Moderate establishment success of Pacific oyster, Crassostrea gigas, on a sheltered intertidal mussel bed

• M. edulis form the settling substrate for C. gigas in the microtidal habitat.
• Recruitment of C. gigas is a frequent event occurring in three out of seven years.
• Forty years after introduction C. gigas pose no negative impact on the native M. edulis.
• C. gigas displays moderate establishment success.
• Due to food limitation, C. gigas is in a prolonged establishment phase.

The Pacific oyster (Crassostrea gigas Thunberg 1793) is introduced into marine ecosystems worldwide. In Denmark, C. gigas was introduced into the micro tidal Limfjord, around 1972 for aquaculture. This study describes the population structure of C. gigas at Agger Tange in 2007, 2009, 2010 and 2011. Here, C. gigas use beds of Blue mussels (Mytilus edulis L.) as primary habitat. The mean abundance (± 1 SD) of C. gigas was unchanged during our study (45 ± 2 indv. m− 2), while it increased for M. edulis from 2010 to 2011 (934 ± 610 to 1434 ± 750 indv. m− 2, respectively). In 2009, a newly settled cohort of C. gigas was present, but in the succeeding years no or negligible recruitment was recorded. However, age cohort analyses, based on individual shell size at different ages, suggest successful recruitment in three out of seven years. A comparison with the course of the bioinvasion in List Tidal Basin, suggests that the population at Agger Tange is not in the expansion phase of the bioinvasion, despite of frequent settlement, high shell growth rates and relatively high abundance. So far, C. gigas has had moderate establishment success. We conclude that C. gigas is still in the establishment phase, but that this is prolonged, presumably due to low food availability.