THE IMPACT OF HARMFUL ALGA BLOOMS ON FINFISH - MORTALITY, PATHOLOGY AND TOXICOLOGY

Ouvrage en anglais Many algal blooms ("red tides" or "brown tides") may be noxious for finfish, either causing anoxia through decay process, clogging the gills or producing specific toxins. Since the 1970's, records of the incidence of injury and mortality to wild and cultured fish due to toxic phytoplankton have increased. It was even demonstrated that toxic algal blooms are spreading in intensity, geographic distribution and number of algal species involved. Thus, algal blooms are becoming one of the major economic threat to fisheries and fish culture industry. This review deals with the literature available (more than 250 references) on the occurrence of fish kills associated with worldwide algal blooms. Their negative effects on fish that result in economic losses are also discussed. Since recent decades, special attention was paid to ichtyotoxicity of algal blooms : organic lesions reported on wild or cultured fish as well as histopathological and physiopathological perturbations induced in laboratory conditions following controlled contamination with toxic cells of toxic extracts. Gill and liver tissues are mainly involved and many cellular disorders (oedema, ulceration, necrosis...) have been reported. Thus, liver injuries, that reveal some hepatotoxic potentialities, were described in few cases. Whether the precise mechanisms of the toxicity of some neuro toxins were analyzed at the molecular level, they were not yet determined for many other toxins. Ecological impact of algal toxicity was investigated, especially when toxins (PSP, DSP, STX, CTX, MTX, OA, gonyautoxin...) were transferred through the food chain, from algae (mainly Dinoflagellates as Gyrodinium, Gymnodinium, Protogonyaulax, Pyrodinium, Noctiluca, Glenodinium, Gambierdiscus and also Cyanobacteria, Raphidophyceae, Prymnesiophyceae, Silicoflagellates and Diatoms...) through vectorial intoxication (herbivorous zooplankton as copepods and pteropods) to fish, sea birds, marine mammals and human consumers. Incidence of mass mortalities, especially fish larvae on dynamics of fish stocks must be emphasized. In conclusion, many facts of the general problem of algal toxicity on fish remain to be explored. An interdisciplinary approach is still needed. Cette revue bibliographique doit être considérée comme un événement d'importance pour l'ensemble des scientifiques, hygiénistes et aquaculteurs intéressés par le problème des microalgues toxiques. Malgré l'abondance de la littérature sur le sujet et la multiplication des groupes de travail et congrès internationaux ces dix dernières années, les ouvrages de synthèse restent rares et la plupart du temps dévolus aux seuls aspects des espèces phytoplanctoniques responsables ou des phytotoxines les plus connues dans le monde, c'est-à-dire très souvent celles qui sont actives par voie alimentaire. Cette synthèse a ceci d'original qu'elle concerne en exclusivité les effets directs de certaines microalgues marines et dulçaquicoles sur les poissons et qu'elle traite non seulement des mécanismes toxicologiques mis en jeu, mais aussi de l'impact économique sur l'aquaculture mondiale. A ce titre, cet ouvrage servira évidemment de référence à tous ceux qui sont concernés par les problèmes de pathologie piscicole, que ce soit sur le plan écologique (incidences sur l'équilibre des écosystèmes côtiers) ou sur le plan de la production alimentaire (fermes d'aquaculture).