Mass mortality of pen shell Atrina maura (Bivalvia: Pinnidae) due to abrupt population increase of tunicate (Distaplia sp.) in a subtropical bay, Mexico

The present study analyses the coverage of epibiont Distaplia cf. stylifera (Ascidiacea: Holozoidae) and five environmental variables (Sal, SST, DO, sea surface Chl-a concentration pH) as potential factors to explain a mass mortality event in restored population pen shell Atrina maura (Bivalvia: Pinnidae) subtropical bay Gulf California, Mexico. Annual autumn surveys 2015, 2016 2017 (Nov–Dec) an additional summer survey (Jun) showed that tunicate was first observed at low densities few sampling sites during 2015 rapidly expanded along banks 2016. Tunicate manual removal carried out two small areas Sep-Nov Jan–Mar 2017. mortalities A. detected resulted high economic loss for regional fishers. A Generalised additive model (GAM) indicated substrate provided by only significant factor associated rapid expansion (measured proportion coverage) D. stylifera. Paradoxically, recovery success increased availability epibiont, facilitating abrupt fast colonisation process on banks, resulting events Our predicts effects begin occur when density is > 47 inds./250 m2. second GAM demonstrated main driver ascidian coverage, salinity, SST DO were secondary factors. We concluded dead increases >8%, salinity >37.3 PSU, <26.9 °C 5.06 mg O2/L. Early harvesting decrease signs growth can avoid losses future.