Gymnodinium catenatum blooms in the northern Gulf of California: From massive mortalities of seabirds and marine mammals to socioeconomic impacts.

Harmful algal blooms (HABs) of the dinoflagellate Gymnodinium catenatum, a producer of paralytic shellfish toxins (PSTs), are a recurrent environmental and socioeconomic problem in the northern Gulf of California (NGC). We describe an extensive HAB that occurred in early 2015 that caused the dead of 1017 seabirds and 249 marine mammals. Also, we analyze bloom recurrence between 2015 and 2024 to identify potential environmental drivers for the development of these phenomena. From January to March 2015, widespread mortality of seabirds and marine mammals was related to an extensive HAB of G. catenatum with maximum cell abundance of 266 × 103 cells l-1 and a maximum toxin concentration of 36 μg PSTs l-1 in particulate matter. Affected organisms exhibited signs of acute PSTs intoxication and these toxins were detected in different organs, gut contents, urine, and feces. Histopathological analyses revealed lesions consistent with systemic hypoxia and neurological effects of PST intoxication, suggesting respiratory paralysis as the primary cause of death. HABs of G. catenatum in the NGC have a clear temporal pattern of occurrence. From 2015 to 2024, G. catenatum proliferated from November/December to March/April every year with high abundances in January. PSTs in geoduck clams were detected in January of all years, except in 2016. Long-term observations indicate that blooms are driven by regional oceanographic processes. Recurrent HABs have generated important socioeconomic impacts through prolonged fishery closures causing job affectations, product losses and direct economic losses to local communities, highlighting the need for integrated monitoring and ecosystem-based management in the NGC.