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 origin is lost in the mists of time is nowadays more frequently invoked as a metaphor for socio-political, scientific, military or corporate incompetence from the top administrative level (Fig. 1). The saying also applies to coastal zone resource management from a socio-economic perspective! But from a purely biological view this proverb is likely untrue; uncleaned healthy fish tend to decompose first from the internal organs, i.e. digestive tract (author: pers. observations of guts as a former professional quality grader for wild-caught Pacific salmon in British Columbia). But what about wild or aquaculture fish killed by exposure to ichthyotoxic blooms, where the gills are often the proximal target organ by putative toxins, allelochemicals or hydromechanical damage? To our knowledge, there has been no concerted scientific effort to examine the fish corpus delecti to determine the time course of tissue decomposition after exposure to fish-killing algae. (A personal anecdote: the author consumed grilled wild Pacific salmon Fig. 1. Acknowledgement to the independent Scottish connoisseurs at Whiskey Moods for inspiration. freshly killed by a bloom of Heterosigma akashiwo. No known measurable biotoxins were found in the flesh. The human test subject survived with no ill effects, to tell the tale....). The question often arises as to whether freshly killed fish by non-toxigenic algal blooms, or where known biotoxins in the flesh are below detection limits, are safe to eat. Human ethical concerns preclude such consumption trials and therefore the precautionary principle prevails. The fishers and aquaculture fish producers (and their sceptical insurers) consider such Agatha Christie detective inquiries as mere sophistry fish killed by algal blooms cannot (legally) be marketed or consumed and hence lose all value. But now back to the fish-killing algal blooms. There is a long and winding trajectory of studies leading from species composition and dynamics of fish-killing algal blooms to ichthyotoxic mechanisms causing mass fish mortalities and other ecosystem effects. Early impetus was provided by an International Workshop on Fish-killing Marine Algae (Oslo, Norway, 2011) hosted by the Norwegian Veterinary Institute as part of the pilot project Algefisk financed by the Research Council of Norway, and cosponsored by the IOC-UNESCO Harmful Algal Bloom Programme (briefly noted in HAN 45 [1]). The technical workshop convened researchers on ichthyotoxic algae, fish pathologists, algal toxin chemists and aquaculture industry representatives to share experiences, ideas and approaches to addressing ichthyotoxic algal events. Frankly, we were inspired by describing the challenges but confronted with a massive deficit in knowledge and understanding about such events at the time. International attention on fish-killing microalgal blooms was next highlighted at the ICES-IOC WGHABD meeting in Oban (2012), and again in Belfast (2013) with establishment of a specific Term of Reference (ToR) on fish-killing algal Content Featured article Long and Winding Sea-lanes for Fish-Killing Algal Events Allan Cembella........................................... Blooms of ichthyotoxic species and Ciguatera Unprecedented Karenia cristata bloom and catastrophic mass mortalities, shellfish toxicity and human respiratory problems in South Australia ......................................... Unprecedented Fibrocapsa japonica bloom in Southern Brittany (France) .................................... 1 6 8 First record of Fukuyoa sp. in Campeche, Mexico................................... 10 Trophic interactions and ciguatera risk in a warming ocean........................ 12 FAO-IOC-IAEA New Guidance for HAB and Toxins monitoring......... 13 Ciliate and Dinoflagellate Blooms in Contrasting Coastal Environments Red and green waters and Mesodinium blooms in Southern Brittany (France) .................................... 14 Noctiluca scintillans blooms and Strombidium in the Gulf of Nicoya, Costa Rica.................................................... 17 Forthcoming Events XMAS 2027 in Xiamen, China ............ 19 ICMSS in Exeter, UK, 6-11 September 2026............................ 20 Networking and Coordination DART, a regional effort against toxic diatoms ............................................ 21 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