Damages on mussel farms potentially caused by fish predation—Self service on the ropes?

A dramatic decline has been recorded in Croatian shellfish production in recent years. Among the many factors that may have contributed to the decline of mussel production, i.e. EU import barriers and consequently small domestic market, great variability in larval dispersion and settlement, it is suspected that fish predation plays an important role in shellfish sustainability. Following this hypothesis, the abundance and diversity of fish assemblages around the Marina mussel farm, situated along the eastern coast of the Adriatic Sea, was monitored for a two-year period, and the mussel recruitment losses, potentially driven from fish predation, after the ‘early thinning-out’ process was quantitatively evaluated by measuring the percentage of consumed or destroyed ropes of longlines. Overall, a total of sixteen fish taxa were recorded at the mussel farm, as opposed to fifteen taxa registered at all control locations. The asymmetrical design of the multivariate analyses revealed a statistically significant impact of the farm on fish assemblages, with expressed seasonal species variation. At mussel farm, the most abundant species observed were gilthead seabream Sparus aurata, sand smelt Atherina hepsetus and mullet Mugilidae species in summer and autumn; sand smelt A. hepsetus and bogue Boops boops in winter; and the sand smelt A. hepsetus, mullet Mugilidae species and seddled bream Oblada melanura in spring. At control locations, characterised with significantly lower fish assemblage abundances, the most abundant species were sand smelt A. hepsetus and mullet Mugilidae species in summer, sand smelt A. hepsetus and seddled bream O. melanura in autumn, sand smelt A. hepsetus and picarel Spicara flexuosa in winter, sand smelt A. hepsetus and bogue B. boops in spring. Gilthead sea bream was extremely abundant at mussel farm, with 5936 individuals censused in 155 of 192 fish counts (80%), and a maximum abundance of 285 individuals per 5000m³. Stomach content analysis confirmed the presence of mussel Mytilus galloprovincialis as the dominant prey. During August (2009), monitoring of 423 ropes revealed that 276m of mussel ropes or approximately 828kg of mussels with an average shell length of 34.3±2.54mm were destroyed within the first week of mussel deposition into the sea. The recorded recruitment losses of 54% in only one month of monitoring indicate a strong negative impact on farm management stability.