Releasing test on reared recruits of sea urchins
\label{releasing-test-on-reared-recruits-of-sea-urchins}
Context
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.
Methods
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.