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Invasive Magallana gigas and native Ostrea edulis oysters in the northern adriatic sea – is there an overlap in their distribution?

In the Adriatic Sea non-native invasive oyster Magallana gigas started to spread mostly in the Northern Adriatic. A spread of invasive species could pose a threat to the ecosystem stability and function that in the case of M. gigas could be expressed through competition for space and food with other species, habitat modification, etc. Given the high invasion success of M. gigas, the aim of this study was to investigate whether the native and non-native oysters are in competition for space in their natural habitats. The distribution and abundance of oysters were determined along Istria west coast by in-situ visual counts at 3 depth zones (surface, 3m, 6m) of 6 locations (2 within the Lim estuary, 2 on Istria coastline north and 2 on coastline south to the estuary). Ostrea edulis mostly occurred as solitary specimens and density slightly increased with depth. Magallana gigas were found exclusively at the surface layer where it formed thick, dense clusters resulting in much higher densities compared to O. edulis in all locations (10.4±4.9 to 141.7±31.2ind/m2 vs. 0.5±1.8 to 1.0±2.4ind/m2 ) except to the north of estuary where the distribution and abundance of two species resembled (0.5±1.8 to 3.6±5.0ind/m2 vs. 1.6±2.8 to 2.1±3.1ind/m2 ). Results suggested that there is no overlap in depth distribution and Magallana gigas were located always in the medio-littoral zone while all recorded O. edulis were below the low–tide line. This study presents first quantitative assessment of M. gigas in the Adriatic and provides a base for monitoring changes in its distribution.

Crassostrea gigas, pacific oyster, Lim Bay, Istria, depth distribution, abundance

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