Proposal idea for a new experiment

Releasing test on reared recruits of sea urchins

In Sardinia (Italy, western Mediterranean) a variety of management tools are used, according to regional laws, for sea urchin fishery, including size limits, closed seasons, gear restrictions and marine reserves (Pais et al. 2007). Despite these regional decrees, the resource is drastically decreasing in several areas with consequences for fishery and may be for the ecological functions of populations and coastal ecosystems. Experiments around the world have repeatedly demonstrated strong effects due to the removal of sea urchins from the environment (Andrew et al. 2002), but few studies are carried out on the effects of the enhancement and recovery of populations for a sustainable fishery. Enhancement of sea urchin populations is divided into three categories: reseeding, habitat enhancement, and transplantation in wild populations (Andrew et al. 2002). About this study, we would like to investigate new issues concerning the reseeding of sea urchin recruits produced and grown in captivity for approximatively sixteen months (diameter without spines 10 -20mm).
The theoretical basis of stock enhancement by reseeding is the belief that populations are recruitment limited (i.e. due to the limited abundance by processes acting on sea urchins before they settle). The premise is that we can raise large numbers of larvae or juveniles and by releasing them into the marine environment, they compensate for the lack of these stages in nature and thereby increase stock size in the late juvenile and early adult stages (Saito 1992, Kitada 1999). Consequently, for reseeding to make sense, mortality of reared recruits have to be no higher than of the wild ones and populations receiving the out-planted organisms have to be not near to the carrying capacity of the environment. In this sense, both of the evaluation of environment carrying capacity and the development of a production process that optimizes the times of the larval growth in according with the lowest possible mortality are the key-points for a successful reseeding.
The active reseeding (or restocking) is based on the production of larvae and post-larvae under controlled laboratory conditions until those animals are potentially ready to be released. At this porpouse, we used optimal diets and we tried to minimize the environmental stress constraining the variation of the physical-chemical conditions to optimize the production and growing of the organisms (Brundu et al. 2016). However, it is still unknown if, through this procedure, mortality of reared recruits is higher respect to the wild ones and, consequently, additional methods of acclimatization should be added before the releasing. Actually, many studies have shown that vertebrate organisms grown in captivity are generally less resistance to the environmental variations than the wild ones (McGinnity et al 2009) and their antipredator behaviour is different, since they are more vulnerable to predators (Meager et al 2011).
In Japanese sea urchin hatcheries, reared individuals with a diameter of about 5mm were directly placed into small mesh cages for intermediate culture in tanks on land or suspended in the sea and no acclimatizing stages were done before reseeding (Tegner, 1989). Previous studies on the effects of sea urchins reseeding (Paracentrotus lividus and Strongylocentrotus franciscanus ) did not carry out acclimatizing stage of animals before their release (Juinio-Meñez et al., 1998; Couvray et al., 2015). Nevertheless, it is possible that intermediate cages or tanks cultures between laboratory and natural environment, as carried out in Japanese sea urchin hatcheries, could influence positively the successful of reseeding. Furthermore, additional methods of acclimatization, to be added before the releasing, could improve the successful of sea urchin restocking, in terms of survival. In this sense, it may be reasonable to check differences in resistance and environmental perceptions among reared and wild sea urchins.
Objective and hypothesis of work
The aim of this study is to evaluate the success of the restocking in a controlled environment with reared recruits (10<TD<20mm) under optimal growth conditions, but without any additional methods of acclimatization.
Since previous studies on the successful of reared sea urchins reseeding are lacking, and in according with the general tendency to skip the acclimatizing stages during the breeding of this invertebrate, we hypothesize that response of reared se urchins to the environment, in terms of mortality and perceptions, will be similar to that of the wild ones.
The International Marine Centre of Oristano (Western Sardinia, Italy) will carry out this study. Approximatively 100 reared individuals with diameter included between 10 and 20mm (corresponding to sixteen months of indoor growing) and the same number of wild individuals, will be stocked without feed in seawater and then released on the natural bottom.
The experiment will be carried out when the seawater temperature reaches approximatively that of laboratory rearing tanks (20 °C, late spring). Tests will take place in a controlled environment easily accessible such as a natural pool of 30m2 surface approximatively, 1.5-2m deep and with a mosaics of algal communities on rocky bottom and bare sand. Experiment consists in two phases addressed to (i) estimate differences in environment perception of reared versus wild recruits and (ii) observe their survival rate.
Phase 1. Environment perception
Differences in perceptions will be estimated as the capacity to recognize a suitable habitat after the reseeding. Specifically, we will estimate the time taken by each single recruit to reach the rocky habitat covered with algae (food and refuge) from a known distance in the bare sand. Observations will be made approximatively on 50 reared and 50 wild recruits, in the same weather conditions (light intensity and no swell).
Phase 2. Estimation of survival rate
Approximately 50 reared recruits and 50 wild recruits will be placed in separate plots of rocky bottom cover by algae (25m2 of surface approximatively in relation with natural density). Each plot will be closed on the edges to prevent sea urchins to leave. The survival rate of both groups will be estimated differentiating the causes of death (e.g. physiological causes or predation). In addition, we will estimate the grazing activity of the two groups by assessing the algal cover before and after the reseeding.

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