Abundance and distribution of waterbirds of the Rotorua lakes, 1985-1996 (Part 1 of 3)

The abundance of nineteen waterbird species was counted during traverses of the entire shorelines of 17 Rotorua lakes in the summers of 1985, 1991 and 1996, by members of the Ornithological Society of New Zealand, assisted by the Department of Conservation and the Eastern Region Fish and Game Council. Also, the locations of all individuals of NZ dabchick were mapped. The overall waterbird community changed little over this decade, both in terms of total numbers of all species combined and species composition. However, 10 of the 19 counted species showed total abundance changes in either direction of Few of these changes are defensibly explicable in retrospect, other than by weak correlations and educated guesses. Canada geese expanded their range from two to five lakes and increased their numbers 20-fold between 1985 and 1996. Little shags and little black shags declined greatly on Lake Rotorua from 1985 to 1991, and again by 1996, during the time of improving trophic state of Lake Rotorua. Red-billed and black-backed gulls declined after 1985, perhaps due to reduced food supply at the Rotorua rubbish dump. Widespread species such as grey/mallard, dabchick, black swan, paradise shelduck and scaup tend to be solitary nesters, shallow water or lake edge feeders, and tolerant of humans. Other species are not widespread for various reasons: Caspian tern are rare; Canada geese and coot are loyal to particular breeding sites; shoveler and grey teal are intolerant of humans; red-billed and black-billed gulls feed and roost in flocks and breed at only one site. Dabchicks were generally confined to shallow (<5 m) water on sheltered shorelines. We recommend that the counts should continue in their present format, with the objective of determining the status of waterbirds on all Rotorua lakes in the medium-long (10-20+ years) terms. We discuss whole-lake and site threats to waterbirds, and suggest priority research and management actions.