Mass fish die-off during a diatom bloom in the Bahía de La Paz, Gulf of California

An unusual fish die-off occurred during a bloom of diatoms from June 10th -July 3rd 2006 in Bahía de La Paz in the Gulf of California. The prevalent organisms were Pseudo-nitzschia spp. (2.4 × 10⁶ cells L^-1), Thalassiosira eccentrica (2.3 × 10⁶ cells L^-1) and Chaetoceros spp. (9.6⁵ × 10⁵ cells L^-1). Three toxic species were identified: P. pseudodelicatissima-complex, P. fraudulenta, and P. pungens. Fucoxanthin was the dominant pigment during the bloom, peaking at 9.3 μg L^-1. Sea surface temperature dramatically increased from 19.0 °C to 27.0 °C during the bloom, with inorganic nitrogen (1.0 ± 0.6 μM) and Si(OH)₄ (15.5 ± 8.0 μM). Low content of domoic acid measured by HPLC-UV from net samples ranged from 24.0 to 52.0 ng per filter and tissue of the chocolate clam Megapitaria squalida (0.55 μg g^-1) and the white clam Dosinia ponderosa (0.06 μg g^-1). Domoic acid analysis of dead fish tissues was negative. Fish necropsy indicated that death by asphyxia was probably caused by Thalassiosira eccentrica and Chaetoceros spp. Alternate mixing processes and instability of the water column, as well as sudden changes of both temperature and nutrients created conditions for proliferation of the diatoms.