Abstract
 

Harmful algal blooms (HABs) occur when colonies of algae overpopulate. HABs can produce toxins, causing adverse effects for humans, fish, shellfish, marine mammals and birds. The decomposition of algae that follows a HAB event can also trigger a depletion of dissolved oxygen (DO) in the water. In the San Francisco Bay Area, a HAB event caused by the algae, Heterosigma akashiwo, lasted from early to late August of 2022 and caused a massive fish kill of an extent that has not been observed before in this system. As a result of this event, numerous white sturgeon (Acipenser transmontanus) and southern distinct population segment (sDPS) green sturgeon (Acipenser medirostris) were killed. White and green sturgeon carcass data were aggregated from citizen science sources, such as iNaturalist, the San Francisco Estuary Institute (SFEI), as well as from Cramer Fish Sciences and CDFW’s open water and shoreline surveys. To control for potential misidentification and multiple reports of the same carcass, records and associated images were manually analyzed. From these data, a total of 864 sturgeon carcasses, including at least 195 white sturgeon and 17 green sturgeon, were identified. These numbers represent minimums, since sturgeon carcasses may sink before they wash up on shore, and large sections of shoreline were not surveyed. Given the potential that many more sturgeon likely died in the 2022 HAB than were recorded, this event may represent a population scale impact for both white sturgeon (a state species of special concern) and sDPS green sturgeon (ESA Threatened). These events may also increase in frequency and severity due to climate change and water management, as associated changes in water temperature and flow create conditions suitable for HABs.