Abundance and distribution of waterbirds of the Rotorua lakes, 1985-1996 John Innes Landcare Research Private Bag 3127 Hamilton New Zealand Kathryn Whaley Landcare Research C/- School of Marine and Environmental Sciences Tamaki Campus The University of Auckland Private Bag 92019 Auckland New Zealand Keith Owen Department of Conservation PO Box 1146 Rotorua New Zealand Published by Department of Conservation Head Office, PO Box 10-420 Wellington, New Zealand This report was commissioned by Bay of Plenty Conservancy ISSN 1171-9834 1999 Department of Conservation, P.O. Box 10-420, Wellington, New Zealand Reference to material in this report should be cited thus: Innes, J., Whaley, K. and Owen, K., 1999 Abundance and distribution of waterbirds of the Rotorua lakes, 1985-1996. Conservation Advisory Science Notes No. 236, Department of Conservation, Wellington. Keywords: Rotorua lakes, waterbirds, survey, threats, bird management, Ornithological Society of NZ Summary 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 Coun- cil. 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 direc- tion of 50% of the previous count on at least one of the count occasions. 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. Wide- spread 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. 1. Introduction Members of the Ornithological Society of New Zealand (OSNZ) assisted by the Department of Conservation (DOC), and the Eastern Region Fish and Game Council (ERFGC) counted all waterbirds on 17 Rotorua lakes during January to April in 1985, and January to February in 1991 and 1996. This report presents the data from this decade of OSNZ et al. counts, and makes first order inter- pretations from them. The results are discussed in relation to other counts of some game species conducted by ERFGC during 1991-1998 and of all waterbirds by Graeme Taylor (OSNZ) during 1981-1983. This report fulfils five tasks posed by the Bay of Plenty Conservancy of DOC: 1. Determine the abundance trends of the waterbird populations on the Rotorua lakes over the 1985-1995 decade, and the reasons for any ap- parent variations. 1 2. Determine the distribution patterns of waterbird species using the lakes over this period. 3. Map the distribution patterns of NZ dabchick on the lakes. 4. Indicate likely risks to waterbird populations. 5. Provide recommendations on the research and management needs of these species, taking account of current threats and perturbations. The physical attributes of the lakes and their catchments, and details of water quality, are summarised by Livingston et al. (1986) and Donald (1997). 2. Methods 2.1 OSNZ ET AL. COUNTS The entire water areas and shorelines of (in order of size) Lakes Rotorua, Tarawera, Rotoiti, Rotoma, Okataina, Rotomahana, Rotoehu, Rerewhakaaitu, Rotokakahi, Okareka, Tikitapu, Rotokawau, Okaro, Ngapouri, Ngahewa, Rotokawa,Tutaeinanga and Opal were traversed by boat or viewed from the shore to count waterbirds in the summers of 1985, 1991 and 1996 (Appendix 1). Large lakes were surveyed from motor-boats travelling 50-100 m from shore at speeds slow enough to identify and count all species present. Five boats were used simultaneously on L. Rotorua, and two on Lakes Rotoiti and Tarawera. Some smaller lakes were surveyed by kayak or from the shore. Counts of individual lakes were always completed within one day, to mini- mise errors from birds moving between count zones and thus being counted twice. Lagoons adjacent to lakes (e.g., at L. Rotoma) were regarded as part of the lake, and counted accordingly. Hamurana Springs waterfowl were added to the L. Rotorua count total. Species counted were dabchick, black shag, little black shag, little shag, white- faced heron, black swan, Canada goose, domestic goose, paradise shelduck, grey/mallard, grey teal, shoveler, scaup, coot, pied stilt, black-backed gull, red- billed gull, black-billed gull and Caspian tern (Appendix 2). Some of these species (e.g., dabchick, scaup, coot) are nearly entirely aquatic but may roost on or among emergent vegetation.The three shag species roost and nest in riparian trees but feed entirely in water. Some other species (e.g., white-faced heron, black swan, paradise shelduck, pied stilt, gulls) may forage on land as well as water; individuals of these species which were on pasture, trees, jet- ties or other structures adjacent to the lakeshores were included in the counts. Conspicuous juveniles of those non-colonial species breeding at the time of the surveys (dabchick, black swan, domestic goose, scaup, coot) were counted separately. 2 2.2 ERFGC COUNTS Entire lake populations of black swan, paradise shelduck and Canada goose were counted by ERFGC staff annually in 1991-1998 on 11 major lakes (Ap- pendix 3). Counts were made from an aeroplane; especially dense aggregations of birds (e.g., a paradise shelduck moulting site) were photographed and counted later from the photograph (M. McDougall, pers. comm.). Coinciden- tally and fortunately, counts were made in late January (ca. 28 January, the same time as the OSNZ counts); this originated as the approximate centre of the moult period for paradise shelduck. 2.3 GRAEME TAYLOR'S COUNTS Graeme Taylor (OSNZ) counted dabchicks on many lakes, and all species on some lakes, during December 1981 to June 1983 (Appendix 4) with the same methods described in 2.1. 3. Results 3.1 ABUNDANCE TRENDS OF WATERBIRD POPULATIONS I N OSNZ COUNTS, 1985-1996, AND REASONS FOR CHANGES The total number of counted birds was 26 331 in 1985, 21 973 in 1991 and 22 992 in 1996 (Appendix 1), and no breeding species noted in 1985 was absent from the later counts (Figs 1-3). This suggests that the waterbird com- munity composition was stable over the count period. Total counts of seven of the species (dabchick, little shag, white-faced heron, black swan, domestic goose, scaup and coot) never varied by more than 50% from one count to the next. However, 10 of the 19 counted species (black shag, little black shag, Canada goose, paradise shelduck, grey/mallard, grey teal, shoveler, pied stilt, black-backed gull, red-billed gull) showed total abundance changes in either direction of 50% of the previous count on at least one of the count occa- sions (Appendix 1; Figs 4a-g). Counts of individual species on all lakes separately are shown in Figs 5a - r. Conspicuous juveniles are excluded from the counts of dabchick, black swan, domestic goose, scaup, and coot. Counts of grey teal, shoveler, pied stilt, black-backed gull and red-billed gull all declined from 1985 to 1991, then increased again by 1996. Grey teal num- bers declined on Lakes Rotorua, Rotoehu, Rotomahana and Rerewhakaaitu after 1985, and bounced back only on Rerewhakaaitu and, to a lesser extent, Rotoehu in 1996 (Figs 4g, 5j). Shoveler (Figs 4g, 5k) declined on Rotorua, Rotoehu, Rerewhakaaitu and Okareka, and increased in 1996 only on L. Rotomahana. Both of these species are highly mobile and disperse widely (Marchant & Higgins, 1990) so that large variations in their numbers at any site are to be 3 expected. Pied stilt (Figs 4d, 5n) declined on Rotorua, Rotoiti, Rotoma, Rotomahana, Rerewhakaaitu and Rotokawa, and increased subsequently on Rotorua, Rotoehu and Rerewhakaaitu. Black-backed gull (Figs 4e, 5o) declined on Lakes Rotorua and Rerewhakaaitu, and 1996 counts increased on Rotorua and Rotomahana. Red-billed gull (Figs 4e, 5p) also declined on Rotorua, and increased again there in 1996, and on L. Rotoiti. These changes for both spe- cies were perhaps due to new management at the Rotorua rubbish dump which reduced their access to food there after 1985. Black shag increased greatly on L. Rotorua in 1991, and there were small changes on other lakes between counts, but the counts were generally low (159-290 total over the decade) and the percentage changes may overstate their importance (Figs 4c, 5b). Little shag declined greatly on L. Rotorua from 1985 to 1991 and had again by 1996 (Figs 4c, 5d). In fact, little shag declined from 1991 to 1996 on every major Rotorua lake except Okareka. Interest- ingly, little black shag numbers (Figs 4c, 5c) dropped greatly also on Lakes Rotorua and Rotoiti from 1991 to 1996, with some small increase in numbers on Rotoehu, Rotomahana and Rerewhakaaitu which may indicate new breed- i ng colonies near those lakes.