The death of a dolphin calf in the Swan River on Monday has exposed the impact of fishing gear discarded in and around the river and highlighted a pattern of similar incidents in the South-West.

Officers from the Department of Biodiversity, Conservation and Attractions were conducting a routine patrol on Sunday when they saw an entangled calf in the lower reaches of the Canning River.

The officers attempted to free the calf from the fishing lines, but its mother was protective and did not allow them to capture it.

On Monday the calf was located in Freshwater Bay and a team from DBCA arrived with additional vessels and vets from Perth Zoo to stage a rescue effort.

DBCA’s Swan-Coastal district manager Mark Cugley said the calf’s entanglement was “very severe”.

“There was fishing line wrapped around its pectoral fins and its peduncle, which is just before its tail, and it had really been cutting through,” Mr Cugley said.

“With the vet’s assessment of that entangled calf, unfortunately it had very little prospect of recovering from its injuries. So the decision was made, for the welfare of the animal, to sedate it and euthanise it.”

Dr Delphine Chabanne, a researcher from the Centre for Sustainable Aquatic Ecosystems at Murdoch University’s Harry Butler Institute, said the loss of the calf was a blow to the river’s small dolphin population of 21 dolphins, including six calves.

“It’s always sad, especially because it’s a very small community so they are all very important for the system,” Dr Chabanne said.

“Dolphins are long-lived animals and generally it takes a long time for a new generation to be produced and we do lose too many calves from human activities such as fishing line.”

Dr Chabanne described the Swan River as being polluted “to an extent” and said she had seen the impacts first hand.

“A lot of birds are also getting fishing line entanglement. Some dolphins have been seen playing with plastic bags.”

In 2019, a calf in the Peel-Harvey Estuary died after it suffered severe injuries from fishing line entanglement, in a similar case to Monday’s death.

Barbara Sing from Estuary Guardians, a Mandurah environmental group that catalogues the Peel-Harvey Estuary dolphins and performs dolphin rescues, was “devastated” to hear about another young dolphin’s death.

The dolphin that died in 2019 had been named Luca by the Estuary Guardians, and had endured several rescue attempts by the Estuary Guardians and teams from DBCA.

The fishing line cut so deep into Luca’s pectoral fin that he suffered a bone infection and could not be saved.

Ms Sing said the death of another dolphin in similar circumstances had brought back sad memories.

“We’re devastated because this shouldn’t be happening,” she said.

Dr Chabanne called for more to be done to prevent fishing litter from entering waterways.

“There needs to be more mitigation around fishing lines that are discarded, as well as any other fishing equipment, crab pots as well,” she said.

“There could be more effort into educating people, making sure that they always take any fishing line back home, and if they do find any discarded fishing line, to take it and discard it off properly.”

Mr Cugley said DBCA had implemented over 65 fishing line disposal bins at popular fishing spots around the Swan and Canning Rivers.