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The RSPCA has questioned the method and motive for Western Australia’s shark cull, amid reports of another catch-and-kill in the state’s South West region.

After thousands rallied on beaches around the country against the controversial Barnett government policy, Australia’s pre-eminent animal welfare organisation put its opposition on the record on Monday.

In a statement posted on its website, the RSPCA said there was no scientific evidence to support the justification for the killings – to reduce the risk to public safety.

“RSPCA Australia does not believe that shark culling is justified. There is no evidence that the increase in attacks is a result of increasing shark numbers,” the statement said.

“Rather, it is consistent with a changing population and human behaviour, that is, there are greater numbers of people in the water.”

Great white, tiger and bull sharks over 3m are being targeted, with baited drumlines set 1km off beaches in Perth and the South West.

Vehement opposition was voiced after the first large shark was caught by a government contractor on Australia Day, with pictures of the animal being shot in the head and dumped further out to sea prompting furious debate.

The RSPCA voiced its strongest concerns about this method, saying there would few operators with the necessary knowledge to shoot a shark humanely.

“The humaneness of shark culling is … questionable. Drumlines with baited hooks can cause considerable suffering, especially when sharks are caught for extended periods of time,” the RSPCA wrote.

“The shark brain is a relatively small target, thus to kill them with a firearm, the shooter must know the position of the brain in the head.

“Only trained operators with knowledge and experience in killing sharks should be used and it is likely that there are limited numbers of personnel with the relevant experience.”

Reports on social media suggested the South West operator, who is being paid more than $600,000 by the state government to set and monitor the lines until April, had caught a 4.4m shark in Eagle Bay, near Dunsborough.

But the government would only say the operators of the drum lines were in “regular contact” with the Department of the Premier and Cabinet (DPC).

“Data on the deployment is being maintained by the operators … and will be forwarded to the Department of Fisheries who in consultation with DPC will make the information publicly available at a later date.”

The RSPCA also questioned claims of the success of baited hooks and nets in reducing attacks in NSW, Queensland and South Africa.