•An acoustically visible gillnet was used in a commercial fishery first the first time.•Bycatch of harbor porpoises was reduced in numbers, but the sample size was low.•The gillnet modification had ...no effect on the catch efficiency of bottom-dwelling species.•A power analysis showed that 130 hauls are necessary for robust statistical assessment.
Bycatch of protected species, particularly small cetaceans, in gillnets is a worldwide concern. One hypothesis for this is that echolocating cetaceans entangle because they do not perceive conventional gillnets as impenetrable barriers, owing to the gillnet’s faint echo. A gillnet modified for improved acoustical visibility was tested in a first pilot trial in a commercial gillnet fishery targeting turbot (Scophthalmus maeoticus) on the Turkish Black Sea coast. This study is the first demonstration of the viability of using a gillnet equipped with small acrylic glass spheres to reduce bycatch of harbor porpoises in a commercial fishery and provides the basis for full-scale sea trials of the gear in commercial fisheries through a power analysis. In these pilot experiments, the focus lied on the handling of the gear and identification of requirements for a full-scale trial, but results include promising bycatch data for an endangered echolocating marine mammal (Phocoena phocoena) and no reduction in catch efficiency of a bottom-dwelling, vulnerable species (Raja clavata).
Abstract
Research surveys are important to evaluate fishery resources’ spatial distribution and abundance. Although the underlying sampling is usually conceived with a focus on specific species, ...efficient designs can also collect data on secondary species. We present a framework to explore and evaluate the adequacy of alternative sampling designs for fishery research surveys aiming to maximize accuracy estimates of the secondary species abundance while maintaining the quality of the abundance estimates of primary species. A geostatistical model-based approach was developed considering the semi-continuous nature of the data and the excess of zero values commonly observed for secondary species. New sampling designs were defined according to optimization weights and evaluated based on the resulting prediction exactness. The framework was applied to the bottom trawl survey conducted along the Portuguese continental coast with European hake, Merluccius merluccius, as the primary species and thornback ray, Raja clavata, as the secondary species. The sampling design URSI provided the best balance between the accuracy for both primary and secondary species. The methodology can be replicated for other bottom trawl research surveys and an extended set of species. We recommend that a decision on which sampling design to adopt in future surveys should consider a cost-efficiency analysis.
Raja clavata
is the most widespread and landed skate species in the Mediterranean Basin. Despite its diffusion and economic importance, several aspects of its life history, such as age and growth, ...are poorly understood. This study evaluated the species’ growth in the South Adriatic Sea (Central Mediterranean Sea) and for the first time attempted an age validation through a tagging experiment. Thin sectioning of vertebral
centra
proved to be a more accurate preparation method in terms of age estimation precision and reproducibility than whole vertebral
centrum
staining (cobalt nitrate and ammonium sulfide technique). Marginal analysis showed a clear seasonal pattern, confirming the hypothesis of a single
annulus
deposition per year. A total of 291 vertebral
centra
were sampled and used for age estimation purposes. The oldest female was estimated to be 12 years old total length (
TL
) = 89 cm, while the oldest male was aged 8 years (
TL
= 79.9 cm). Females were also found to be characterized by a slightly wider longevity range (ω
L
= 11.5, ω
U
= 16.8 years) than males (ω
L
= 7.8, ω
U
= 11.2 years). The von Bertalanffy growth curve fit the age and length data more accurately than the Gompertz and logistic models. Eighty-three thornback rays were tagged and released, of which two were recaptured. In both recaptured specimens, oxytetracycline marks were clearly visible. The band deposition after oxytetracycline injection and growth during the freedom period (about 1 year) were consistent with the age estimation method and criteria used and with the obtained growth results. Thus, the analysis of the vertebral
centra
extracted from the two recaptured specimens confirmed the hypothesis of the deposition of a single
annulus
per year and in general the age estimation criteria used in this study.
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•Chondroitin sulfate was isolated from fish processing industry by-products.•CS Mw was determined by GPC and sulfation position by NMR and SAX-HPLC.•CS was isolated from Scyliorhinus ...canicula, Prionace glauca, and Raja clavata.•4S/6S ratio below 0.20 was observed for P. glauca and R. clavata.
Chondroitin sulfate (CS) was isolated from Scyliorhinus canicula (fin, head and skeleton), Prionace glauca (head), and Raja clavata (skeleton) by-products from fish processing industry using environmentally friendly processes. The molecular weight was determined by gel permeation chromatography and the sugar composition and sulfation position by NMR and SAX-HPLC after enzymatic digestion. The CSs showed a prevalent 6S GalNAc sulfation for the 3 species (4S/6S ratio lower than 1). A higher 6S sulfation was observed for P. glauca head and R. clavata skeleton (4S/6S ratio below 0.20) than for S. canicula (4S/6S ratio ca. 0.6). The existence of CS samples with such low 4S/6S ratio has only been observed before in a rare species of shark (Mitsukutina owatoni, globin shark). The good extraction yields achieved make S. canicula, P. glauca and R. clavata fish industry by-products a useful source of 6-sulfated chondroitin sulfate.
•Different gelatin hydrolysates from thornback ray (TRGHs) skin were prepared using various proteases;•TRGHs exhibited varying in vitro antioxidant activities depending on the test used;•TRGHs showed ...in vitro ACE inhibitory activity;•TRGHs could be used as ingredients to formulate functional foods.
The angiotensin I-converting enzyme (ACE) inhibitory activities and antioxidant properties of thornback ray gelatin hydrolysates (TRGHs) prepared by treatment with proteolytic proteases from Bacillus subtilis A26, Raja clavata crude alkaline protease extract, Alcalase and Neutrase were investigated. All gelatin hydrolysates showed different degrees of hydrolysis and hydrophobic/hydrophilic peptides ratio. Moreover, they possess high protein content (70.04 ± 0.55–74.14 ± 0.28%). The antioxidant activity was assayed using various in vitro tests. The highest antioxidant activity was observed with hydrolysate obtained by treatment with A26 proteases (TRGH-A26) which exhibited a 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging activity with a concentration that produces 50% of inhibition (IC50) of 1.98 ± 0.02 mg/ml of sample, reduced the ferric ions with an absorbance at 700 nm of 0.962 ± 0.07, prevents bleaching of β-carotene with 73.02 ± 1.90% inhibition and gave an antioxidative efficacy of 180 ± 0.08 µmol/ml α-tocopherol equivalents at 5 mg/ml in the phosphomolybdenum assay. However, gelatin hydrolysate treated with Alcalase (TRGH-Alcalase) was the most potent to prevent DNA oxidation. For the ACE inhibitory activity, all hydrolysates displayed ACE-inhibitory activity. TRGH-A26 and TRGH-Alcalase exhibited the highest activity with 85 ± 0.65 and 82 ± 0.49%, respectively, at 5 mg/ml. The results revealed that TRGHs could be used as ingredients to formulate functional foods.
We studied the glycogen content in erythrocytes of two Black Sea cartilaginous fishes (Raja clavata, Dasyatis pastinaca) and four teleost species (Trachurus mediterraneus ponticus, Spicara flexuosa, ...Diplodus annularis, Scorpaena porcus). Glycogen accumulation in erythrocytes of teleost fish was about twice as high as in the ray fishes. Due to the unique ability of thornback ray R. clavata erythrocytes to well preserve their cell integrity in vitro in the cold (4°C), the dynamics of glycogen expenditure in erythrocytes was tracked over 11 days of storage until cell disintegration. Erythrocytes spent glycogen economically: during the entire storage period, only 52% of glycogen were spent. In the black scorpionfish S. porcus, erythrocytes were far less tolerant to being stored in the cold. The stability period of S. porcus erythrocytes suspended in physiological saline did not exceed two days. During this period, no significant changes in the glycogen concentration were observed.
Fish movements are fundamental to their ecology and survival. Understanding the causes and consequences of the spatial behaviour of fish is of high relevance as it provides critical knowledge for ...conservation purposes. Skate (Rajidae) populations face an unprecedented global decline due to overfishing. In this study, we used acoustic telemetry to track the movements of nine individuals of the near threatened thornback skate (
Raja clavata
) around the Cíes Islands, a small marine protected area in the northwest of the Iberian Peninsula. Our results demonstrate the significant impacts of biotic and abiotic drivers on the spatial behaviour of
R. clavata
. Overall residency inside the study array was low (0.073), differed between sexes (higher for females) and over the course of the year (peaking in summer). The analysis of the direction of the excursions performed by
R. clavata
individuals revealed high consistency in the exit and entry areas and a strong connectivity with inshore waters connecting with the Ría de Vigo, as compared with offshore waters connecting with the open ocean. Finally, the activity space of
R. clavata
also varied over the time of the year with a peak in summer. This study provides an important baseline information for understanding the spatial behaviour of
R. clavata
that can serve as a starting point for planning future conservation actions or studies.
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.
Trophic ecology studies on predator–prey interactions reveal insights into ecological communities and help understand a species’ role in the food web by contributing to improved fisheries management ...and conservation capabilities. Understanding the ecological role of overexploited and endangered predators is essential to deciphering how their feeding behaviour influences food web dynamics. In this study, the authors investigated the feeding behaviour of the common and IUCN‐listed Near Threatened (NT) thornback ray Raja clavata, using carbon and nitrogen stable isotope and stomach content analysis (SCA). It has recently suffered an 87% decline in reported catches from the Sea of Marmara within the last decade. These results show that thornback ray mainly feeds on teleost species, except in summer, with both methods showing this species changes its diet ontogenetically by SCA. This ontogenetic diet shift was at lengths 40–50 cm by changing group preferences from Crustacea to Teleostei. MixSIAR results showed that both adult and juvenile individuals of R. clavata feed mainly on the crustaceans, but the contribution of teleosts represented by Trachurus sp. was very low (<15%). The trophic position increased total length and was higher than other batoid species in the Sea of Marmara.
The lack of information on age validation often affects ageing studies in cartilaginous fish, even in the most common species. Recently, the annual growth band deposition pattern has been directly ...validated for thornback ray in the Mediterranean Basin, thus providing a highly reliable protocol for age reading. In this regard, taking advantage of this new information, the present study aims to investigate this species growth for the first time over a wide area of the Western and Central Mediterranean Sea, involving four different FAO geographic sub-areas. In order to provide an accurate description of the species growth, different models were fitted to the age-length data obtained from the observation of vertebral centra thin sections. A total of 720 specimens were analysed (358 females and 362 males) with total length ranging between 11.5 and 86.4 cm. The corrected Akaike's Information criterion (AICc) indicated, among the tested models, the three-parameter von Bertalanffy function as the most robust in describing the species growth. Growth modelling outcomes revealed the thornback rays as capable of growing relatively fast during the first years; the growth rate gradually slows down allowing maximum sizes of about 90–100 cm in total length to be reached. Significant discrepancies in the growth pattern were found between sexes in all investigated areas with the only exception being in the Northern Tyrrhenian Sea. Differences in growth patterns were observed between areas, specifically in the estimated growth rate of the species for each sub-region. The present study, confirmed the importance of the availability of a validated ageing protocol and testing multiple growth models. Moreover, our results highlighted the urgent need to investigate a species growth in a wide geographic area, searching also for possible differences at sub-region level. Providing this information could indeed allow eventual management plans to be adapted to the exact growth pattern that the species exhibit in the region, in this way increasing their effectiveness.
•The growth pattern of the Thornback ray (Raja clavata) was examined.•A wide study area of western-central Mediterranean Sea was investigated.•Annual band counts in R. clavata vertebrae is a highly reproducible process.•Multiple growth models were applied to the obtained age-length data.•The von Bertalanffy function was to be the best model to describe R. clavata growth.
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