Settlement of an exotic oyster and other epibenthos on plastic substrates at a northern Brazilian mangrove island

Abstract

Management and culture of mangrove oysters may reduce harvesting pressure on natural populations in northern Brazil. Densities of oyster seed, scars and other epibenthos on plastic sheets and bottles were observed over two years at a mangrove island (Ilha Canela), Bragança, Pará, northern Brazil. The only oyster species found settling on the substrates was a possibly exotic oyster (Crassostrea sp. canela) that is closely related to Indo-Pacific Crassostrea and that also occurs in southern China. Densities of seed and scars of Crassostrea sp. canela on bottles were about half those on sheets. Seed and scars predominated (>95% relative abundance) on both collectors. Although densities of other epibenthos were low and irregular, these were positively correlated with densities of seed on both collectors. Oyster seed and barnacles on sheets were more abundant in the wet season. However, in the dry season, seed predominated on the inside of bottles, a refuge from predation or desiccation or both, whereas oyster scars and barnacles were more abundant on the outside. The target species for oyster culture in Pará is Crassostrea gasar. Oyster growers are concerned about potential impacts of Crassostrea sp. canela, the larvae of which appear to prefer high salinity. Seed is regular and abundant as densities exceed published figures for several other Crassostrea species. Knowledge of the timing and density of settlement of this oyster and associated epibenthos is important in order to minimize interference during seed collection of target species. The timing and location of seed collection is therefore critical. We also suggest experiments with collectors conditioned with adult Crassostrea gasar shell remnants, as well as those where limited establishment of epibenthos, such as barnacles, has been allowed, in order to maximize spat collection of the target species, C. gasar, along the Pará coast.