Reports of mature salmon washing up at beaches across more than 40 kilometres of the region's coastline have sparked online debate among concerned residents.
The Department of Primary Industries and Regional Development confirmed it had received a report of about 10 to 20 dead salmon at Le Grand Beach and Lucky Bay Beach on June 1.
An online thread in the Esperance Community Page confirmed further sightings at seven other locations, including Dunn Rocks, West Beach, Twilight Beach, Fourth Beach, Wharton Beach, Rossiter Bay and Observatory Beach.
"I saw about 5 at Dunns over the weekend," Jason Meredith said.
"I found one about 60 cm long on Saturday at West Beach - had a wound across its tail but did not look like a reason for its death," another commenter wrote.
Resident Tim Bailye said he had seen a large salmon die at Observatory Beach on Thursday, June 6, after flapping around offshore.
"It was flapping around in a circle on its side about 30 metres offshore for about thirty seconds," Mr Bailye said.
"Twenty minutes later, it washed up on the beach. No injuries."
Former Esperance resident Jannalea Piercey had been visiting Lucky Bay with family when she spotted a kangaroo feeding on a salmon that had washed ashore.
Despite having lived in the region for 21 years, former resident Jannalea Piercey said she hadn't seen anything like it before.
"I did think it was strange, especially considering kangaroos are herbivores," she said.
"I'd certainly be curious to know why, whether it's something they're lacking in their diet or out at Lucky Bay."
At this stage, a cause has not yet been determined.
A Department of Primary Industries spokesperson encouraged the public to report fish kills.
"We urge members of the public to be watchful and report any fish kill they see to our special FishWatch reporting line on 1800 815 507," the spokesperson said.
"The key advice around any fish kill is that you do not touch the affected fish or the water in their vicinity, or collect any fish for samples or use the dead fish as bait.
"It's also important not to consume any fish that have been caught in the area of the fish kill."
The Shire of Esperance also confirmed it had received a report and encouraged the public to report fish kills to FishWatch.
For more information on fish kills, visit http://www.fish.wa.gov.au/Sustainability-and-Environment/Fisheries-Science/Aquatic-Animal-Health/Pages/Fish-Kills.aspx