Abstract
The European flat oyster, Ostrea edulis, is a keystone species suffering major population declines due to overfishing, habitat loss and parasite diseases. Knowledge of its fine-scale population genetic structure and connectivity, needed for effective conservation, restoration and management, is largely lacking. Along the eastern Adriatic Sea, genotyping of 1178 O. edulis individuals at 12 microsatellite loci was conducted, grouping the sampled populations by geographical origin (North, Middle, South Adriatic), shell-farm association (farmed, farm-impacted, wild oysters) and sampling year (2017, 2018), in order to explore spatio-temporal genetic variation and potential footprint of known human-mediated spat translocation events for aquaculture purpose. Short-term temporal genetic structuring of O. edulis populations was less pronounced compared to their spatial variability, which showed genetic discontinuity between O. edulis populations from different geographical regions, with the main boundary separating the North from the Middle and South Adriatic, and the weaker one limiting the flow between the Middle and South Adriatic. While the present culture practise and ongoing spat translocation promotes genetic heterogeneity in the investigated farms, reduced genetic diversity and smallest effective populations size of impacted, i.e. farm-associated O. edulis was consistently recorded in all geographical regions. Taken together, the results reflect regional oceanographic features, ongoing spat translocation and intensive harvesting, which might have reduced the wild O. edulis densities below the critical threshold for reproductive success, compromising settlement and favoring unidirectional gene-flow towards higher density farmed O. edulis. Genetic structure of Adriatic O. edulis populations revealed some concerning demographic changes and farm-wild oyster interactions and hence further investigation and management recommendations are given.
Highlights
Population connectivity and spatial structure information provide a basis for understanding marine species population dynamics, and play a key role in the conservation and management of fisheries (Reiss et al, 2009)
Collection included: (i) wild oysters sampled from eight natural beds covering 600 km of Croatian coastline; (ii) farmed oysters sampled from the three main aquaculture areas along the coastline, and (iii) three farm-associated populations sampled from natural beds located in the vicinity of oysterfarm installations
Human-mediated translocation of wild or hatchery-born O. edulis from one location to another, for the purpose of restoration of stocks depleted by intense exploitation and/or outbreak of disease, has been increasingly used in recent decades as a conservation management strategy for endangered species (Seddon et al, 2014; Bromley, 2015)
Summary
Population connectivity and spatial structure information provide a basis for understanding marine species population dynamics, and play a key role in the conservation and management of fisheries (Reiss et al, 2009). The European flat oyster (O. edulis) is one of the bivalve species with the longest tradition of harvesting and aquaculture (e.g., Caceres-Martinez and Figueras, 1997; Edwards, 1997; Goulletquer and Heral, 1997). It is a sessile, filter-feeding bivalve mollusc with a distribution ranging from Norway to Morocco in the Atlantic Ocean, and in the Mediterranean Sea and extending into the Black Sea. In the wild, O. edulis lives from the intertidal to 90 m depth and on different types of bottoms. It has been introduced into other parts of the world, including the United States and Canada (Carnegie and Barber, 2001; Vercaemer et al, 2006)
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