Hard time to be parents? Sea urchin fishery shifts potential reproductive contribution of population onto the shoulders of the youngest
Background. In Sardinia, as in other regions of the Mediterranean Sea, sustainable fisheries of the sea urchin Paracentrotus lividus have become a necessity. At harvesting sites, the systematic removal of large individuals (diameter ≥ 50 mm) seriously compromises the biological and ecological functions of sea urchin populations. Specifically, in this study, we compared the reproductive potential of the populations from two Mediterranean coastal sites which have different levels of protection.
... ls of protection. The sites were Su Pallosu, where fishing pressure is high (take zone) and at Tavolara-Punta Coda Cavallo Marine Protected Area (no-take zone) where the pressure is negligible. Methods. Reproductive potential was estimated by calculating Gonadosomatic Index (GSI) from June 2013 to May 2014 both for individuals of commercial size (diameter without spines, TD ≥ 50 mm) and the undersized ones with gonads (30 ≤ TD < 40 mm and 40 ≤ TD < 50 mm). Gamete Output was calculated for the commercial-size class and the undersized individuals with fertile gonads (40 ≤ TD < 50 mm) in relation to their natural density (Gamete Output per m2). Results. The reproductive potential of populations was slightly different at the beginning of the sampling period but it progressed at different rates with an early spring spawning event in the take zone and two gamete depositions in early and late spring in the no-take zone. For each fertile size class, GSI values changed significantly during the year of our study and between the two levels of protection. Although the multiple spawning events determined a two-fold higher total Gamete Output of population (popTGO) in the no-take zone, the population Mean Gamete Output (popMGO) was similar in the two zones. In the take zone, the commercial-sized individuals represented approximatively 5% of the population, with almost all the individuals smaller than 60 mm producing an amount of gametes nearly three times lower than the undersized ones. Conversely, the high density of the undersized individuals released a similar amount of gametes to the commercial-size class in the no-take zone. Discussion. Overall, the lack of the commercial-size class in the take zone does not seem to be very alarming for the self-supporting capacity of the population, and the reproductive potential contribution seems to depend more on the total density of fertile sea urchins than on their size. However, since population survival in the take zone is supported by the high density of undersized sea urchins between 30 and 50 mm, management measures should be addressed to maintain this size and to shed light on the source of the larval supply.