Effects of High-density Stocking in a Recirculating Aquaculture System on Gill Morphology ofHybrid Striped Bass (Morone saxatilis x M. chrysops)

The types and distribution of gill lesions observed in hybrid striped bass (Morone saxatilis x M. chrysops) reared in a commercial-scale recirculating aquaculture system are described. When placed in the system as fingerlings and reared there for eight months at typical stocking density, the gills of all examined fish presented a variety of extensive, non-specific lesions typically resulting from poor water quality. Lesions included epithelial cell hyperplasia, infiltration of the interfilamental region by mixed inflammatory cells, hyperplasia of mucous and lamellar epithelium, lamellar fusion and occasional filamental fusion. Up to 76% of the gill sample surface of individual fish was affected, with lesions being most severe in the distal filamental regions. Fish transferred to and maintained at low stocking densities in water of superior quality demonstrated that all lesions were fully reversible by five weeks post-transfer. This study demonstrates that culture of hybrid striped bass under intensive aquaculture management induced pathological changes in the gills, and suggests that maintenance of fish under improved water quality conditions will reduce gill lesions, which could potentially increase the fishes' performance.