastal waters

Updating knowledge on the Critically Endangered white skate Rostroraja alba (Lacepède, 1803) through the first record in North-East Sardinian coastal waters

Horizontal and vertical movements of the critically endangered Rostroraja alba in a coastal marine protected area

Abstract Elasmobranchs are slow‐growing marine predators whose populations have declined due to their susceptibility to overfishing, especially affecting species of large size. Rostroraja alba is a large skate that is currently classified as critically endangered. To address the effects of overfishing and promote the recovery of marine species and habitats, marine protected areas (MPAs) are a commonly implemented management strategy. A critical aspect of their success is using information on species' movement patterns; however, this is frequently not assessed for elasmobranchs. In this study, acoustic telemetry was used to monitor 30 R. alba in the Professor Luís Saldanha Marine Park (Portugal) to assess their long‐term movements in the context of a coastal MPA and to evaluate the contribution of this kind of spatial management to the conservation of this species. Immature individuals were more frequently captured than adults, which postulates this area as a potential nursery. The average residency was moderate and relatively consistent throughout the year, as was area use. No marked size or sex‐based differences in movement were noted. Individuals were more active and occurred in shallower waters at night and twilight, and occasional quick vertical movements into the water column, isolated or in sequence, were also noted. In general, the LSMP seems to offer stable protection to immature R. alba, with no strong seasonal variation. These results provide relevant input for adaptive management measures of this and other similar MPAs and highlight the contribution of these efforts to the protection of nursery areas.

RAJONCHOCOTYLE CERFONTAINE, 1899 (MONOGENEA: HEXABOTHRIIDAE) SPECIES FROM SOUTH AFRICA, WITH DISCUSSION OF THE LITERARY ACCOUNTS OF R. EMARGINATA (OLSSON, 1876).

Rajonchocotyle Cerfontaine, 1899, species are blood-feeding parasites on the gills of rajiforms. Eight species are considered valid, the most recent of which was described just after World War II. Many of the original descriptions of Rajonchocotyle species are limited in diagnostic value, and comparative museum material is scanty. The genus requires revision, and in support thereof, we provide detailed redescriptions for Rajonchocotyle albaCerfontaine, 1899, from the type host Rostroraja alba (Lacepède, 1803) and Rajonchocotyle emarginata (Olsson, 1876), Sproston, 1946, from 2 new host records, Raja straeleni Poll, 1951, and Leucoraja wallacei (Hulley, 1970), from South Africa, a new locality record. The generic diagnosis for Rajonchocotyle is amended to include greater details of the male reproductive system and confirms Paul Cerfontaine's and Nora Sproston's historic observations of the morphology of the male copulatory organ consisting of a separate proximal seminal vesicle and a distal cirrus. The lectotype of Rajonchocotyle kenojeiYamaguti, 1938, is assigned, and a comprehensive Rajonchocotyle-host species list is provided, highlighting species records that require further verification, and the purported global host range of R. emarginata is discussed.

Total mercury concentrations in sharks, skates and rays along the South African coast.

Global declines in elasmobranch populations resulting from several stressors raises conservation concern. Additionally, apex predators bioaccumulate high concentrations of total mercury (THg), due to biomagnification. Although South Africa is considered one of the top ten contributors of Hg emissions globally, information on Hg concentrations in elasmobranchs is limited. The aim of this study was to evaluate the THg concentrations in 22 species of elasmobranchs along the South African coastline. Concentrations ranged between 0.22 and 5.8mg/kg in Haploblepharus pictus (dark shysharks) and Rostroraja alba (white skates) on the south coast, respectively. Along the east coast it ranged between 0.21 and 17.8mg/kg in Mobula kuhlii (shortfin devil rays) and Sphyrna lewini (scalloped hammerheads), respectively. Mercury concentrations on the east coast were in the same range or higher compared to the same species sampled between 2005-10 from the same region, with generally higher concentrations compared to the same species sampled globally.

Threatened, host-specific affiliates of a red-listed host: Three new species of Acanthobothrium van Beneden, 1849 (Cestoda: Onchoproteocephalidea) from the endangered white skate, Rostroraja alba (Lacépède).

The white skate, Rostroraja alba (Lacépède), is listed as an endangered species, the second-highest category before being declared extinct in the wild, in the International Union for Conservation of Nature's (IUCN) Red List of Threatened Species. This species is heavily affected by anthropogenic impacts such as capture induced stress by overfishing and by-catch, habitat degradation and pollution that caused a drastic decline in populations in recent years. As part of a larger study on elasmobranch affiliates in southern Africa, two specimens of R. alba were screened. Three species of the tapeworm genus Acanthobothrium van Beneden, 1849 (Cestoda: Onchoproteocephalidea) were discovered. Application of Ghoshroy and Caira's classification system facilitated the differentiation of congeners through a combination of specific morphological characteristics. As a consequence, three species new to science are described herein, namely Acanthobothrium umbungus n. sp., Acanthobothrium usengozinius n. sp., and Acanthobothrium ulondolozus n. sp. In light of these new discoveries from an endangered host, it is apparent to address the conservation status of its affiliate species, that co-evolved with their elasmobranch hosts for millions of years, thereby creating unique and intimate host-parasite interrelationships. Currently, altering environmental conditions caused by anthropogenic pressures have direct impacts on this host-parasite system with increasing risks of extinction. As merely 9% of elasmobranchs in South African waters have been examined for endohelminths and other affiliate taxa, extensive studies on these organisms and their hosts implementing multisource approaches are needed. This will provide a better understanding on the intimate nature of host-parasite systems that may lead to new prospects in conservation science and the preservation of threatened host species, such as R. alba, together with their unique fauna of affiliate species.

Differences in the occurrence and abundance of batoids across an oceanic archipelago using complementary data sources: Implications for conservation.

Batoids, distributed from shallow to abyssal depths, are considerably vulnerable to anthropogenic threats. Data deficiencies on the distribution patterns of batoids, however, challenge their effective management and conservation. In this study, we took advantage of the particular geological and geomorphological configuration of the Canary Islands, across an east‐to‐west gradient in the eastern Atlantic Ocean, to assess whether patterns in the occurrence and abundance of batoids varied between groups of islands (western, central, and eastern). Data were collected from shallow (<40 m, via underwater visual counts and by a local community science program) and deep waters (60–700 m, via ROV deployments). Eleven species of batoids, assessed by the IUCN Red List of Threatened Species, were registered, including three “Critically Endangered” (Aetomylaeus bovinus, Dipturus batis, and Myliobatis aquila), three “Endangered” (Gymnura altavela, Mobula mobular, and Rostroraja alba), two “Vulnerable” (Dasyatis pastinaca and Raja maderenseis), and two “Data Deficient” (Taeniurops grabata and Torpedo marmorata). Also, a “Least Concern” species (Bathytoshia lata) was observed. Overall, batoids were ~1 to 2 orders of magnitude more abundant in the central and eastern islands, relative to the western islands. This pattern was consistent among the three sources of data and for both shallow and deep waters. This study, therefore, shows differences in the abundance of batoids across an oceanic archipelago, likely related to varying insular shelf area, availability of habitats, and proximity to the nearby continental (African) mass. Large variation in population abundances among islands suggests that “whole” archipelago management strategies are unlikely to provide adequate conservation. Instead, management plans should be adjusted individually per island and complemented with focused research to fill data gaps on the spatial use and movements of these iconic species.

On the presence of the Endangered white skate Rostroraja alba in Sardinian waters

The white skate Rostroraja alba is assessed by the International Union for Conservation of Nature as Endangered globally and is now considered rare in the Mediterranean. This species has never been recorded in Sardinian seas (central western Mediterranean), but an empty eggcase was collected on a sandy bottom off the western Sardinia coasts during the MEDiterranean International Trawl Survey (MEDITS) program in 2019. A morphometric and morphological description of the Sardinian eggcase, after a comparison with measurements from other Mediterranean and Atlantic specimens, showed it to be clearly ascribable to Rostroraja alba.

Preliminary insights into the spatial ecology and movement patterns of a regionally critically endangered skate (Rostroraja alba) associated with a marine protected area

ABSTRACTThe implementation of spatial protection measures is currently a priority in batoid species' conservation strategies, but their spatial ecology remains largely unknown. We provide some preliminary insights into the movement patterns of the white skate (Rostroraja alba), a batoid classified as Critically Endangered in European waters. Three individuals (two females: one mature, one immature; and one immature male) were monitored with acoustic telemetry in a marine protected area (Portugal). The mature female remained present in the study area throughout the whole monitoring period (20 months). Residency analyses revealed this specimen spent more than 70% of the time within this coastal marine park. The immature female and the immature male were only detected during three and four months, respectively. Whether the uncovered movement patterns are common within the population remains unclear, but the present study provides useful information to better plan future research on the movement ecology of a rather unstudied species.

Range Extensions and New Records from Alaska and British Columbia for Two Skates, Bathyraja Spinosissima and Bathyraja Microtrachys

Recent deep-water surveys of the continental slope in the Bering Sea and the eastern North Pacific, conducted by the US National Marine Fisheries Service, Alaska Fisheries Science Center, and Fisheries and Oceans Canada, have broadened our understanding of the ichthyofauna in the region. Herein, we report significant new records of 2 species of skates based on morphological and molecular data. For Bathyraja spinosissima, the White Skate, we describe 2 specimens: a neonate male that constitutes a northern range extension for the species to Alaska in the Bering Sea; and an adult male representing a new record for the species in British Columbia. For B. microtrachys, the Fine-spined Skate, we describe 5 males taken in a single haul that represent the 1st record of the species in British Columbia. These specimens had been previously misidentified as B. violacea, the Okhotsk Skate, a species known only from the western Aleutian Islands and the western North Pacific.

Skating on thin ice: Identifying the need for species‐specific data and defined migration ecology of Rajidae spp.

AbstractSkates (class Chondrichthyes; subclass Elasmobranchii; order Rajiformes; family Rajidae) comprise one quarter of extant chondrichthyans, yet have received little attention in the scientific literature likely due to their relatively low economic value and difficulties in species identification. The absence of species‐specific information on catch, life history and migration of skates has often precluded the development of single‐species stock assessments and led to the use of cursory multispecies assessments, which lack the ability to track species‐specific catch and abundance trends. This has resulted in undetected local extirpations, as has been previously reported for common (Dipturus batis, Rajidae), white (Rostroraja alba, Rajidae) and long‐nose (Dipturus oxyrhinchus, Rajidae) skates in the Irish Sea. Here, we (a) use case studies to illustrate how the perception of skate population structure and stock status has historically been masked through multispecies assessment and management practices, (b) review current information on the movement of skates and identify gaps in knowledge, and (c) identify biases associated with the use of various tagging technologies, which have confounded our understanding of movement and migration ecology of skates. Considering that over 40% of extant Rajidae species are listed as “Data Deficient” by the International Union for Conservation of Nature, we illustrate a critical need to broaden the current understanding of skate life history, movement and migration ecology by providing recommendations on the further application of electronic tags and biogeochemical natural tags in movement studies and highlight the benefits that studies using these approaches have for novel management frameworks.

Descriptions and revised key to the eggcases of the skates (Rajiformes: Rajidae) and catsharks (Carcharhiniformes: Scyliorhinidae) of the British Isles.

Updated descriptions and measurements for the eggcases of 10 rajiform and three scyliorhinid species occurring in the shelf seas around the British Isles are given, based on museum material, specimens collected during the 'Great Eggcase Hunt' (a Shark Trust recording project), and specimens obtained from fishery surveys. Quantitative data are given for Amblyraja radiata (n = 94), Dipturus batis (n = 24), D. cf. intermedia (n = 33), Leucoraja naevus (n = 94), Raja brachyura (n = 53), R. clavata (n = 52), R. microocellata (n = 57), R. montagui (n = 52), R. undulata (n = 52), Rostroraja alba (n = 5), Galeus melastomus (n = 7), Scyliorhinus canicula (n = 52) and S. stellaris (n = 58). An updated key for the identification of elasmobranch eggcases is provided, incorporating recent changes in skate taxonomy.

Are critically endangered fish back on the menu? Analysis of U.K. fisheries data suggest post-ban landings of prohibited skates in European waters

Skates (Rajidae) have been commercially exploited in Europe for hundreds of years with some species’ abundances declining dramatically during the twentieth century. In 2009 it became “prohibited for EU vessels to target, retain, tranship or land” certain species in some ICES areas, including the critically endangered common skate and the endangered white skate. To examine compliance with skate bans the official UK landings data for 2011–2014 were analysed. Surprisingly, it was found that after the ban prohibited species were still reported landed in UK ports, including 9.6t of common skate during 2011–2014. The majority of reported landings of common and white skate were from northern UK waters and landed into northern UK ports. Although past landings could not be validated as being actual prohibited species, the landings’ patterns found reflect known abundance distributions that suggest actual landings were made, rather than sporadic occurrence across ports that would be evident if landings were solely due to systematic misidentification or data entry errors. Nevertheless, misreporting and data entry errors could not be discounted as factors contributing to the recorded landings of prohibited species. These findings raise questions about the efficacy of current systems to police skate landings to ensure prohibited species remain protected. By identifying UK ports with the highest apparent landings of prohibited species and those still landing species grouped as'skates and rays’, these results may aid authorities in allocating limited resources more effectively to reduce landings, misreporting and data errors of prohibited species, and increase species-specific landing compliance.

Age, growth and length-weight relationship of the white skate, Rostroraja alba (Linnaeus, 1758) (Chondrichthyans: Rajidae), from the Gulf of Gabes (Tunisia, Central Mediterranean)

Age, growth and length-weight relationship of the white skate, Rostroraja alba (Linnaeus, 1758) (Chondrichthyans: Rajidae), from the Gulf of Gabes (Tunisia, Central Mediterranean)

Age, Growth, Mortality, Longevity and Reproductive Biology of the White Skate, Rostroraja alba (Chondrichthyes: Rajidae) of the Gulf of Gabès (Southern Tunisia, Central Mediterranean)

The Age, growth, mortality, longevity and reproductive parameters were studied for Rostroraja alba from the Gulf of Gabes (Southern Tunisia, central Mediterranean Sea), collected monthly during 2007 from commercial fisheries. 43.75% of the composition was females and 56.25% males. A total of 140 females (44.5 - 160 cm TL) and 180 males (39.5 - 150 cm TL) were examined to study the reproductive cycle. A subsample of 115 specimens (65 females and 50 males, ranging from 39.5 to 160 cm TL) was used for age and growth estimation derived from vertebral centra. The oldest female was 35 years and 160 cm TL, whereas the oldest male was 32 years and 150 cm TL. The von Bertalanffy growth parameters were L∞= 177.6±3.61 cm, k= 0.06 ±3.08 year-1 and t0= -1.28±1.97 10-1 year for females and L∞= 199.6±3.66 cm, k= 0.04± 1.9610-3 year-1 and t0= -1.47±0.35 10-1 year for males. Growth was not significantly different between sexes. The maturity size (TL 50%) was 119.3 cm (94.20 cm DW) for males and 129.4 cm (95.77 cm DW) for females. The maturity age (A50%) was estimated to 19.69 and 23.47 years respectively for males and females. Size and age maturity were not significantly different between sexes.

Associations of demersal fish with sponge grounds on the continental slopes of the northwest Atlantic

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 477:217-230 (2013) - DOI: https://doi.org/10.3354/meps10127 Associations of demersal fish with sponge grounds on the continental slopes of the northwest Atlantic E. Kenchington1,*, D. Power2, M. Koen-Alonso2 1Fisheries and Oceans Canada, Bedford Institute of Oceanography, 1 Challenger Drive, Dartmouth, Nova Scotia B2Y 4A2, Canada 2Fisheries and Oceans Canada, Northwest Atlantic Fisheries Centre, PO Box 5667, St. John’s, Newfoundland A1C 5X1, Canada *Email: ellen.kenchington@dfo-mpo.gc.ca ABSTRACT: Association of demersal fish taxa with Geodia-dominated temperate sponge grounds was examined in 104 research vessel survey trawl sets from 500 to 1500 m depth along the continental slopes of the Grand Banks and Flemish Cap, northwest Atlantic. The total number of fish taxa and their total biomass were negatively correlated with sponge biomass and catch depth, and sponge biomass increased with depth. ANOSIM identified significantly different fish communities (biomass and abundance composition) associated with 3 sponge catch weight classes, and multidimensional scaling plots showed separation of trawl sets labelled by this factor to be strongest for abundance. SIMPER analysis identified the fish assemblage associated with the sponge grounds (≥250 kg km-1): roughhead grenadier Macrourus berglax, roundnose grenadier Coryphaenoides rupestris, blue hake Antimora rostrata, longnose eel Synaphobranchus kaupii, turbot Reinhardtius hippoglossoides, black dogfish Centroscyllium fabricii and deep-sea cat shark Apristurus profundorum. These contributed to 92 and 81% of the similarity (~50%) among trawl set catches using biomass and abundance data, respectively. Four additional taxa contributed to 92% similarity in abundance: common grenadier Nezumia bairdii, shortnose snipe eel Serrivomer beanii, lanternfish (Myctophidae) and goitre blacksmelts Bathylagus euryops. SIMPER dissimilarity identified 6 fish taxa with larger biomass in catches with high sponge catch weight: shortnose snipe eel, deep-sea cat shark, eelpout Lycodes spp., spinytail skate Bathyraja spinicauda, white skate Dipturus linteus and deepwater chimaeras Hydrolagus affinis. The first 3 species were also more abundant in those catches and are considered to be strongly associated with the sponge grounds. Future research focussing on these species identified as being associated with the sponge grounds may provide greater insight into the strength of the associations and the ecological linkages between these benthic and pelagic communities. KEY WORDS: Demersal fish assemblages · Sponge grounds · Trawl catch Full text in pdf format Supplementary material PreviousNextCite this article as: Kenchington E, Power D, Koen-Alonso M (2013) Associations of demersal fish with sponge grounds on the continental slopes of the northwest Atlantic. Mar Ecol Prog Ser 477:217-230. https://doi.org/10.3354/meps10127 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 477. Online publication date: March 12, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Molecular barcoding of skates (Chondrichthyes: Rajidae) from the southern Northeast Atlantic

Serra‐Pereira, B., Moura, T., Griffiths, A. M., Gordo, L. S. &amp; Figueiredo, I. (2010). Molecular barcoding of skates (Chondrichthyes: Rajidae) from the southern Northeast Atlantic. —Zoologica Scripta, 40, 76–84.Due to their vulnerability to fishing pressure, many species of skate (Rajidae) in the Northeast Atlantic are undergoing declines in abundance. The assessment of stock status and subsequent proposal of management measures are often complicated by high levels of species diversity and endemism, coupled with morphological and ecological conservatism, which makes distinguishing between species difficult. To improve the identification of skates and investigate the phylogenetic position of endemic species the cytochrome c oxidase subunit I (COI) was sequenced in 12 species (Dipturus oxyrinchus, Leucoraja naevus, Leucoraja circularis, Neoraja iberica, Raja brachyura, Raja clavata, Raja maderensis, Raja microocellata, Raja miraletus, Raja montagui, Raja undulata, Rostroraja alba) inhabiting the Portuguese waters. Based on sequence divergence R. maderensis and R. clavata only differ by 1% of the 652 bp COI sequence, questioning the recognition of R. maderensis (considered to be endemic to Madeira and the Azores), as a reproductively isolated species. Otherwise, there was clear phylogenetic support for the different genera and all the remaining species, although the genetic divergence was low compared to other chordates. In particular, COI analysis allowed clear identification of the morphologically similar species R. brachyura and R. montagui.

Taxonomic confusion and market mislabelling of threatened skates: important consequences for their conservation status

Abstract The iconic European common skate (Dipturus batis) has been described as the first clear case of a fish species brought to the brink of extinction by commercial fishing. Its listing was upgraded to Critically Endangered on the 2006 IUCN Red List of Threatened Species. According to FAO fishery statistics, France is responsible for 60.2% of the 502 tonnes reported as ‘D. batis’ in the 2005 world landings. Noticeable phenotypic differences within the species and inconsistencies in published data on its sexual maturation required careful re‐examination of its taxonomy. Morphology, genetics, and life history reveal that two distinct species have been erroneously confused since the 1920s under the single scientific name D. batis. Here it is argued that they should be resurrected as two valid species. The common skate D. batis species‐complex is split into two nominal species, the blue skate (provisionally called D. cf. flossada) and the flapper skate (D. cf. intermedia) with maximum lengths of 143.2 cm and 228.8 cm respectively. This taxonomic confusion puts into question all previously accumulated data based on D. batis. Its endangered status highlights the need for an extensive reassessment of population collapses with accurately identified species. In 2006/2007 an extensive survey (4110 skates, 14.081 tonnes by weight) was conducted in the main French ports of the D. batis species‐complex and relatives (D. oxyrinchus, D. nidarosiensis and Rostroraja alba) that are mixed together in landings under the names ‘D. batis’ and ‘D. oxyrinchus’. The survey reveals that official fishery statistics mask species‐specific declines, due to the mislabelling of five species under only two landing names. Trends in landings since the 1960s and the life history of these species suggest a dramatic decline and collapse of the spawning stock, preventing the recovery of relict populations. The risk of extinction of these depleted species may be higher than previously assessed and might be unavoidable without immediate and incisive conservation action. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Southern African skate biodiversity and distribution

The skates (Family Rajidae) have 12 genera and possibly 28 species off southern Africa (southern Angola, Namibia, South Africa and Mozambique). The geographic and bathymetric distribution and the taxonomic composition of the southern African skate fauna are analysed and the distribution mapped. The southern African skate fauna is best known off the temperate west coast of South Africa from the intertidal to approximately 1,200 meters, but poorly known below 1,200 m and sketchily known in warm-temperate and tropical parts of the area. Southern African skates of the temperate continental shelves above 100 m are not diverse and regularly include one species of the genus Dipturus, one species of Leucoraja, two species of Raja (including R. straeleni, the most abundant skate in southern African waters) and the giant skate Rostroraja alba. All of these skates are ‘shelf overlap’ species that range onto the outer shelves and uppermost slopes, and none are confined to inshore environments. Skate diversity increases on the outer shelves and upper slopes. At least half of the skate species are endemic to the southern African region; other species also occur off East or West Africa, a few extend to European waters, and records of one species, Amblyraja taaf, appear to be of strays from nearby sub-Antarctic seas. The genus Bathyraja and softnose skate group (Arhynchobatinae) are surprisingly limited (a single species) in deep-water off southern Africa (unlike other regions including the Antarctic), and almost all of southern African skates are members of the Rajinae. Amongst rajines, the tribes Amblyrajini (Amblyraja, two species, Leucoraja, two species, and Rajella, five species) Rajini (Dipturus, six species, Okamejei, one species, Raja, two species, and Rostroraja, one species), and Anacanthobatini (Anacanthobatis, two species, and Cruriraja, three species) predominate, while Gurgesiellini has a species of Neoraja and possibly two of Malacoraja.

Changes in the demersal fish assemblages of British coastal waters during the 20th century

Catches of demersal fish from research vessel surveys in three areas around the British Isles from 1901 to 1907 were compared with survey catches in the same areas from 1989 to 1997. Mesh size and other characteristics of the trawls used suggested that some of these data were comparable, and could be used to describe changes in demersal fish assemblages over the last 80 years. In Start Bay (NW English Channel) and the Irish Sea, species diversity was the same in both periods, although the most abundant species in each period were not the same. In English coastal regions of the southern North Sea, fish populations became more diverse, as plaice Pleuronectes platessa and whiting Merlangius merlangus became less abundant, and the relative abundance of several non-target species such as dragonet Callionymus spp., bib Trisopterus luscus, and bull-rout Myoxocephalus scorpius increased. The proportion in the catch of small fish species (maximum body length 30 cm) in catches decreased in all regions during the time period, except in the Irish Sea where plaice replaced grey gurnard Eutrigla gurnardus as a dominant species. There was a decline in abundance of large sharks, skates and rays, including the common skate, Raja batis, white skate R. alba and the angel shark Squatina squatina. During historic surveys, 60% of the elasmobranch fauna consisted of thornback ray Raja clavata, whereas in contemporary surveys the lesser spotted dogfish Scyliorhinus canicula was the most abundant elasmobranch. Changes in length-frequency distribution of fish in both target and non-target categories, and other observed changes, were thought to be a response to commercial exploitation, and corresponded to similar observations recorded elsewhere. (C) 2000 British Crown Copyright.

Fishery Stability, Local Extinctions, and Shifts in Community Structure in Skates

Abstract:Skates are arguably the most vulnerable of exploited marine fishes. Their vulnerability is often assessed by examining fisheries catch trends, but these data are not generally recorded on a species basis except in France. Aggregated skate catch statistics tend to exhibit more stable trends than those of other elasmobranch fisheries. We tested whether such apparent stability in aggregated catch trends could mask population declines of individual species. We examined two time series of species‐specific surveys of a relatively stable skate fishery in the northeast Atlantic. These surveys revealed the disappearance of two skate species, long‐nose skate ( Dipturus oxyrhinchus) and white skate ( Rostroraja alba) and confirmed a previously documented decline of the common skate ( D. batis). Of the remaining five skate species, the three larger ones have declined, whereas two smaller species have increased in abundance. The increase in abundance and biomass of the smaller species has resulted in the stability of the aggregated catch trends. Because there is significant dietary overlap among species, we suggest the increase in abundance of the smaller species may be due to competitive release as the larger species declined. A consequence of this kind of stability is that declining species cannot be detected without species‐specific data, especially in taxa exhibiting competitive interactions. This may explain why previously documented disappearances of two species of skates went unnoticed for so long. The conservation of skates and other elasmobranchs requires species‐specific monitoring and special attention to larger species.