Knowledge of life-history traits is increasingly recognized as an important criterion for effective management and conservation. Life history studies provide important information for conservation of a species by helping to define conservation units on the basis of biological/ecological units.
The main objective of this thesis was the definition of the life history of R. clavata through the application of different approaches: study of the reproduction, assessment made by demographic models and morphological definition of the species.
A background of the current knowledge available on the thornback rays is presented with the aim of highlight which information are needed in understanding the life cycle of this species. One of this was the understanding of the processes relate to the reproductive system. Then a comparison between macroscopic and microscopic observation of the reproductive system in both sexes is presented. This research allowed the description of the principal phases of the gametogenesis in R. clavata and the validation of the first stages of the maturity scale in use during the MEDITS trawl survey for this species. The value of this study consisted in that macroscopic and the corresponding microscopic observations were made exactly on the same individual. Thus, thanks to the photographic registration of the macroscopic observations, the two characteristics were associated and illustrated.
A part of this research dealt with the quantitative approach to the study of reproduction. Reproductive and mortalities parameters are required both for population demography model and stock assessment of species harvested. The total body mass at size, length at maturity and mortality were estimated with the aim of providing information that has been utilized in the next chapter. The length at which the 50 % of females were mature was about 78 cm, while length at maturity in males was estimated as 66 cm TL. The natural mortality rate (M) estimated in the present work is 0.16, while the mean instantaneous mortality coefficient Z estimated from research vessel catches was 0.4.
For the modelization of the status of population the so called demographic models can be a good alternative for sharks and rays population, for which often data on catch, effort and age composition in the catch are not available. In this study a demographic analysis of R. clavata undertaken by standard life history table methods was presented. This is a “per-recruit analysis” that allows foreseeing the response as change in the structure of the stock due to mortality (both natural and by fishing) and it mainly utilised information on life history traits. Two simulations were made considering different fecundities. In case of low fecundity the value of r is very near to zero, indicating a population not able to increase.
Moreover, a yield-per-recruit model was used for a preliminary assessment of the current status of exploitation of the species. The analyses performed in the present study allowed examining the likely consequences on yields and on the population capability of self-renewal following change in F and/or in the age of first capture. This is the first time that such analyses have been applied to elasmobranch fish in the Mediterranean Sea.
The value of the intrinsic rate of population increase, r, estimated for the thornback ray, 0.29-0.36, is coherent with that estimated for other rays of similar maximum total length, more then 100 cm. Thornback ray is characterized by a very low age at first capture, and both analysis, demographic and YPR models, highlighted that gains are much greater from increasing the age at first capture then from increasing fishing effort. Furthermore, the reference points evaluated indicate a current mortality rate higher then advisable. These results indicate a high vulnerability of this species.
Morphometric description of R. clavata has been carried out for the first time through a geometric approach. In this method homologous landmarks are used to explore biological shape through mathematical and statistical analyses. Thornback ray samplings were collected in north Tyrrhenian, Adriatic and North Sea, however the Adriatic samples were numerical insufficient for a further analysis. Landmark selected on dorsal side allowed to describe differences in shape between sexes and to hypothesize a shape variation between areas. The differences between sexes and areas appear to be due to the cephalic region; however a possible “Pinocchio effect” might be done to the method of superimposition used. The ontogenetic variation of males and females was illustrated through the thin-plate representations, showing one of the main advantages of the geometric morphometrics methodology, its ability to illustrate the variation of shape with size. Lastly, the shape differences between R. clavata and R. polystigma were briefly described with the aim of testing the applicability of the landmark selected in R. clavata to other species. The morphological approach is potentially a very useful tool to combine with the genetic evaluation of the population structure of the rays species.
This research helped build upon current knowledge about the life strategies of R. clavata, revealed to be a vulnerable species due to its late age at maturity, and low rate of population increase. This information can then be incorporated into fishery management plans to ensure that these species, and other similar, are managed and protected in the most efficient and effective manner.