Co-occurrence of nativeOstrea edulisand non-nativeCrassostrea gigasrevealed by monitoring of intertidal oyster populations

Abstract

Coastal ecosystems are particularly vulnerable to alien invasions. Regular, standardized, targeted monitoring of coastal areas helps to detect the arrival of non-native species early, identify sites most vulnerable to invasion, and assess potential for further spread. This study quantified the spread and changes in distribution of non-native oyster,Crassostrea gigas, populations around the coast of Ireland. In total 37 sites were surveyed, in areas which either currently or previously harboured cultivatedC. gigas, for the presence and abundance of ‘wild’C. gigas. Wild populations were identified at 20 sites and at four additional sitesC. gigaswas observed as recently discarded from aquaculture activity. Five of the invaded sites were identified as being highly suitable for a population expansion based on their current population status. Importantly, we also identified individuals ofC. gigasand native European oysters,Ostrea edulis, co-occurring within the same shore at five sites. This is the first record to our knowledge of such co-occurrence within Europe. This evidence of co-existing oyster species raises concerns regarding the potential impact ofC. gigason recoveringO. edulispopulations. In Ireland, however,C. gigasdoes not typically spread extensively from introduction points, and although self-containing populations exist, they are currently sustained at a much lower density than those observed in other regions such as the Wadden Sea or French Atlantic coasts. We suggest, therefore, that to protect native oyster populations,C. gigasshould be eradicated where co-occurring withO. edulisand recommend continuous monitoring of invaded sites.