Filling in knowledge gaps: Length–weight relations of 46 uncommon sharks and rays (Elasmobranchii) in the Mediterranean Sea

Large sharks and rays are generally understudied in the Mediterranean Sea, thus leading to a knowledge gap of basic biological Mediterranean Sea, the length–weight relations (LWR) are available for 28 (37%) of them, usually for common small-sized species estimation of LWR of rare and uncommon sharks and rays in the Mediterranean Sea using the information from single records or Mediterranean regions. Museum collections and sporadic catch records of rare emblematic species may provide useful biological information with the use of appropriate Bayesian methods.


Introduction
According to a recent gap analysis on the biology of Medare among the least studied species in the Mediterranean Sea and the lack of knowledge is higher for the less abundant large sharks and rays that are protected and rarely (Dimarchopoulou et al. 2017). Despite their conservation 2006). Due to the scarcity of samples, the records on the biology of sharks and rays in the Mediterranean are spohowever, because of the importance of large sharks and rays in marine ecosystems and their conservation status (Dulvy et al. 2014), there are several single-catch records available of emblematic species such as the great white shark Carcharodon carcharias (Linnaeus, 1758) (see Kabasakal 2020). The majority of Mediterranean records originate from Tunisia and Turkey owing to the lifetime Kabasakal 2020 among others) and also, in the case of Tunisia, to the higher species richness of sharks and rays in the western Mediterranean Sea (Dulvy et al. 2016). The majority of biological characteristics of small and com-Scyliorhinus canicula (Linnaeus, 1758)-and rays-e.g., thornback ray, Raja clavata Linnaeus, 1758-that are marketed and not protected in the Mediterranean Sea are better studied (Froese and Pauly 2020).
Length-weight relations (LWR 2006) are important in stock assessments for converting length measurements to weight and estimation of stock biomass (Froese et al. 2011). The LWR is the most studied biological characteristic and studies are available for in the Mediterranean Sea (Dimarchopoulou et al. 2017). However, rare sharks and rays are underrepresented (Fro-cords deviate from the cube law dynamics (Froese 2006) because of the small sample size and narrow length and weight distributions (Froese et al. 2011).
A Bayesian hierarchical method for estimating length-by body form (Froese et al. 2014). According to this methb) and the intercept (a and are available in FishBase (Froese and Pauly 2020). Recently, a new approach for estimating LWR from single the length and weight records has been proposed and applied to preserved museum specimens for species for which LWR are not available in the literature (Hay et al. 2020). Based on this approach the LWR of several available but their LWR were not, were generated (Hay et al. 2020).
Overall, out of the 43 species of sharks and 33 species of rays that are present in the Mediterranean Sea (Dulvy et al. 2016), LWR are available for 28 species (37%) based on FishBase records (Froese and Pauly 2020) and the literature (updated dataset of Dimarchopoulou et al. 2017). The aim of the presently reported study was to combine these two approaches and generate LWR for 46 rare shark and ray species in the Mediterranean Sea based on single records (or records of a few individuals) that were available in the literature and for which LWR are lacking at a global or regional scale. Thus, the gap between available and desired knowledge on the LWR of Mediterranean ma-

Methods
We collected the single specimen records (or few specimens) of total length (L) [cm] and total weight (W) [g] data for 46 uncommon shark and ray species in the Mediterranean Sea that belong to 10 orders and 16 families from the literature. Then, we estimated their LWR (Table 1) using the Bayesian estimate available for the species and Pauly 2020). The number of specimens ranged from a single (17 cases) and double specimens (10 cases) to 27 specimens of the great white shark that are available in a recently published book (Kabasakal 2020). The mean number of specimens was 3.65. All length data were converted to cm of total length and all weight data to g of total weight.
In species for which a range of sizes was not available to estimate the parameters of LWR, such as with museum specimens, Hay et al. (2020) assumed that the LWR were isometric, i.e., that the parameter b = 3, which is a good ap-ilar approach was followed in the presently reported study with b with similar body form (Froese and Pauly 2020) following a Bayesian hierarchical method (Froese et al. 2014).
When total length (L) and total weight (W) measurements from single specimen were available, the parameter a was estimated as (Hay et al. 2020 timate available for the species based on LWR of related species (Froese and Pauly 2020) or body form when a Bayesian estimate was not available (Froese et al. 2014).
When several specimens (n) were available, but not enough to support a valid LWR, because of a small sample size or narrow length range, the mean value of parameter a was estimated as (Hay et al. 2020 timate available for the species based on LWR of related species (Froese and Pauly 2020) or body form when a Bayesian estimate was not available (Froese et al. 2014).

Results
The LWR of 46 uncommon sharks and ray species are reported for the Mediterranean Sea based on published of the specimens was preserved or in a museum collec-LWR for the Mediterranean Sea (Table 1). The remain-ern (nine species), eastern Mediterranean (one species) regions, and Aegean Sea (one species). A Bayesian estimate of parameter b based on LWR of related species was available for 41 species (Froese and Pauly 2020), while for the remaining 5 ones-Hexanchus griseus (Bonnaterre, 1788), Cetorhinus maximus (Gunnerus, 1765), Odontaspis ferox (Risso, 1810), Aetomylaeus bovinus Oxynotus centrina (Linnaeus, 1758)-an estimate based on their body form was used (Froese et al. 2014). In cases where more than one specimen was available, the range of length, weight, and parameter a is provided (Table 1).

Discussion
Although isometric growth (b = 3) is the most common type of growth in the majority of families (Froese 2006), strong deviations from isometry have been observed due to a narrow range of sizes and/or low sample size (Froese ality in b values is strong along with the corresponding estimates of a (Froese 2006), a limited sampling period, even with a large sample size of the entire somatic range of the species may lead to varying LWR (Hay et al. 2020).
viviparous sharks that deviate a lot in somatic weight during their gestation period (Castro 2000).
Length data for a few specimens of some other very

Pristis pristis
dusky shark, Carcharhinus obscurus (Lesueur, 1818) (see Bilecenoglu et al. 2013)-but weight data are lacking as part of the body was "available" usually the head, or are based on photos or anecdotal records from newspapers and magazines. Anecdotal evidence and historical records may be really valuable in reconstructing the history of these iconic predators (Ferretti et al. 2016) and gaining insight into the previous status of marine ecosystems and ecosysinformation from museum collections (Hay et al. 2020) to rare and sporadic catch records of emblematic species such as large sharks and rays for which biological information is lacking, at least in the Mediterranean Sea.
cient areas and species is highlighted along with the need for recording the basic biological information (length, uncommon sharks and rays. As the majority of these species are rare and protected (although illegally landed in many areas of the Mediterranean Sea), sometimes such sakal and Bilecenoglu 2020   Ali M, Saad A, Reynaud C, Capapé C (2013)  Ben Souissi J, Golani D, Mejri H, Ben Salem M, Capapé C (2007)