Filling biological information gaps of the marine topshell Phorcus sauciatus (Gastropoda: Trochidae) to ensure its sustainable exploitation

Abstract

Topshells play a pivotal role in intertidal rocky ecosystems and are adapted to harsh thermal and hydric stress. Phorcus sauciatus, a common grazer in the Macaronesian region (Madeira and the Canaries), has rarely been studied due to its restricted geographic distribution compared with Phorcus lineatus. Monthly samples were taken throughout 2017 to analyse biological parameters and evaluate the harvesting effect on the stocks of this species in Madeira. Individuals of the first age classes (<4 years) were dominant (similar to 89%), while immature individuals were more abundant during the summer season. The spawning season occurs between March and August, especially from March to June. The size at first maturity was 12.95 mm long (1.68 years), with a continuous recruitment pattern throughout the year. Yield-per-recruit analysis (Y/R) showed that maximum production is achieved at a fishing mortality of 1.7 year(-1), corresponding to a Y/R of 0.023 g. Currently, Phorcus sauciatus seems to be moderately exploited in Madeira, but urgent conservation measures, such as a landing obligation, the establishment of a minimum catch size of 15 mm length, and a closed season (February-May), are warranted to preserve stocks of this species in the medium to long term.