Review of progress in our understanding of fish-killing microalgae: implications for management and mitigation

Fish-killing algal species are responsible for much greater global economic impacts than HAB species leading to seafood biotoxin contamination. Yet the precise mechanisms of how microalgae kill fish remain poorly understood. Progress has been hindered by the use of widely different bioassay systems and lack of analytical methods to quantify and characterize so-called “ichthyotoxins”. All high biomass blooms even of nontoxic phytoplankton can cause significant stress for finfish contained in intensive aquaculture systems. Highly potent fish-killers include the taxonomically unrelated flagellate groups Cochlodinium, Karenia, Chattonella, Pseudochattonella, Heterosigma, Prymnesium which all readily lyse upon impact on the sensitive gill tissues of fish. A key mechanism for fish-gill damage being proposed is HAB cell lysis releasing free fatty acids (EPA, DHA, OPA) which in synergism with reactive oxygen species generate labile (min to hrs) lipid peroxidation products. Cell lysis is critical for Karlodinium and Alexandrium ichthyotoxicity, and high ROS producing strains (eg. Chattonella, Chilean A. catenella) cause greatest gill damage. With perhaps a single exception (Florida Karenia brevis), none of these ichthyotoxins are of human health significance, meaning that recently killed fish are still fit for human consumption. Finely ground bentonite clays at environmentally acceptable concentrations can effectively mop up ichthyotoxins and offer great potential as a HAB emergency response tool.