Mysterious mass sea hare beaching theory released

In February 2021, thousands of dead slug-like sea hares washed up on the beaches of the Hardy Inlet in Augusta causing local community concern.

Sea hares are molluscs, which are dark-coloured and grow to up to 60cm. They are toxic and while they do not pose a threat to human health, they are dangerous to dogs if consumed. 

Scientist Dr Joanna Browne from the Department of Water and Environmental Regulation was contacted to investigate the cause of the event. She has recently co-authored a scientific paper detailing what could have caused the deaths of the creatures. 

Working with Marine Mollusc specialist, Adjunct Professor Fred Wells, Dr Browne used Healthy Estuaries WA fortnightly monitoring results to investigate what could have caused the thousands of molluscs to collect on the beaches. 

“Our monitoring found that water quality conditions were fairly standard for that time of year,” Dr Browne said.  

Dead sea hares near Tuner Caravan Park.
Photo credit: John McKinney, Augusta-Margaret River Shire

“There were no unusual or excessive algae; nutrients were consistent with what we usually sample; and oxygen levels were within a healthy range. 

“What we did notice when looking at the wider weather conditions was that there had been several weeks of strong easterly winds blowing, which were unusually persistent. 

The conclusion was the strong winds caused currents that concentrated the hares and pushed them onto the beaches close to the mouth of the inlet.

The beaching was likely a natural phenomenon and caused by large-scale climatic patterns, Dr Browne said. 

Scientific paper:

Mass mortality of the black sea hare Aplysia gigantea (Gastropoda: Aplysiidae) at Augusta, Western Australia in the austral summer of 2021: Molluscan Research: Vol 41, No 3 (