Publication Date
Fall 2025
Abstract
The ongoing harmful algal bloom in South Australia that began in March 2025, which resulted in over 106 marine animal mortalities across 550 taxa, was recently identified as having brevetoxins, which can harm marine mammals and human beings (Murray et al., 2025). This algal bloom is currently affecting Kangaroo Island, South Australia, a key habitat for the endemic Australian sea lion (Neophoca cinerea), as well as long-nosed fur seals (Arctocephalus forsteri) (Goldsworthy et al., 2021), (Goldsworthy and Shaughnessy, 1994). Because these pinniped species were observed eating algal bloom affected prey, previous research on other pinnipeds suggests that the subject pinnipeds could be physically harmed or killed by the bioaccumulation of brevetoxins (Hall et al., 2024), (Goldstein et al., 2007). After the initial massive prey mortality events in March – May 2025 that drew large numbers of both species, anecdotal evidence indicated the abundance of those pinnipeds in once reliable sites had noticeably declined (Horbelt, 2025). To test this observation and the potential for toxin-induced pinniped mortalities, during November 2025 a 19-day beach community survey was conducted in Stokes Bay to monitor the development of the bloom and potential mortality events, together with 4 days of pinniped boat surveys to 4 historical haul-out sites and 1 novel haul-out site previously surveyed by ecotourism operator Research and Discovery Kangaroo Island (RAD KI).
This study revealed that mortality events during November were lower in frequency and consisted of only a few species, compared to the numerous and diverse mortality events of March – May 2025. When compared to average abundance occurring in previous years as observed by RAD KI, Australian sea lions and long-nosed fur seals first increased in abundance during the March – May 2025 mortality events, specifically with a notable increase in adult female Australian sea lions, suggesting that the abundance of prey drew females away from their historical breeding sites (Osterrieder et al., 2015). After this period of high abundance, there was a stark decline during the low prey mortality event period of October – November 2025 for both species. With the existing knowledge that prey availability is a major driver for haul-out site selection for pinnipeds, the observed boom-bust cycle of 2025 is likely due to the initial ease of catching dead prey during the height of the bloom, and then in contrast, the decrease in prey availability in October – November 2025, leading to an exodus of historical haul-out sites in search of more prey (Osterrieder et al., 2015), (Salgado Kent et al., 2024), (Corrales-Guerrero et al., 2025). Although no pinniped mortalities were observed during the study, the potential for toxin-induced deaths remain. To understand the finer details of how this bloom could affect the pinnipeds and humans of Kangaroo Island, future research must focus on identifying the specific algae species present in Kangaroo Island’s waters, testing toxin bioaccumulation in pinniped tissues, monitoring long term pinniped abundance trends, and identifying the breeding colonies of the north coast’s Australian sea lion communities.
Disciplines
Life Sciences
Recommended Citation
Cassidy, Rose, "Blooming questions: the potential impact of the ongoing harmful algal bloom on pinnipeds on Kangaroo Island, South Australia" (2025). Australia: Rainforest, Reef, and Cultural Ecology. 15.
https://digitalcollections.sit.edu/ase2/15