Council water production manager Gerard McCombie said the council had noticed seagulls were thriving at Mt Grand reservoir in Dunedin.
''The big black-backed gulls have congregated at the reservoir on several occasions and three weeks ago, there were about 500 of them.''
He had never seen so many seagulls at any council reservoir. The birds were a nuisance and fouled the black polythene on the sides of the reservoir, but finding a solution was proving difficult, he said.
In the past, they had put lines across reservoirs to try to stop birds landing and had used bird-scaring guns, but both methods were ineffective, he said.
And loud blasts from a bird scarer would be annoying to neighbours, so other options were being explored, he said.
''... we would like to try and keep the raw water as clean as possible before we treat it, but we do have a very good treatment plant, which can cope with the additional faecal loading.''
A trained falcon could be used to scare the seagulls, he said.
''It's something we will be having a look at. We just don't have a solution for the problem at the moment, so we are relying on the robustness of our water treatment plant.''
