Climate-Driven Harmful Algal Blooms and Aquaculture: Early Warning, Fish Mortality, Exposure Mechanisms and Farm Adaptation
Sangeeta Hansda *
Department of Zoology, Ranchi University, Ranchi, Jharkhand- 834008, India.
*Author to whom correspondence should be addressed.
Abstract
Harmful algal blooms (HABs) are a recurrent hazard to marine and freshwater aquaculture, yet the phrase ‘climate-driven HAB’ can obscure the diversity of taxa, mechanisms and environmental pathways involved. This critical narrative review evaluates how climate variability and long-term change alter bloom opportunity, how harmful algae translate into fish mortality, how farms are exposed, and whether monitoring and adaptation measures provide actionable protection. Peer-reviewed literature published from 1 January 2000 to 4 July 2026 was considered, with older material retained only where conceptually necessary. Evidence was synthesised across climate–bloom attribution, ichthyotoxic mechanisms, operational early warning, aquaculture risk and mitigation. The strongest evidence indicates that warming can expand growth windows or geographic niches for some harmful taxa, while hydroclimatic extremes, stratification, altered flushing and nutrient delivery can trigger severe regional events. These relationships are not universal: corrected global event datasets do not support a uniform worldwide increase, and cold-adapted fish-killing taxa illustrate why species- and region-specific attribution is essential. Fish mortality is likewise mechanistically heterogeneous. Gill injury, direct contact toxicity, reactive oxygen species, lipid-derived compounds, pore-forming toxins and oxygen stress may act alone or in combination, with strong strain and host dependence. Early-warning capacity is improving through integrated microscopy, molecular assays, remote sensing, automated imaging and hydrodynamic forecasting, but technological detection is not equivalent to decision-useful warning unless thresholds, lead time and farm actions are locally validated. Evidence for farm adaptation is weaker than evidence for hazard detection: strategic siting, exposure avoidance and contingency planning are defensible, whereas reactive interventions such as clay treatment, pumping or other physical controls remain context dependent and incompletely evaluated at commercial scale. Freshwater cyanobacterial risks add chronic toxin exposure and eutrophication feedbacks to the acute fish-kill paradigm. The principal research need is therefore not a single universal HAB predictor or remedy, but validated, species-specific risk systems linking climate-sensitive hazard, exposure, fish vulnerability and feasible farm response.
Keywords: Aquaculture biosecurity, climate variability, fish-killing algae, harmful algal bloom forecasting, ichthyotoxicity, mariculture, cyanobacterial blooms, risk adaptation