Testing the vulnerability of juvenile sea bass ( Dicentrarchus labrax ) exposed to the harmful algal bloom (HAB) species Pseudo-nitzschia delicatissima

A 45-day exposure experiment was carried out to assess whether the harmful algal blooming Pseudo-nitzschia delicatissima (i.e. needle shaped diatom and potential domoic acid producer) is involved in recurrent mass mortality events at a coastal fish farm in the southern North Sea. Juvenile sea bass (Dicentrarchus labrax) were exposed to different concentrations of P. delicatissima, ranging from 1×106 to 5×106cellsL−1. Despite exposure to high concentrations of P. delicatissima, sea bass weight and length based growth rates (0.05±0.02gd−1 and 0.18±0.05mmd−1, respectively) and condition (Fulton K=0.98±0.06mgmm−3) were not significantly reduced suggesting that the fish were not affected during the 45-day exposure period. However, signs of gill mucosa irritation (i.e. mucus overproduction) were observed, showing that this diatom induced a mechanical stress, which can affect gill functions or increase susceptibility of fish to secondary infections (e.g. bacterial and viral infections, parasitism). Results are discussed with regard to consequences for wild and farmed fish, and possible synergic noxious effects when P. delicatissima is associated with other potentially harmful taxa (e.g. Phaeocystis globosa) during the phytoplankton spring bloom in the eastern English Channel–North Sea ecosystem.