Waterborne gill-toxic potency as an effect-based endpoint for monitoring fish-killing harmful algal blooms

Abstract Fish-killing harmful algal blooms (HABs) continue to inflict severe economic losses on marine aquaculture worldwide, yet monitoring systems designed around microalgal cell counts frequently fail to anticipate acute mortality events. The dominant surveillance paradigm was inherited from shellfish-toxin monitoring, where a filter-feeding bivalve integrates exposure to intracellular phycotoxins and toxin burden in tissue triggers regulatory action. This architecture is structurally non-transferable to fish-killing HABs, since fish are exposed directly through their gills to a dynamic water mass in which the relevant compounds are predominantly extracellular, conditionally produced, and structurally unresolved for most ichthyotoxic taxa. Field evidence from the 2016 Pseudochattonella verruculosa event in Chile and the 2024 non-toxic Prymnesium parvum bloom in the Oder River, both at high cell densities but with contrasting mortality outcomes, confirms that abundance and hazard are not equivalent. This Perspective proposes that operational monitoring should progress toward a tiered effect-based framework centred on waterborne gill-toxic potency, defined as the measurable capacity of a water mass to damage fish-gill tissue under environmentally relevant exposure. The framework combines depth-resolved sampling, standardised gill-cell assays applied to paired water fractions, and effect-directed analysis coupled with non-target high-resolution mass spectrometry for chemical discovery. Its wider contribution is to formally connect fish-killing HAB science with the ecotoxicology of emerging contaminants, closing an epistemic delay recognised institutionally by the IOC-UNESCO Task Team on Fish-Killing Microalgae. Substantial validation is required before the framework can support regulatory decisions, and the priorities for that work are set out here.