HALIFAX - As Canada's fisheries minister contemplates whether to approve a five-year cull of 140,000 grey seals on the East Coast, his own department has released a study that concludes there's little evidence to show such slaughters actually work.

Keith Ashfield is under pressure from the fishing industry to do something about the stalled recovery of overfished cod stocks, particularly in the southern Gulf of St. Lawrence, where there's indirect scientific evidence suggesting hungry grey seals are to blame.

The cod in the area are on the verge of disappearing even though large-scale commercial cod fishing has been banned there since the early 1990s.

Last month, a federal advisory panel consisting of industry representatives and scientists recommended a cull that would eliminate 70 per cent of the grey seals that feed in the area.

Since then, biologists and animal welfare and marine conservation groups have come forward to condemn the proposal, saying it is being driven by politics, not science.

Proponents counter that the cull is supported by peer-reviewed research — compiled by scientists at a workshop last fall — and would be subject to strict controls and monitoring.

However, the Canadian Science Advisory Secretariat, a branch of the Fisheries Department, released a review of scientific literature earlier this year that concludes there has been very little study of marine and land-based culls around the world.

"Many consider it obvious that removing predators should increase prey populations, but predator-prey interactions are far too complex to assume this," says the review, written by Fisheries Department researcher Don Bowen and Damian Lidgard of Dalhousie University in Halifax.

"Despite the widespread use of culling to manage carnivore populations with respect to food production, there is rather limited scientific evidence that such management is generally effective."

Still, the practice has been used across Canada for almost a century.

In British Columbia, an average of 2,900 harbour seals were killed each year for bounty between 1914 and 1963. Bounties for harbour seals were also offered in Nova Scotia between 1927 and 1976, and grey seals were included between 1967 and 1983, the study says.

As well, a cull of grey seal pups in the Gulf of St. Lawrence was sanctioned from 1978 until 1990.

"In each case, there appears to have been no analysis of the benefit of these long-standing culls," the study says.

The world's largest seal cull occurs in Namibia, where about 80,000 Cape fur seal pups are killed annually, ostensibly to protect fish stocks. Again, there appears to be no published scientific analysis of the cull, the authors say.

"Science evidence needed to justify a cull is usually highly uncertain and indirect," the study says.

"This is mostly because of the difficulty in obtaining direct evidence for the negative effects of marine mammal predation on prey populations because predation can rarely be observed and is inferred from estimates of diet."

The study also said that culls sometimes lead to unintended consequences for other species.

Repeated requests for an interview with Ashfield were declined.

Jeff Hutchings, a biology professor at Dalhousie University in Halifax, said he was part of the workshop that concluded grey seals may be hindering the recovery of groundfish in the southern Gulf.

"There was some pretty careful analysis," he said in an interview. "It is a scientifically defensible position to say that ... the elevated levels of natural mortality may well be caused by predation by grey seals."

However, he said more research needs to be done to justify a cull.

The problem, he said, is that a crucial piece of evidence — an analysis of grey seal stomach contents taken off the north coast of Cape Breton — has yet to be repeated.

"It's one year of data," Hutchings said. "Any scientist would tell you that you would want to repeat that sampling to see whether it holds up in other years."

But Hutchings said it would still be next to impossible to draw conclusions from a cull in a complex, multi-species ecosystem like the southern Gulf.

"Even if you have all of these (monitoring) criteria in place, the suggestion that we will know precisely what the consequences of the removal will be is simply not a statement that would receive strong scientific support," he said.

Gerard Chidley, chairman of the advisory panel that recommended the cull, said the proposed slaughter would be different from previous culls because it is supported by scientific evidence and it would be rigorously monitored.

"This is a scientifically monitored removal to test the hypothesis that the grey seals are having an impact in the southern Gulf of St. Lawrence," said Chidley, chairman of the Fisheries Resource Conservation Council.

"When this removal is done in a controlled fashion, scientifically done and monitored, the information from that should give you enough evidence one way or the other. ... We have to prove it or disprove it."

Chidley, a fisherman for 38 years who still operates a boat out of Renews, N.L., said the proposed cull is a small one, considering the size of the grey seal population off the East Coast is at least 330,000.

He said unless something is done to control the population, the seals will eventually have a large impact on the region's lucrative lobster fishery, adding that previous culls and bounties should have been extended.

"We would not have the problem that we're talking about today."