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A combination of molecular and morphological characteristics was used to investigate populations of wild and farmed gilthead sea bream Sparus aurata from the largest Croatian shellfish production area, Mali Ston Bay. Identification of farmed escapees was performed in the wild in order to evaluate and compare the detection sensitivity of molecular and morphological tools. Using a traditional set of measurements and a truss network system, morphometric trait analysis of gilthead sea bream showed clear body shape differences between individuals from the wild and farmed populations. Microsatellite markers only revealed weak neutral genetic differentiation between farmed and wild samples from Mali Ston Bay (Wright's F ST = 0.019, p > 0.05; Jost's D EST = 0.022), despite the fact that the farmed gilthead sea bream were of Atlantic origin. Using assignment tests, morphological (16%) and molecular (25%) analysis identified farmed escapees in the samples of wild gilthead sea bream in the vicinity of aquaculture sites. Morphological data showed good potential for distinguishing recent escapees in wild populations, since these characteristics are affected by the environment. This study demonstrates the presence of escapees in the local wild population in this region, and suggests the potential for genetic interaction.

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Hybridizations between domesticated escaped farmed fish and wild conspecifics may increase genetic diversity or break down locally adapted gene complexes, thus reducing competitive ability and overall fitness. We examined the genetic structure of six farmed populations of European seabass, Dicentrarchus labrax, of different geographical origin, and ten neighbouring wild populations in the Adriatic Sea using 10 microsatellite loci. All loci were polymorphic, with mean expected heterozygosity >74% in all 16 populations. Overall number of alleles as well as short-term effective population size were smaller in farmed (A = 18.7; Ne = 56.1) than in wild populations (A = 21.2; Ne = 180.6). The global FST of 0.0672 across loci showed significant population subdivision. Strong genetic differences between farmed fish and their wild conspecifics enabled the identification of seabass escapees back to their origins and the estimation of the extent of hybridization. Bayesian assignment analyses clustered wild populations together, whereas each farmed populations was assigned to a separate cluster. Intraspecific hybridization was highest in wild populations in areas impacted by fish farms, where highly admixed populations had decreased genetic diversity. The results of this study represent a solid foundation required to establish a genetic register of European seabass wild and farmed populations in the eastern Adriatic Sea that are required to establish a seabass DNA register.

The abundance and size structure of wild fishes aggregated around the sea-cages of two commercial Thunnus thynnus farms, including control locations, were assessed and compared over a 1 year period. The T. thynnus farms were located in the eastern Adriatic Sea, offshore of the islands of Ugljan and Brač. Fish assemblages were evaluated through visual census using scuba at 2 month intervals at two sites within each farm. The data suggest that wild fish assemblages at the study sites differed greatly; 20 species occurred at the Ugljan farm and 17 at the Brač farm, while only seven species were observed at the control locations. The abundance and diversity of wild fish assemblages were greater at the farms in comparison to control locations. The most abundant families were Sparidae and Belonidae (>80% of aggregated fishes). At both farms, the abundance and diversity of wild fishes were highest during summer, while diversity was lowest in winter and was mainly characterized by schools of bogue Boops boops and garfish Belone belone. Variability was also detected in spatial assemblages between farms; B. boops and B. belone were the most abundant species for the overall study at the Brač farm, while B. belone and saddled bream Oblada melanura were the most abundant at the Ugljan farm. The settlement also played a significant role in farm-associated fish assemblages, as both juveniles and advanced juveniles were common residents at farms. The majority of species which settled at the farms belonged to the sparids. Results indicate that aggregations of wild fishes at T. thynnus farms are persistent year-round, though the assemblage compositions and size structures of dominant species vary in respect to location and season.