The Barnett Government's shark kill policy is likely to have no effect on the environment, according to previously undisclosed advice from the Fisheries Department.
In revelations that contradict claims about the environmental consequences of the policy, the department said the measure would have only a "negligible" effect on most species.
Under the policy, baited drum lines are being set 1km off popular Perth and South West beaches to catch great white, tiger and bull sharks bigger than 3m.
The policy, devised after seven fatal shark attacks off WA in three years, came into force late last month and will run until April 30.
Rick Fletcher, the department's research director, wrote in his advice to the Government that tiger sharks were the most likely target species to be caught but not in numbers significant enough to affect their population.
Dr Fletcher said even fewer great white sharks were likely to be caught, predicting the number would be less than 10 including undersized sharks.
The Environmental Protection Authority released the advice yesterday after a third party referred the policy to the watchdog for assessment.
"Current research on the population size of the western population of white sharks in Australia . . . suggests that this is in the order of few to several thousand," Dr Fletcher said.
"Consequently, even if the total number of white sharks killed this program up to the end of April is in the order of 10 to 20 then this is still likely to have only a negligible impact on the total stock size of this population of white sharks."
According to Dr Fletcher, there was little to no risk of other sea life such as seals and sea lions, turtles, whales and dolphins being killed by the drum lines.
However, he said there was a potential risk of the policy affecting the dusky shark population, which was recovering and could not afford to lose bigger, breeding stock.
Despite this, Dr Fletcher said the policy posed little environmental risk, though would need to be reassessed if implemented again.
"Given the short time period of this program, the small footprint . . . the relative numbers of individuals that may be captured compared to the total stock sizes of the affected species, this program would not have any measurable effect on broader ecosystem functioning," he said.