Distribution patterns of two co-existing oyster species in the northern Adriatic Sea: The native European flat oyster Ostrea edulis and the non-native Pacific oyster Magallana gigas. (June 2020)

• Record Type: Journal Article
• Authors: Stagličić, Nika; Šegvić-Bubić, Tanja; Ezgeta-Balić, Daria; Bojanić Varezić, Dubravka; Grubišić, Leon; Žuvić, Luka; Lin, Yaping; Briski, Elizabeta
• Is Part Of: Ecological indicators. Volume 113(2020)
• Publication Date: 2020-06
• Subjects: Crassostrea gigas -- Mediterranean Sea -- Non-indigenous species -- MPA -- Size structure -- Oyster population density; Environmental monitoring -- Periodicals; Environmental management -- Periodicals; Environmental impact analysis -- Periodicals; Environmental risk assessment -- Periodicals; Sustainable development -- Periodicals; 333.71405
• DOI: 10.1016/j.ecolind.2020.106233
• Languages: English
• ISSNs: 1470-160X

Abstract

Highlights: Native flat and non-native Pacific oysters coexist in the Northern Adriatic Sea. Flat oyster populations are sparse, while Pacific oysters locally form dense reefs. Two oyster species inhabit different water depths. Currently interspecific competitive displacement is unlikely. Abstract: The Pacific oyster Magallana gigas, globally one of the most translocated marine species, has never been commercially farmed in any part of the Croatian eastern Adriatic Sea, where the native flat oyster Ostrea edulis is the only cultured oyster species. The Pacific oyster has, however, established populations on the west coast of the Istria peninsula, in the northeast Adriatic Sea. Current distribution, abundance and size structure of the flat and Pacific oysters were studied there along spatial and depth gradients in natural and artificial habitats to assess their potential for coexistence. This is the first quantitative assessment of native and non-native oyster populations on a Mediterranean-wide scale providing a baseline for determining future changes in their distribution. Both species were omnipresent, with the Pacific oyster displaying a more pronounced variability in abundance and size in relation to survey regions and depths. The population density of flat oysters was low, generally less than 1 individual/m 2, with no difference among regions. The density of the Pacific oyster was significantly higher, being on average 5 individuals/m 2 . Dense, reef-forming aggregations of Pacific oysters were contained in Lim Bay, a nationally-important shellfish aquaculture area, where its mean density was 107 individuals/m 2 . Along the open coastline its densities were considerably lower and followed a latitudinal gradient. The observed abundance and size distribution patterns of the Pacific oyster suggest that Lim Bay was the introduction point, with feral settlement likely originating from short-lived experimental aquaculture trials and subsequently dispersing by prevailing local currents. The flat oysters were larger in size and settled at different depths compared to the Pacific oysters. The vertical range of Pacific oysters was mostly contained in the tidal zone, while flat oysters were present in the subtidal exclusively. Such spatial partitioning likely resulted from introduced oysters occupying a vacant ecological niche and not due to interspecific competitive exclusion. Habitat type had a strong effect on proliferation of Pacific oysters. Artificial hard substrata harboured more abundant and larger Pacific oysters than the natural rocky shore habitat. The possibility of multiple local introduction pathways in ports and marinas is also discussed. Currently, there seems to be no spatial competition between the two oyster species, but as it is hard to predict future possible impacts of non-native species, we strongly suggest regular monitoring of the Pacific oyster, prioritising sensitive and protected areas as well as areas at the edge of its distribution.