By Janine Baker, Marine Ecologist / Educator / Citizen Science Project Manager
Over the past 7 months, South Australia has experienced the largest recorded mass mortality of leafy seadragons (Phycodurus eques) and weedy seadragons (Phyllopteryx taeniolatus). Thousands of these iconic fishes have died during an extended harmful algal bloom (HAB), dominated by the dinoflagellate Karenia mikimotoi, along with smaller concentrations of related Karenia and Karlodinium species, and other harmful algae. The combination of major nutrient sources, warmer than average sea and air temperatures and very calm, clear conditions provided ideal conditions for the harmful algae to start blooming last summer. Since that time, the HAB has spread over 4,500km and become self-perpetuating.
Although seadragon specimens from South Australia have not yet been tested for toxins from Karenia or related species, the wash-ups coincide over space and time with the confirmed HAB locations. This pattern has been consistent over seven months, supported by both satellite imagery of chlorophyll-a density, and water sampling across multiple sites.
In locations where seadragons have died, a spatially and temporally variable suite of harmful microalgae have been detected in the water, particularly Karenia species. Seadragons and pufferfishes were some of the first fish species found dead and washed up in the impacted areas. To date, mass deaths of around 190 bony fish species have been recorded.
“Slow-moving, site-attached, longer-lived fishes are most at risk of extirpation,” said Janine Baker, Marine Ecologist, Manager of Dragon Search South Australia.
Why seadragons are among the hardest Hit
The syngnathids (the fish family that includes seahorses, pipefish and seadragons) have small gill pores that can easily be clogged by harmful microalgae. Unlike fast-swimming pelagic fishes, our iconic fishes, unfortunately, cannot escape dense algal blooms. The seadragons’ light, flat and thin body also means that they easily wash ashore when dead.
Thousands of dead pipefishes, particularly the seagrass-dwelling species Brushtail Pipefish (Leptoichthys fistularius), and hundreds of Short-headed Seahorses (Hippocampus breviceps) have also been documented in bloom-affected regions.
The effect of algal toxins
Blooms dominated by Karenia species can harm fishes in several ways. Recent research shows that the various species of Karenia and related dinoflagellates in the South Australian HAB mix are known to produce cytotoxins (including haemotoxins), and neurotoxins. Direct contact between algal cells and fish fills can potentially:
Damage and break down gill tissue
Cause immune reactions such as excessive mucus production, leading to suffocation.
The gill damage may allow salt water to enter the bloodstream (Emer. Prof. I. Gibbins, pers. comm. August 2025)
Disrupt oxygen pathways
Expose the fish to neurotoxins produced by the HAB species
There may be interactions between the toxins in the various species in the bloom mix, and potential for novel harmful actions not previously considered. Finding the exact causes and actions for each HAB species requires specialist necropsy and molecular-level toxicological research that is not yet available in South Australia, and commercially significant species have been prioritised for current testing at an overseas facility.
Lost generations
Most, but not all, of the seadragon deaths occurred during South Australia’s autumn and winter, just before their breeding season in spring and summer. As a result, none of the thousands of adult seadragons that have died in South Australia’s HAB bloom will reproduce this year, or ever again. This not only removes breeding adults, but also wipes out the next generation of seadragons.
In heavily bloom-impacted regions such as western and south-western Gulf St Vincent, southern Yorke Peninsula, northern Kangaroo Island, and the Encounter Bay coast, thousands of young seadragons that would otherwise be born in late 2025 and early 2026 will now be lost,
The Importance of citizen scientists
Seadragons are not evenly distributed across South Australia’s coast. Both the leafy and weedy seagdraon live in several habitats including seagrass beds (Amphibolis and Posidonia seagrass), seaweed forests (Ecklonia and Sargassum), even built structures like jetties. Not only have habitats been damaged by the HAB, populations of small crustacean food sources for seadragons have likely also been impacted.
Because seadragons live across such varied habitats, it can be difficult to capture the full scale of their losses, and this is where citizen scientists play a crucial role. In early April 2025, a bloom-related mortalities project was set up on the citizen science portal iNaturalist.org. Since then, thousands of dead weedy seadragons have been logged in almost 400 records (some records representing dozens of animals), and hundreds of dead leafy seadragons (logged in 63 records), have been submitted from around bloom-impacted parts of South Australia. “These numbers under-represent total deaths, as many carcasses in remote or inaccessible areas would have gone unrecorded,” added Baker.
More than 130 people have reported and uploaded seadragon images to iNaturalist, from around Gulf St Vincent, southern Yorke Peninsula, Encounter Bay coast, and northern Kangaroo Island.
A milestone for seadragon research
In the early stages of the bloom, some people were also hesitant to report wash-ups, since seadragons are protected under legislation in South Australia, and possession without a permit is prohibited. This barrier was overcome in May 2025, when the government agency PIRSA granted a one-year Ministerial Exemption to IUCN SSC Seahorse, Pipefish & Seadragon Specialist Group representative Professor David Booth and his agents. At the same time, PIRSA also issued a joint protected species possession permit for Professor Booth, who’s also a fish ecology lab staff at University of Technlogy Sydney, and Janine Baker, Dragon Search South Australia’s citizen science project coordinator.
This allowed for specimens being collected and shipped by agents around South Australia to UTS for ageing (via otoliths) and for inclusion in the national seadragon mortalities database.
"The citizen science recordings and collection efforts coordinated by Janine Baker has been critical in understanding the impacts of the algal bloom on seadragons,” said Prof. Booth.
The situation now, and what the future might look like
Although the seadragon beach wash-ups have eased in some areas, such as Encounter Bay and southern Yorke Peninsula, mortalities are still being reported where the HAB remains active or is expanding. As of September 2025, dead seadragons were still being recorded periodically on beaches along the metropolitan eastern side of Gulf St Vincent and upper Fleurieu Peninsula.
Tonnes of seagrass and large seaweeds have washed up dead over the past 7 months in bloom affected locations. Large scale benthic habitat restoration will be needed across bloom affected locations in SA. That includes re-establishing critical seagrass and canopy seaweed habitat for seadragons and other species. It will be a long and costly process.
However, some hope for the future comes from recent sightings of several live seadragons at two locations on the Fleurieiu. Although one of the main locations for monitoring (Rapid Bay jetty) is currently closed due to safety concerns, the SA dive community has been discussing with government the need for research access. It is hoped that the dozens of seadragons that Dragon Search SA divers have identified, individually named and monitored in recent years at Rapid Bay and other locations, have survived the bloom. Time will tell.
There has been State, national and international discussion of aquarium breeding to help restock seadragons populations in areas of SA where they have been severely depleted.
As the bloom subsides, and available data from around the central South Australian coast are further collated, a more detailed picture of population and habitat impacts will become available, and next steps can be decided.