La riproduzione dei condroitti come elemento chiave per la loro conservazione e gestione nel Mediterraneo centro-occidentale

Chondrichthyes make up one of the oldest and most ecologically diverse vertebrate lineages: they arose at least 420 million years ago and rapidly radiated out to occupy the upper tiers of aquatic food webs (Compagno, 1990; Kriwet et al., 2008; Dulvy et al., 2013). Today, this group of predators plays important functional roles in the control of coastal and oceanic ecosystems (Stevens et al., 2000; Ferretti et al., 2010; Heithaus et al., 2012). They are an important resource, and constitute a significant fraction of the by-catch, which for decades has been undergoing an increase in mortality that has altered the relative abundance and structure of the their worldwide population. This is mainly due to their K-selected life-history strategy that makes them extremely vulnerable and not very resilient to the high rates of mortality due to fishing (Musick, 1999; Cortés, 2002; Dulvy and Forrest, 2010). The knowledge of the biological parameters related to the reproductive biology (life-history traits) and the state of the populations of these marine organisms are essential to plan proper management of fisheries. Sardinian seas represent a small biodiversity hotspot in the central western Mediterranean, as well as being an important junction of the two basin sides (West and East). The high richness of chondrichthyan species which inhabit these waters combined with the absence of protection measures, has provided an opportunity to undertake a study on the status of these marine predators, in order to establish a starting point for a correct management, studying biological parameters. This aim was achieved conducting specific analysis on sexual maturity going to deepen aspects on their reproductive strategies. The results on the abundance trends of demersal cartilaginous fish inhabiting the Sardinian waters, were at first useful to identify the most abundant species for which the information on the reproductive cycle and parameters are scarce or fragmented in the literature (Dipturus oxyrinchus, Etmopterus spinax, Galeus melastomus, Raja brachyura, R. miraletus and Squalus blainville). All these species are included in the IUCN red lists. The adoption of standardized maturity scales for oviparous and viviparous species, through the observation of macroscopic reproductive systems, supported by histological analysis, were crucial for the correct attribution of maturity stages that allowed to discern the different phases of their life cycle and to identify the main reproductive parameters. The latter revealed a generalized vulnerability to fish mortality in all studied species. Specifically, skates and sharks shown a very late achievement of sexual maturity, in both sexes, with L50 generally higher than 80% of their maximum size. This feature, compared with those reported in the Atlantic (~ 75% of the maximum total length, Cortés 2000) suggests that the fishing pressure in our seas is worrying. The reproductive potential of the species seems to depend closely to the hepatic activity that supports the development of ovaries and testes, and in the two viviparous sharks analyzed (E. spinax and S. blainville), guarantees also the embryos' growth, thanks to the production of specific lipids, during the pregnant phases. The determination of the breeding season is the key for a proper management. Among the oviparous species studied, D. oxyrinchus and G. melastomus showed an extended reproductive period with one and two peaks respectively during the year. A reproductive cycle limited to well-defined period, however, has characterized the two coastal skates (R. brachyura and R. miraletus). E. spinax and S. blainville displayed a reproductive cycle that consists of two different periods (vitellogenesis and gestation) with a quite extensive gestation ranging from 1.5 to about three years. It is evident that the capture of these species, in particular females, especially during the breeding period, would lead to a slow decline of these populations. A quite extensive reproductive cycle in the two viviparous sharks was further confirmed by the analysis of the oviducal gland micro-architecture, with a 'long-term storage' of sperms (about two years) in special compartments called Sperm Storage Tubules (SST). This observation would lead to hypothesize a reproductive strategy that would benefit them in survival in deep environments, where the chances of mating are rather scarce, thus increasing the reproductive fitness. The limited reproductive period encountered in R. miraletus has been validated by a 'short-term storage' of sperm cells in the oviducal gland, with a rather rapid preservation of the same. Instead, D. oxyrinchus, that showed an annual reproductive cycle, appeared to preserve the sperm for a longer period of time (in terms of months). The over-exploitation of a species can lead to changes density-dependent (Fahy, 1989) and, in some populations, the answer to the excessive fishing effort could be offset by the reduction of the medium maturity sizes (Ellis and Keable , 2008). Considering these aspects, the identification of specific spawning areas and the analysis of time trends on the medium sizes of spawners, highlighted that the west coast of Sardinia is the one with a higher concentration of them. In particular, R. miraletus and G. melastomus sizes seems to be temporally undergoing a decline often beyond the L50. For this reason we proposed specific areas that need protection measures and we indicated the minimum size of capture for each species. Finally, the analysis conducted on the morphology and morphometry of the most common Rajid species egg capsules, led to the creation of a dichotomous table useful for the specific identification of those found stranded or during fishing operations. This table could allow to obtain information on the spatial and temporal distribution and identify any nursery areas in the Mediterranean. In conclusion, the overall picture of the reproductive status of the six species investigated in Sardinian waters represents a first step in order to impress management measures in the island and later in the Mediterranean. It is hoped that this information can be used for the creation of appropriate management policies by the competent authorities. Moreover, in the near future, there is a commitment to improve the monitoring, including the development of targeted sampling protracted. Finally, we propose to extend the research to the other chondrichthyan species present in the Sardinian seas to have a more complete picture, so we can safeguard these important predators, because without them, marine ecosystems would be destined to collapse.