Gwyn Morgan: Stopping the fowling of lakes

Black swans and Canada geese can pose a problem to our lakes and waterways. Photo / File

The numbers of Canada geese and black swans are increasing and Federated Farmers is concerned about the impact these non-farming sources of nutrient are having on our lakes and waterways.

Canada geese numbers have reached an estimated 60,000, with black swans estimated at 50,000.

This is despite Wildlife Act amendments to control Canada geese, which effectively meant they are no longer protected and can be hunted.

Black swans are partially protected and are hunted in seasons according to regional limits. In Tauranga Harbour, 300-1000 black swans are culled every year to reduce the impact on sea grass beds.

Still, numbers continue to rise rapidly as neither the Canada geese nor the black swan have natural predators. Canada geese lay up to nine eggs and black swans up to 14 eggs every year.

Twenty years ago, Lake Rotorua was receiving 107 tonnes of bird droppings (guano) per year from waterfowl. This is the equivalent of 10 large tip trucks of bird droppings being tipped in Lake Rotorua, with three truck loads from black swans alone.

That is five tonnes of nitrogen and 1.8 tonnes of phosphorus going into Lake Rotorua from waterfowl. There were about 1483 black swans that made up 30 per cent of the waterfowl biomass then.

At present, the actual numbers of black swans and Canada geese on Lake Rotorua is unknown but perhaps it's time for closer monitoring as history shows numbers can multiply quickly.

If we do not acknowledge invasive species sufficiently and identify the problem, we could end up with a situation like we have in the Waikato. There, koi carp are having a devastating effect on lakes such as Lake Waikare. There are an estimated 466,000 adult koi carp with a combined biomass of 851 tonnes.

To avoid a similar situation in Lake Rotorua, the Rotorua District Plan and Bay of Plenty Regional Pest Management Strategy must acknowledge the impacts so they can monitor numbers and control invasive species such as large exotic waterfowl.

Gwyn Morgan - Federated Farmers