ins were added during the IPHAB XVII Intersessional (2025-2026), but the total meagre reported ichthyotoxins score (by March 2025) (zero goniodomins, zero prymnesins, one karlotoxin [sterolysin]) has increased dramatically (by February 2026): seven goniodomins, four prymnesins, one karlotoxin, and more sterolysins (e.g., leadbeaterins) are expected soon. The IOC-FAO IPHAB and subsidiary regional groups (IOC/IOCARIBE-ANCA, IOC/WESTPAC-HAB, IOCAFRICA/HAB, FANSA, ANCA) together with associated international bodies (ICES, PICES and their Working Groups) have played a critical role in knowledge acquisition and scientific development studies on fish-killing algal bloom events for more than two decades. The IOC-SCOR GlobalHAB programme leads the HAB science agenda on this theme. Collectively these organisations have convened, sponsored or co-hosted literally dozens of symposia, meetings, webinars, discussion groups, training workshops and courses, and Special Sessions at national and international conferences. Major programme presentations and special sessions on Fish Killing Algae and Ichthyotoxins have been featured, at least since the ICHA15 meeting (Korea, 2012) and at each successive international conference until ICHA21 (Punta Arenas, Chile, 2025). Presentations on combining meta-Omics approaches for ichthyotoxic HABs demonstrated for the first time the functional interactions of ichthyotoxic species in plankton communities and the (toxic) mechanisms of collateral damage to fish and other marine fauna (ICHA21 conference highlights from HAN81 [6]). Functional models of toxigenicity of groups of putative ichthyotoxins, such as sterolysins, are now applied to explore their membrane-disruptive effects on fish gills and plankton predators and competitors. In contrast to the quasi-predictable HAB events linked to shellfish toxins in many global coastal regions, mass fish-killing bloom events appear to defy forecasting and predictions based on current knowledge and paradigms of oceanographic and ecological theory. Recent apparently stochastic fish events would include the sudden devastating salmon mortality event in northern Norway (2019) caused by Chrysochromulina leadbeateri after decades of few fish4 Fig. 5. Reported mass fish mortalities caused by a Prymnesium parvum bloom in the Oder River basin between Germany and Poland (BBC News International, 2022). killing events caused by haptophytes of any Chrysochromulina or Pyrmnesium species. But C. leadbeateri blooms recurred in 2025 albeit with lesser fishkilling consequences. A sudden massive ichthyotoxic bloom of Prymesium parvum in the Oder river basin shared by Poland and Germany in 2022 caused millions of freshwater fish deaths (Fig. 5). Since then, annual P. parvum occurrences are endemic to the region but have caused only minor fish mortalities. A large geographical scale and persistent bloom of Karenia cristata along the South Australia coast initiated in 2025 caused mass mortalities of marine life and respiratory irritation in humans (HAN 83 [7]). This species has been found previously off Newfoundland in Atlantic Canada and South Africa but never associated with harmful events. Discovery of brevetoxins in isolates from this extensive bloom were suspected to account for both human and environmental effects, but apparently the ichthyotoxic potential by K. cristata is orders of magnitude higher than can be explained by its brevetoxin content. This underscores the critical importance of defining true ichthyotoxins that may be distinct from known biotoxins. Are fish-killing blooms becoming more toxigenic? Are they shifting their chemical ecological strategies in response to environmental changes? Or are we merely mis-characterising the fishkilling chemical agents in an ecological context? In any case, the fish-killing algae problem will not go away indeed it continues to inspire and require further research because most fundamental questions are unresolved, and solutions for monitoring, mitigation, control are elusive. Meanwhile global demand for safe and secure seafood is increasing the pressure on declining wild fish stocks, and the growing dependency on aquaculture fish. More seafood resources are thereby at risk from targeted submesoscale fish-killing blooms. Primary scientific research has yielded substantial knowledge dividends in the last decade we are beyond the naive questions (who killed the fish? How many? How fast did they die?). But what is the outcome of all this research effort? We now know the structure and mode of action of a handful of true ichthyotoxins, e.g. a few serolysins [89], and even a plausible scenario on how ichthyotoxic blooms may function in ecological interactions in the case of the Chrysochromulina bloom in northern Norway [10]. Conceptual fish-based models HARMFUL ALGAE NEWS NO. 83 / 2026 Harmful Algae News An IOC Newsletter on Toxic Algae and Algal Blooms No. 83 June 2026 https://hab.ioc-unesco.org/ Long and Winding Sea-lanes for Fish-Killing Algal Events An ancient idiom dead fish rot (or stink) from the head down possibly attributable to Turkish or Persian fishers but the orig Fig. 2. Programme for the Advanced International Colloquium and Technical Workshop on fish killing marine algae and their effects. blooms. The WG also decided to revise the classic but outdated Cooperative Research Report [2] on HAB effects on mariculture and marine fisheries published in 1992 for Chilean government, through CORFO and cooperation of CREAN-IFOP (reported in HAN 63 [3]) (Fig. 2). The colloquium convenors invited international experts to Puerto Varas, Chile in 2019 to review disciplinary knowledge on all aspects of fish-killing algae and associated mortality events (Fig. 3). A p ins were added during the IPHAB XVII Intersessional (2025-2026), but the total meagre reported ichthyotoxins score (by March 2025) (zero goniodomins, zero prymnesins, one karlotoxin [sterolysin]) has increased dramatically (by February 2026): seven goniodomins, four prymnesins, one karlotoxin, and m (admittedly controversial) explain how toxigenic blooms may directly kill fish in aquaculture operations [e.g., 11]. Access to comprehensive time-series databases on HAB events (HAEDAT, HAIS/ OBIS) has allowed for interpretation of fish-killing events over decades on a regional geographical basis, e Catastrophic marine mass mortalities, shellfish toxicity and human respiratory problems from a Karenia cristata dinoflagellate bloom in South Australia, 20252026 Fig. 1. Satellite chlorophyll image from March 2024 showing widespread offshore diatom blooms in response to a massive upwelling event du from which the species name cristata is derived), and a longer hypocone with the right lobe slightly longer than the left. Scanning electron microscopy revealed that the crest was formed by a slight elevation of the right side of the apical groove. On the dorsal side, the apical groove extended to o Unprecedented bloom of Fibrocapsa japonica on French coasts Fig. 1. Map of the different bays in southern Brittany monitored as part of the REPHY program. Since 1987, the REPHY (French Phytoplankton and Hydrology Monitoring Network in Coastal Waters), operated by IFREMER, has conducted long-term m Fig. 3. Light microscope images of living Fibrocapsa japonica cells. Scale bars = 20 μm. French waters. REPHY data [9] indicate that the previous maximum abundance occurred in 2013 in the Vilaine estuary, reaching 1.9 x 105 cells L1, approximately 25 times lower than values than maximum concentrati First Record of Fukuyoa sp. (Gambierdiscoideae) in the Northeastern Region of Términos Lagoon, Campeche, Mexico Fig. 1. Map of sampling stations at Términos Lagoon, Campeche, Mexico. The station where Fukuyoa sp. was found is circled in red. Términos Lagoon is a region of substantial economic rele Fig. 2. (AB). Fukuyoa sp. in ventral view. (C). Dorsal view. (D). Antapical view showing plates S. d. p., 1, 1. (EF). Ventral view showing plates 1, 1,2, 7, S. d. p., 1,1. (GH). Antapical view showing plates 1p,2,3, 1, 2. Scale bars = 20 μm. in Australia [6] recorded F. paulensis at temperatures of Trophic interactions shape ciguatera risk in a warming ocean Ciguatera is one of the most widespread marine poisonings worldwide, caused by the consumption of fish that bioaccumulate ciguatoxins (CTXs) produced by dinoflagellate species belonging to the genus Gambierdiscus. Its expansion into non-en drivers such as temperature variability and habitat disturbance. Calibration and validation with empirical data on cell densities, toxin concentrations, and ecosystem dynamics will be essential to generate robust predictions. The inclusion of spatial structure and coupling with human health risk mod Red and green waters in southern B rittany (France) in March 2026 linked to a bloom of Mesodinium spp. Fig. 1. Map of reported water discoloration events in southern Brittany (1012 March 2026) based on PHENOMER observations and direct reports. Locations include the Bay of Audierne and the Bay of C Fig. 3. Sentinel-2 satellite image (11 March 2026) showing the spatial extent of the bloom in southern Finistère, with burgundy-red patches indicative of high Mesodinium surface concentrations. are more stable and impart a green coloration. This transformation can occur within minutes, explaining t Fig. 6. Cells of Mesodinium major. 13. Different views of living cells. 46. Representation of pigment dynamics during Mesodinium cell degradation: transition from phycoerythrin-dominated red coloration to chlorophyll-dominated green coloration following cell lysis. All images to scale. accompanied Who turned on the light? First report of extensive bioluminescent blooms of the heterotrophic dinoflagellate Noctiluca scintillans with low abundance of bioluminescent bacteria in the Gulf of Nicoya, Costa Rica Historically, the Gulf of Nicoya has experienced recurrent algal blooms, including events Fig. 2. Dominant dinoflagellate of the Noctiluca scintillans algal bloom around Cedros Island. (A) Bloom of N. scintillans. (B) Ciliate fed on by Noctiluca, Strombidium sp. (C) Culture of bioluminescent bacteria. (D, E) bioluminescence observed at night on Cedros Island and surrounding areas (photog estuaries supporting fisheries, tourism, and biodiversity. Monitoring these processes allows differentiation between benign and harmful blooms and supports the preservation of ecosystem integrity. Ultimately, such phenomena not only expand scientific understanding but also underscore the resilience International Conference on Molluscan Shellfish Safety (ICMSS) Dear colleagues, We wanted to draw your attention to the upcoming International Conference on Molluscan Shellfish Safety (ICMSS), taking place 611 September 2026 at the University of Exeter, UK. Further details, including registration (w DART in action: Scientists launch regional effort against toxic diatoms Scientists from across Asia have come together to tackle the growing threat of toxic diatoms that produce neurotoxin Domoic Acid (DA). On 1718 March 2026, researchers from China, Malaysia, and Singapore gathered in Qingdao, Chin Fig. 2. Kick-off meeting and research presentations. (A) Nancheng Chen, the lead PI, delivers the opening address. (BD) PIs from participating countries presenting their national research plans. (EF) Selected presentations delivered during the workshop. molecular tools can greatly improve our abilit IOC-FAO Intergovernmental Panel on Harmful Algal Blooms (IPHAB) Extraordinary online Session 27 October 2026 and 18th Session (IPHAB-XVIII), 1820 March 2027, FAO, Rome The Intergovernmental Panel on Harmful Algal Blooms (IPHAB) was established in 1992 to strengthen the scientific, managerial, and fi