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Oceanological and Hydrobiological Studies Oceanological and Hydrobiological Studies International Journal of Oceanography and Hydrobiology Volume 50, No. 2, June 2021 pages (115-127) ISSN 1730-413X eISSN 1897-3191 Biology of the thornback ray (Raja clavata Linnaeus, 1758) in the North Aegean Sea by Abstract 1, The study deals with aspects of the population Koray Cabbar *, dynamics in the thornback ray (Raja clavata L., 1758), one 2 Cahide Çiğdem Yiğin of the most abundant cartilaginous fish caught in the North Aegean Sea. Females accounted for 73.08% and males 26.92% of all individuals. Total length of females and males ranged between 50.2 and 89.9 cm (disc width: 33.4–62.0 cm), and between 43.1 cm and 82.7 cm (disc width: 30.7–64.2 cm), respectively. Relationships between total length (TL) and total weight (TW), and between DOI: 10.2478/oandhs-2021-0011 disc width (DW) and total weight (TW) were described by 3.10 3.03 Category: Original research paper the equations: TW = 0.0041 TL and TW = 0.0178 DW , respectively. Age data derived from vertebrae readings Received: September 23, 2020 were used to estimate growth parameters using the von Accepted: December 1, 2020 −1 Bertalanffy function: L∞ = 101.71 cm, K = 0.18 y , t0 = −0.07 −1 y for males and L∞ = 106.54 cm, K = 0.16 y , t0 = −0.28 y for females. The maximum age was 8 years for males 1Çanakkale Onsekiz Mart University, and females. Total length at first maturity of males and females was 70.9 cm and 81.2 cm, respectively. Based Graduate School of Natural and Applied on the gonadosomatic index and gonadal macroscopic Sciences, Department of Marine Science and observations, it was determined that the spawning period Limnology, Çanakkale, Turkey lasted throughout the year. Stomach content analysis showed that crustaceans (53.03% IRI) and teleosts (14.70% 2 Çanakkale Onsekiz Mart University, IRI) were the most preferred prey. Marine Science and Technology Faculty, Department of Marine Science and Limnology, Çanakkale, Turkey Key words: Raja clavata, age and growth, reproduction, feeding, North Aegean Sea * Corresponding author: [email protected] online at www.oandhs.ug.edu.pl ©2021 Koray Cabbar & Cahide Çiğdem Yiğin. This work is licensed under the Creative Commons Attribution- NonCommercial-NoDerivs License (http://creativecommons.org/licenses/by-nc-nd/4.0/) 116 Oceanological and Hydrobiological Studies, VOL. 50, NO. 2 | JUNE 2021 Koray Cabbar, Cahide Çiğdem Yiğin 1. Introduction 2. Materials and methods Elasmobranchs are among the top predators in the Samples were collected between January 2016 and marine environment, thus they affect populations of March 2017 by commercial fishing vessels operating both fish and invertebrates at lower trophic levels (Ellis in the North Aegean Sea. A total of 104 specimens et al. 1996). The thornback ray Raja clavata (Linnaeus were brought to laboratory cold storage conditions. 1758) is a common skate in the eastern Atlantic, For each specimen, total length (TL, cm), disc width distributed from Norway to South Africa, including the (DW, cm), clasper length (CL, cm), total weight (TW, g) Mediterranean and the Black Sea (Stehmann & Burkel and ovarium weight (OW, g) were determined to the 1984). Although the reported depth ranges vary from nearest 0.1 cm and 0.1 g, respectively. The TL–W and 10 to 300 m, the species is most common in coastal DW–W relationships were determined using allometric waters at a depth of 10–60 m (Stehmann & Burkel 1984; equations (Sparre & Venema 1992), W = a TLb and Wheeler 1978). It is usually found on muddy, sandy and W = a DWb, respectively. gravelly bottoms, rarely on rougher bottoms. Due to A vertebral segment containing at least five its wide distribution and abundance, thornback ray is vertebrae, between the 10th and 20th vertebral one of the most studied skate species. However, little segments, was extracted. The section was then information is available on its biology. thawed and three vertebrae were dissected, carefully R. clavata has low growth, maturity and fecundity separated and excess connective tissue, as well as rates compared to most teleostei. The thornback neural and hemal arches were removed and placed in ray is oviparous and its reproductive activity occurs a 5–25% solution of sodium hypochlorite. Vertebrae throughout the year (Jardas 1973). The reproduction were left in this solution for 5–30 min depending on peak occurs in June–July. Average production is their size and then scrubbed to remove any residual estimated at 140 eggs year−1 (Holden 1975), but more tissue (Kusher et al. 1992). The vertebrae were then recent studies indicate lower fecundity, ranging from embedded in epoxy resin to enable cutting. To prepare 48 to 74 eggs (Ellis & Shackley 1995). Egg cases hatch the epoxy resin, Araldite epoxy GY502 and hardener after about 4–5 months, and pups are about 11–13 HY 956 were mixed at a ratio of 5:1 and poured over cm in total length (Pawson & Ellis 2005). Embryos the vertebrae in silicone molds and allowed to dry for feed exclusively on yolk (Dulvy & Reynolds 1997). 24 h (Campana 2014). An IsoMet low-speed diamond Raja clavata is a predator species in the marine bladed saw was used to prepare vertebral sections. environment. Thus, it plays an important role in marine Growth bands in vertebral sections were counted ecosystems. R. clavata feeds mainly on crustaceans, using an Olympus SZX7 stereomicroscope. The age teleosts and cephalopods (Saglam & Bascınar 2008; of each specimen was determined from the number Yıgın & Ismen 2010; Kadri et al. 2014b), and occasionally of opaque and hyaline bands deposited on vertebral on gastropods and polychaetes (Kadri et al. 2014b). centra. The thornback ray is one of the most abundant Growth was modeled using von Bertalanffy’s (1938) elasmobranch species in Turkish coastal waters. Over growth equation: the past ten years, catches of thornback ray have decreased from 668 t in 2010 to 82.6 t in 2018, which L = L (1 − e−K(t−t0)) corresponds to a decrease of 87% (Tüik 2018). In t ∞ Turkish waters of the Black Sea and the North Aegean Sea, sharks and rays are caught as bycatch by otter where L∞ is the asymptotic TL, Lt is the TL at age t, trawls, purse seines, bottom longlines and gillnets. As K is the growth curvature parameter, and t0 is the reported in a related circular, there has been a recent theoretical age at which fish would have 0 total length. increase in sensitivity to shark and ray fishing in Turkey. The VBGE was calculated using FISHPARM, a computer Thornback rays are prohibited to be caught according program for parameter estimation of nonlinear models to Fisheries Regulation Notice No. 2016/35 in all Turkish with Marquardt’s (1963) algorithm for least-square waters. An important factor in the decline of thornback estimation of nonlinear parameters (Prager et al. 1987). rays is their longer life span, slower growth rate, lower Sex ratios were compared to the 1:1 ratio using the fecundity and late sexual maturity. These factors make chi-square (χ2) test. them susceptible to changes in the ecosystem. In this Maturity of males was determined based on context, this study provides important information the state of mixopterygia (claspers) development on age, growth, reproduction and feeding strategy of in relation to the edge of the pelvic fin. Maturity of thornback rays in the North Aegean Sea. females was determined by internal examination (Holden & Raitt 1974; Paesch & Oddone 2008). www.oandhs.ug.edu.pl Journal owner: Faculty of Oceanography and Geography, University of Gdańsk, Poland Oceanological and Hydrobiological Studies, VOL. 50, NO. 2 | JUNE 2021 117 Biology of Raja clavata To study the food intake, stomachs of the sampled The TL–W and DW–W relationships were assessed fish were immediately dissected and stored in a separately for females and males and are presented as 4% formaldehyde solution. In the laboratory, the W = 0.003 TL3.18 (95% CI for b: 3.01–3.35) and W = 0.0187 identification of ingested prey was carried out to the DW3.01 (95% CI for b: 2.84–3.18), and W = 0.0223 TL2.68 order level. Identification of ingested prey was carried (95% CI for b: 2.29–3.07) and W = 0.0091 DW3.21 (95% out systematically (Yığın & Ismen 2010). The following CI for b: 2.85–3.57), respectively. A negative allometric references were used in the identification of food growth (b < 0.05) was determined for males and a items: Murduchay-Boltouski (1969), Fischer (1973), positive one (b > 0.05) for female individuals. Whitehead et al. (1986a,b), Fischer et al. (1987a,b). Stomach contents were analyzed using the numerical 3.2. Age and growth percentage (N%) and frequency of occurrence (F%), percentage by weight (W%; Hyslop 1980) and the The age of 78.8% of the sampled 104 thornback Index of Relative Importance (IRI; Pinkas et al. 1971). rays was determined for the purpose of the study. The index of relative importance (IRI) is used to A total of 82 individuals, ranging in age from 4 to determine important food items in the diet of fish. The 8 years, were examined; 76% were females and formula used was IRI = F% (N% + W%) (Pinkas et al. 24% were males. Each species had similar band 1971; Windell & Bowen 1978). morphology, with opaque bands appearing as relatively thick, regularly spaced, dark-colored bands 3. Results along each sagittal section axis, separated by narrower lighter-colored translucent bands (Fig. 2). 3.1. Length distribution Sex ratios of specimens were determined as M:F = 1:2.71 and the difference between sexes was statistically significant (χ2 =18.1; df =1, p < 0.05; SPSS 21.0).
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