Long-term monitoring of the critically endangered limpet Patella ferruginea Gmelin, 1791: new ecological insights and first demographic results

Patella ferruginea is considered to be the most endangered marine invertebrate of western Mediterranean rocky shores. Its critical risk of extinction calls for the need to monitor the remaining populations and evaluate the status of the species at a local level. We have initiated a long-term demographic monitoring programme of the P. ferruginea population in Ceuta (North Africa, Strait of Gibraltar), one of the most important remaining populations in terms of size and density of individuals. Here we provide the longest demographic information available for this species. Seven sites were selected, with different degrees of accessibility. At each site, three 10-m transects were delimited and all the P. ferruginea individuals were counted and measured yearly. During the first 5 years of the monitoring programme (2007-2011), we recorded a 61.9% increase in the overall number of individuals. Site accessibility (positively correlated with the impact caused by human collection) is a good predictor of P. ferruginea population structure and density; while subpopulations located in easily accessible sites showed an increase of 8.7% in the number of adults (>25 mm), other subpopulations with more difficult access increased by 80-100% in the 5-year interval. Results also provided new insights regarding recruitment processes in the species, suggesting that the number of adults located within the immediate vicinity influences larval settlement. We conclude that legal measures such as the creation of artificial marine micro-reserves should be considered in order to develop a network of small reserves that may help the viability of this species.