Debate over effects of swans and geese

Canada geese and black swans are out of control on some Northland lakes with the regional council looking at ways of reducing their numbers. Photo / File

Black swans and Canada geese are out of control in the Far North and options for managing them need to be investigated, Northland Regional Council says.

However, an expert has said the populations in question are stable and the public perception they are damaging the environment is wrong.

Birds converging on harbours, dune and inland lakes in the district have some locals and iwi concerned about the impact on fisheries, human health and farm productivity, the NRC's environmental management committee has heard.

Canada geese were an unprotected species and could be shot freely, but as the two species often co-habitated they needed to be considered together.

A Northland representative for Fish & Game - the organisation authorised to issue permits to cull the protected Black Swan - said shooting the birds could actually cause populations to balloon.

Basic population ecology suggests the birds would increase reproduction if the population was culled to below the environment's "carrying capacity", said Fish & Game Northland field officer Nathan Burkepile.

"It's a tough issue. A lot of the problems are perceived ... There's been no science behind what [NRC] say are the problems. They say there's issues with water quality and I haven't found anything to support that."

He said in some cases problems arose where swans and geese settled in lakes on farmland and then ate grass in competition with livestock.

"The easiest way to deal with that is to fence off dune lakes and waterways. If you fence off and plant a buffer around them, we won't have the issue of swans moving up into the paddocks.

"You can't just go willy nilly shooting native species. We manage them as a renewable resource."

NRC biosecurity senior programme manager Don McKenzie said there were no immediate plans to cull the birds, with further investigation needed.

"There is no real evidence and I think we're saying the same thing," he said. "Our staff view is we're listening to community concerns about these birds and maybe we need to put a bit more of a spotlight on these impacts ... because they haven't been fully investigated."

Communities, including iwi and Te Tai Tokerau Maori Advisory Committee, had expressed concern over faecal contamination of waterways while some farmers had reported a loss of production due to the birds' grazing, Mr McKenzie said.

A 2008 study by Northland Fish & Game showed Canada geese populations had doubled across the North Island over the previous 20 years.

Mr McKenzie is looking at organising a public workshop on the issue.