ASPECTS OF THE FEEDING ECOLOGY OF MALLARD AND BLACK SWAN IN A SMALL FRESHWATER LAKE Kerry John Potts Submitted for the degree of Doctor of Philosophy in Zoology at the Victoria University of Wellington November 1982 i CONTENTS---------------------------- _ -------- ---------------------------- Page LIST OF FIGURES . v LIST OF TABLES .. .. vii LIST OF PLATES .. viii ABSTRACT .. .. ix GENERAL INTRODUCTION .. .. .. .. X GENERAL DESCRIPTION OF STUDY AREA .• .. .. XV A. Physiography, vegetation, drainage, fertility xv B. Waterfowl .. .. .. .. .. xx C. Climate .. .. .. .. .. SECTION 1: GENERAL LIMNOLOGY AND HABITAT EVALUATION WITH SPECIAL REFERENCE TO (a) THE FACTORS REGULATING THE BALANCE BETWEEN MACROPHYTES AND PHYTOPLANKTON (INCLUDING THE MECHANISMS INVOLVED IN MACROPHYTE DECLINE), AND (b) THE POTENTIAL OF SWAN GRAZING TO STIMULATE CHANGE IN THE DIRECTION OF PHYTOPLANKTON DOMINANCE Ch. 1 INTRODUCTION .. .. .. .. .. 1 C h . 2 METHODS . .. .. .. .. 5 2.1 Bathymetry .. .. .. .. .. 5 2.2 Aerial photography .. .. .. .. 5 2.3 Weather and water levels .. .. .. 5 2.4 Sampling stations and times .. .. 5 2.5 Water chemistry .. .. .. .. 7 2.6 Macrophyte sampling .. .. .. 7 2.6.1 Rake method .. .. .. .. 2.6.2 Trial comparing the efficiencies of the rake and modified Gerking methods to the direct hand cutting method .. 9 (a) Objectives .. .. 9 (b) Methods .. .. 9 (c) Results .. 11 (d) Conclusions .. .. 11 2.7 Zooplankton sampling .. .. 14 2.7.1 Field sampling .. .. 14 2.7.2 Laboratory preparation and counting 14 2.8 Phytoplankton and epiphytic algae sampling 15 2.8.1 Phytoplankton productivity .. 15 2.8.2 Phytoplankton and epiphytic algae identification .. .. 16 2.9 Aquatic invertebrate sampling .. 16 2.9.1 Gastropod - macrophyte relationship 16 2.9.2 General seasonal availability of plant-associated invertebrates 17 VICTORIA U N IVERSITY OF WELLINGTON 11 Page Ch. 3 RESULTS .. .. .. .. .. 18 3.1 Bathymetry .. .. .. .. .. 18 3.2 Water levels, flow, rainfall, air and water temperatures .. .. .. .. 18 3.3 Water chemistry .. .. .. .. 22 3.4 The occurrence of macrophytes .. .. 22 3.4.1 Macrophyte development in 1977/78 .. 24 3.4.2 Macrophyte development in 1978/79 .. 29 3.5 Aquatic invertebrates .. .. .. 35 3.5.1 Gastropod-macrophyte relationship .. 35 3.6 Zooplankton .. .. .. .. 38 3.7 Phytoplankton .. .. .. .. 39 3.8 Spring epiphytic diatom pulse .. .. 39 Che 4 DISCUSSION .. .. .. .. .. 43 Part 1 - General Limnology .. .. .. 43 4.1 Water and soil quality .. .. .. 43 4.1.1 Water chemistry .. .. .. 43 4.1.2 The relationship between specific water chemical parameters, primary and secondary productivity, and waterfowl habitat quality .. .. .. 45 4.1.3 The relationship of water chemistry to soil chemistry and the role of Typha orientalis (raupo) in terms of nutrient recycling, organic debris contribution, and invertebrate production .. .. 47 4.2 Macrophytes 50 4.2.1 Macrophytes as trophic status indicators 50 4.2.2 The balance between P. crispus and P. pectinatus in deep open water .. 50 4.3 The seasonal availability of plant-associated macroinvertebrates .. .. .. .. 51 4.4 Zooplankton community structure .. .. 54 4.5 Phytoplankton community structure .. .. 55 4.6 Spring epiphytic diatom pulse .. .. 55 Part 2 - The balance between macrophytes and phytoplankton and the potential of swan grazing to stimulate changeovers from macrophyte to phytoplankton dominance 57 4.7 The balance between macrophytes and phyto plankton .. .. .. .. .. 57 4.7.1 The mechanisms involved in changeovers from macrophyte to phytoplankton dominance 64 4.7.2 Could storm or heavy swan grazing damage trigger a sudden change to phytoplankton dominance at Pukepuke? .. .. 73 l u Page SECTION 2: THE FEEDING ECOLOGY OF MALLARDS Ch. 5 INTRODUCTION • • • • • • • • • • /y 81 Ch. 6 METHODS .. • • • • • • • • • • 82 Ch. 7 RESULTS .. • • • • • • • • • • 7.1 Winter - spring 1977 # . • • • • 82 7.2 Summer - autumn 1977/78 • • • • • • 84 7.3 Winter - spring 1978 • . • • • • 87 7.4 Summer - autumn 1978/79 • • • • • • 88 99 Ch. 8 DISCUSSION • • • • • • • • • • Part 1 - Seasonal feeding patterns • • • • 99 99 8.1 Winter - spring feeding • . • • • • 8.1.1 Unpaired birds .. •• •• 99 8.1.2 Paired birds .. •• •• 106 8.1.3 Indicators of winter (post-solstice) food adequacy .. .. •• 107 8.1.4 The possibility of energy stress during the hunting season (pre-winter solstice) 112 8.2 Summer - autumn feeding .. .. •• 114 8.2.1 The feeding relationship between mallard and black swan in the summers of 1977/78 and 1978/79 .. .. l21 Part 2 - Breeding and brood production .. .. 124 8.3 The meaning of local production .. .. 124 8.4 The food requirements of ducklings. Is food likely to be limiting to duckling production on Pukepuke Lagoon? .. •• •• 128 8.5 The possible evolutionary significance of close-to-the-water nesting .. .. •• 131 SECTION 3: FACTORS CONTROLLING THE PRESENCE AND DISTRIBUTION OF BLACK SWAN IN THE PUKEPUKE-CENTRED WETLAND COMPLEX Ch. 9 SEASONAL OCCURRENCE AND PRODUCTION IN PUKEPUKE RESERVE .. .. .. .• •• • • 134 9.1 Introduction .. .. •• •• 134 9.2 Methods .. .. .. •• •• 135 9.3 Results .. .. •• •• •• 135 9.4 Discussion .. .. .. •• •• 141 9.4.1 The relationship between Pukepuke Lagoon and outlying waters .. •• •• 141 9.4.2 Territorial breeding .. •• 141 9.4.3 The relationship between flock birds and breeding pairs .. •• •• 142 9.4.4 Production, survival, carrying capacity and timing of breeding .. •• 143 iv Page Ch. 10 AN HYPOTHESIS TO ACCOUNT FOR FLOCK BIRD USE OF VARIOUS WATERS IN THE PUKEPUKE-CENTRED WETLAND COMPLEX 147 10.1 Introduction .. .. .. .. 147 10.2 An experiment to test the hypothesis that flock swans will only leave Pukepuke Lagoon in the spring if they are forced to do so by territorial aggression .. .. .. 14 9 10.2.1 Objective .. 149 10. 2.2 Methods .. 150 10.2.3 Results .. 15 2 10.2.4 Conclusions 154 SUMMARY .. 155 ACKNOWLEDGEMENTS 16 2 REFERENCES 163 APPENDICES 173 Appendix 1 Monthly maximum densities of macrophytes obtained by the 0.1 m 2 rake method n 1977 (g. dry wt.) .. ... .. .. 173 Appendix 2 Monthly maximum densities of macrophytes obtained by the 0.1 m2 rake method in 1978 (g. dry wt.) .. .. .. 174 Appendix 3 Oesophagal contents of mallards shot on Pukepuke Lagoon .. .. .. 175 Appendix 4 Winter grain trapping attempts at Pukepuke , 176 Appendix 5 Mallard winter sex ratios in New Zealand: An examination of the major forces operating to produce these results, including the sex- selective effect of hunting .. .. 178 V LIST OF FIGURES Page 1. Map of Pukepuke/Omanuka catchment .. .. xviii 2.4.1 Location of sampling stations .. .. .. 6 3.1.1 Depth contours of Pukepuke Lagoon as surveyed in January 1976 .. .. .. .. 19 3.2.1 Seasonal pattern of water level change in Pukepuke Lagoon from 1969 to 1979 .. .. .. 20 3.2.2 Monthly rainfall, water level, air temperature and surface water temperature .. .. .. 21 3.4.1 Occurrence and seasonal development of macrophytes sampled in 1977 .. .. .. .. .. 25 3.4.2 Summary of macrophyte development and decline in Pukepuke Lagoon in 1977/78 .. .. .. 26 3.4.3 Occurrence and seasonal development of macrophytes sampled in 1978 .. .. .. .. 31 3.4.4 Summary of macrophyte development and decline in Pukepuke Lagoon in 1978/79 .. .. .. 32 3.5.1 Gastropods per g. dry wt. of macrophyte - predominantly P. crispus - from four sites sampled in 1977 .. .. .. 36 3.6.1 The seasonal pattern of zooplankton occurrence, phytoplankton productivity, and Secchi disc extinction in Pukepuke Lagoon from 1977 to 1979 .. 40 4.7.1 Phytoplankton productivity in relation to water levels from March 1977 to April 1979 .. .. 60 4.7.2 Hypothesis to account for decline in macrophyte populations when lakes are fertilised .. .. 67 7.1.0-7.2.5 Diurnal feeding patterns of mallards as determined by scan counts undertaken at monthly intervals from July 1977 to April 1978, and generally at two-monthly intervals from June 1978 to March 1979 7.1.0 July 20-23, 1977. Counts at two-hourly intervals over four days .. .. .. * .. .. 90 7.1.1 August 22-26, 1977. Counts at hourly intervals over five days .. .. .. .. .. 91 7.1.2 September 19-23, 1977. Counts at hourly intervals over five days .. .. .. 92 7.1.3 October 23-27, 1977. Counts at hourly intervals over five days .. .. .. .. .. 92 vi 7.1.4 November 21-25, 1977. Counts at hourly intervals over five days .. .. .. .. .. 93 7.1.5 December 14-18, 1977. Counts at hourly intervals over five days .. .. .. .. .. 93 7.1.6 January 18-22, 1978. Counts at two-hourly intervals over five days .. .. .. 94 7.1.7 February 22-26, 1978. Counts at two-hourly intervals over five days .. .. .. 94 7.1.8 March 16-19, 1978. Counts at two-hourly intervals over four days . .. .. 95 7.1.9 April 11-15, 1978. Counts at two-hourly intervals over five days .. .. .. 95 7.2.0 June 20-24, 1978. Counts at two-hourly intervals over three days . .. .. 96 7.2.1 August 15-17, 1978. Counts at two-hourly intervals over three days .. .. 96 7.2.2 October 27-29, 1978. Counts at one and two- hourly intervals over three days .. .. 97 7.2.3 December 11-13, 1978. Counts at two-hourly intervals over three days .. .. .. 97 7.2.4 February 20-21, 1979. Counts at two-hourly intervals over two days .. .. 98 7.2.5 March 28, 1978. Count at two-hourly intervals over one day .. .. .. .. .. 98 9.3.1 to 9.3.5 The occurrence of black swan on Pukepuke Lagoon from 1971 to 1978 9.3.1 1971/72 .. .. .. .. .. .. 136 9.3.2 1973/74 .. .. .. .. .. .. 136 9.3.3 1974/75 .. .. .. .. .. .. 137 9.3.4 1975/76 .. .. .. .. .. .. 137 9.3.5 1977/78 .. .. .. .. .. .. 138 10.1.1 The occurrence of flock swans on Pukepuke Lagoon in relation to an induced delay and restriction in hatching .. .. .. 153 vii LIST OF TABLES Page 2.6.1 Two-way analysis of variance of macrophyte data obtained by (a) the hand cut and rake sampler methods and (b) the hand cut and modified Gerking sampler methods .. .. .. .. .. 12 3.3.1 Water chemical analyses undertaken from 1976 to 1979 23 3.5.1 Biomass of macrophyte samples obtained for gastropod macrophyte study in 1977 .