Farmed bivalve loss due to seabream predation in the French Mediterranean Prevost Lagoon

M Richard,J Bourjea,Cw Mckindsey,Md Callier,F Forget,Am Weise,S Mortreux,P Le Gall,A Mignucci

doi:10.3354/aei00383

Abstract

Bivalve predation by seabream has been observed worldwide and is a major concern for bivalve farmers. Farmed bivalve-seabream interactions must be better understood to ensure the sustainability of bivalve aquaculture. The objectives of this study were to characterize gilthead seabream Sparus aurata presence in a bivalve farm in Prevost Lagoon (Mediterranean Sea) using acoustic telemetry and to evaluate monthly losses of mussels Mytilus galloprovincialis and oysters Crassostrea gigas due to seabream predation over an 18 mo period inside the farm and at an unprotected experimental platform. Large (281 to 499 mm TL) seabream were more commonly detected in the bivalve farm than were small (200 to 280 mm TL) seabream. In contrast to small seabream, 90% of large seabream returned to and spent extended periods in the study area the following year, suggesting inter-annual site fidelity for large fish that used the bivalve farm as a feeding site. Signs of predation were observed on mussels and oysters throughout the year at the unprotected experimental platform. Farmers noted losses in the farm from April to September. Maximal losses (90 to 100%) were observed post-oyster ‘sticking’ and mussel socking. Despite the deployment of nets as mechanical protection to reduce predation, oyster losses represented 28% of the annual value of oysters sold while mussel losses were estimated at ca. 1%. These results suggest that bivalves must be protected by nets throughout the year to avoid predation, particularly post-handling. A collaboration between shellfish farmers and fishermen could be a sustainable solution for bivalve farming, by regularly fishing for seabream in farms, between tables and inside protective nets.

Highlights

Fisheries and aquaculture production from marine and coastal environments accounted for a global annual production of 171 million t in 2016, of which 28.7 million t were from mariculture
Seabream have been shown to feed on farmed mussels and oysters (Saito et al 2008, Šegvić-Bubić et al 2011); little information is available on the magnitude of bivalve culture losses due to seabream predation
In French Mediterranean lagoons, oysters Crassostrea gigas and mussels Mytilus galloprovincialis are reared on ropes suspended from metal structures, hereafter referred to as ‘tables’ (Gangnery et al 2003)

Summary

Introduction

Fisheries and aquaculture production from marine and coastal environments accounted for a global annual production of 171 million t in 2016, of which 28.7 million t were from mariculture. 85% of surveyed bivalve farmers protect bivalves against seabream by deploying nets around the tables. This incurs significant financial and logistical costs, including purchasing nets, their deployment and maintenance, and potentially reduced bivalve growth due to reduced current flow and food delivery. Despite this physical protection, bivalve farmers have estimated mean mussel losses due to seabream predation at 26% (Gervasoni & Giffon 2016). Predation on juvenile oysters was observed but losses were not estimated (bivalve farmers pers. comm.)

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Conclusion