Uncommon biological patterns of a little known endemic Mediterranean skate, Raja polystigma (Risso, 1810)
Introduction
Despite representing only 0.32% of the oceans’ surface, the Mediterranean Sea is a hotspot of marine biodiversity containing on average 6.4% of the world’s marine species, of which approximatively 20.2% are endemic (Coll et al., 2010).
Out of 73 chondrichthyans, 15 species of skates (Rajifomes: Rajidae) are currently identified in the Mediterranean Sea, three of which are classified as endemic and found nowhere else: the Malteseskate Leucoraja melitensis (Clark, 1926), the Rough skate Raja radula Delaroche, 1809, and the Speckled skate R. polystigma Regan, 1923 (Dulvy et al., 2016). In this regard, rare and poorly studied species, such as endemic Mediterranean skates, are therefore prone to high rates of extinction and threat (McKinney, 1999) also because of non-optimal reporting and sampling that could affect the power of analyses needed for a correct management (Roberts and Hawkins, 1999, Dulvy et al., 2003). Raja polystigma is a small-sized skate reported to inhabit soft bottoms on the continental shelf and upper slope of the western Mediterranean Sea, in the Sicilian Channel and Adriatic Sea (Serena, 2005, Serena et al., 2010). Specimens of the Speckled skate have been frequently misidentified as R. montagui Fowler, 1910 due to the high level of external morphological similarity observed both in juveniles and adults (Ramírez-Amaro et al., 2018). Therefore, the resulting lack of accurate species-specific landing data represented a significant issue in knowing the size of the population and in developing proper conservation and management measures. Recent molecular genetic tools were useful to describe their differential geographic distribution. Indeed, R. polystigma is present and abundant only in the central-western Mediterranean basin (showing the presence of a single, almost panmictic population) and with R. montagui found exclusively in the westernmost area of the Mediterranean (Algerian basin) and in the Atlantic Ocean (Cannas et al., 2008, Frodella et al., 2016, Ramírez-Amaro et al., 2018). The population genetic features of this species, besides their intrinsic vulnerability as K-selected strategy species (Ellis et al., 2010), render these organisms highly vulnerable to fishing pressure, and thus, calling for focused regional studies to better understand its life-history traits and population status.
Elasmobranchs constitute a common by-catch fraction of many fisheries in the Mediterranean Sea (Ferretti and Myers, 2006). Furthermore, while species targeted by professional fisheries have often been intensely studied, by-catch species such as R. polystigma are often ignored. In particular, in Sardinia (Italy, central-western Mediterranean) even if sharks and other chondrichthyans are not the target of fisheries, they are often caught as by-catch and then may be discarded (Marongiu et al., 2017). Raja polystigma, in Sardinian bottom trawling fishery, is, among Rajiformes, the second most abundant species found between 10 and 800 m of depth (Density Index, 38.7 ± 13.0 N/km2) and the first one in the continental shelf (200 m, Density Index, 53.2 ± 25.5 N/km2) (Marongiu et al., 2017, Follesa et al., 2019a).
Nevertheless, life-history information on R. polystigma population is very limited and is needed as input to formulate fishery management decisions. Currently, the only available data are from biological studies carried out along Tunisian coasts more than 30 years ago (Capapé and Quignard, 1978) on the reproductive cycle and in the Balearic Sea on its feeding ecology (Valls et al., 2011, Mulas et al., 2019). Despite the paucity of natural history data, the species is listed as of Least Concern according to the International Union for the Conservation of Nature (IUCN) Red List criteria (Dulvy et al., 2016).
In this context, a new knowledge of life-history traits allows the construction of age-based population models and, considering other ecological aspects like reproductive cycle and size at maturity, can eventually lead to an assessment of the population status of this data poor species (Cortés, 2002).
In addition, looking at different reproductive trends among skates like seasonality and bimaturism (e.g., Mabragaña et al., 2015, Porcu et al., 2015), detailed knowledge on these aspects is required to provide sound scientific advice for the implementation of conservation and management strategies.
Given its abundance as by-catch of small and large professional fisheries, the objective of this research was to establish baseline data, which are mostly unknown, on the Speckled skate providing critical information for this species conservation in order to assess properly its possible vulnerability to such fishing pressure. In particular, we focused on: (i) age and growth through the comparison of different growth models; (ii) estimating maturation, mating and egg-case laying periods; (iii) estimating size at maturity for both sexes and describing the morphological changes in reproductive organs during maturation; (iv) testing a possible bathymetric segregation with respect to sex and sexual maturity.
Section snippets
Sampling
We conducted the study in Sardinian seas (central-western Mediterranean) between 2005 and 2017 at depths between 18 and 660 m through both commercial and scientific (MEDITS) trawl surveys (Fig. 1).
From each Raja polystigma individual, the total length (TL, mm), the total weight (TW, g) and the sex were noted.
The population sex-ratio (SR, females:males) was calculated and the significance of its deviation from the 1:1 condition was tested as a null hypothesis through the test (Zar, 1999).
Processing the ageing structures
Sex ratio and length frequency distributions
Of the 2312 specimens of Raja polystigma collected, 1173 were females and 1139 were males; sexes were equally distributed: SR 1.03; ; p 0.05.
In females, the TL ranged from 103 to 600 mm (mean S.D. 326.28 ± 93.29) and TW from 5.58 to 1580.0 g (mean S.D. 225.60 ± 231.46) and in males, the TL ranged from 112 to 610 mm (mean S.D. 327.31 ± 90.97) and TW from 6.86 to 1480.0 g (mean S.D. 214.41 ± 198.91) with both females and males attaining substantially the same size (
Discussion
The present study represents the first assessment of age, growth, maturation and reproduction of the Speckled skate Raja polystigma, endemic to the Mediterranean Sea, and therefore contributes towards the limited information available on the species throughout its distribution range.
Conclusion
The growth patterns found in this study indicate that R. polystigma is a relatively short-lived and fast-growing species if compared with other Rajids (e.g. Porcu et al., 2015, Bellodi et al., 2017). These features could ascribe this skate as more resilient to exploitation than other elasmobranchs for any given level of fishing pressure. Similarly, reproductive aspects of Raja polystigma (e.g., late size and age at maturity, low fecundity and high dependence on the coastal environment, seasonal
CRediT authorship contribution statement
Cristina Porcu: Conceptualization, Methodology, Writing - original draft, Writing - review & editing, Software, Investigation. Andrea Bellodi: Conceptualization, Methodology, Writing - original draft, Software, Investigation. Alessandro Cau: Formal analysis, Investigation. Rita Cannas: Formal analysis, Investigation. Martina F. Marongiu: Formal analysis, Investigation, Writing - review & editing. Antonello Mulas: Formal analysis, investigation. Maria C. Follesa: Conceptualization, Methodology,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was financed by Autonomous Region of Sardinia within the frame of the research project ‘Approccio multidisciplinare per la conservazione e gestione della selacofauna del Mediterraneo’ (LR7 CRP- 25321) and carried out within the Data Collection Regulation and Framework - module trawl surveys MEDITS (Mediterranean International Trawl Survey).
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