West Coast Estuaries - Black Swans and Canada Geese

Abstract from report: The exotic Canada goose and the native black swan are present in Waikato estuaries, and there are concerns that bird numbers are increasing. This report reviews available information on the use of estuaries on the west coast of the Waikato region by Canada geese and black swans, including the potential effects of these species on estuarine ecosystems. A systematic literature review was carried out using key search terms and survey data from Fish & Game New Zealand and Birds New Zealand were used to estimate bird populations in these areas. Bird surveys show that Canada goose numbers are increasing in western Waikato lakes, wetlands and estuaries. Black swan numbers have declined in the western Waikato over the past three decades, though use of estuarine habitats may be increasing. Further surveys are required to understand whether black swan numbers are increasing in estuaries, and whether this is associated with declines in lake and wetland habitats. These waterfowl have various potential ecological effects, the most significant of which is damage to seagrass, a key biogenic habitat provider that has been declining over the last 50 years. Both species feed on seagrass, and may damage seagrass beds, though it is not clear whether either species feeds preferentially on seagrass in Waikato estuaries. At high densities, waterfowl can also contribute to reductions in water quality. Waterfowl can contribute to eutrophication of waterbodies through the deposition of faecal matter. The nutrient contribution of swans and geese to Waikato estuaries is as yet undetermined, though it could be calculated based on parameters from relevant literature. Other potential ecological impacts of these birds include changes to other estuarine vegetation, disease transmission, propagule transport and aggression towards other birds. Swans and geese should be managed via an integrated approach focused primarily on reducing the adult population. Given the mobile nature of these species, management would probably need to be nationally coordinated to ensure that vacated habitats are not recolonised.

Literature review prepared for Waikato Regional Council. Looked at swans and geese: - population estimates, trends and distribution - habitat utilisation, diet and feeding - population management

Waikato estuaries on West Coast (Kawhia, Raglan and Aotea)

Data examined from 1980s to present as available.

A systematic approach was used to search the relevant literature regarding geese and swans using Google Scholar and Google. Results included peer-reviewed articles, technical reports, client reports, theses and websites (government, restoration groups, and press). The following databases were also accessed: New Zealand Birds Online, iNaturalist, and Ebird.

BirdNZ data can be variable, possibly because only targets known roost sites and birds seen on the way to and from these roosts. Weather/tide and seasonal differences may affect counts.

Data gathered due to concerns about increasing numbers of Canada geese and swans in western Waikato estuaries, and associated ecological effects.

Data available from online databases, and summarised in report.

Data collection ongoing.