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Habitat Conservation Trust Fund Proposal

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Habitat Conservation Trust Fund Proposal Draft THE WESTERN GREBE ON SWAN, OKANAGAN, AND SHUSWAP LAKES by Richard J. Cannings, R.P.Bio. and Kayla Feldman March 14, 2000 Cannings Holm Consulting 1330 Debeck Road R.R. # 1, S. 11, C. 96 Naramata, B.C. V0H 1N0 This project has been funded by the Habitat Conservation Trust Fund. This fund was created to preserve, restore and enhance key areas of habitat for fish and wildlife throughout British Columbia. Hunters, anglers, trappers and guides contribute to HCTF enhancement projects through licence surcharges. Tax deductible contributions to assist in the work of HCTF are also received. i SUMMARY Western Grebes nest at only 3 sites in British Columbia. A former breeding site, Swan Lake near Vernon, seems to be still suitable for Western Grebes. We undertook this study in an attempt to understand why the species abandoned the site in the late 1970s or early 1980s. We compared conditions at Swan Lake with two nearby colonies at the north end of Okanagan Lake and at Salmon Arm on Shuswap Lake. At all sites we studied various habitat parameters, disturbance by boaters, prey availability, lake level fluctuations and presence of potential nest predators. Western Grebes were seen on Swan Lake, but only during spring migration in May. The colonies at Okanagan Lake and Salmon Arm were both unsuccessful this year. The Okanagan Lake colony was abandoned and partially predated for unknown reasons; the Salmon Arm colony was flooded by extreme high water levels that persisted from mid-June through late July. Habitat analyses indicated that Swan Lake is suitable for Western Grebe colonization—sufficient stands of open Scirpus marsh are available and small fish populations seem high. Disturbance by boats on Swan Lake is low, particularly that involving high-powered ski boats. Water level fluctuations at Swan Lake are the least of the three sites and well within tolerance limits for grebes. Predator numbers are also relatively low at Swan Lake. We noticed that incubating birds at the Okanagan Lake colony left their nests when we approached by kayak to within 50 to 100 m of the colony. Since repeated disturbance of this sort may be detrimental to any grebe colony, we feel that it would be ii worthwhile to consider limiting boat activity of all types on a portion of Swan Lake to minimize such disturbance. An assessment of habitat availability and obvious points of boat access suggests that the north end of the lake would be best for this purpose, with perhaps a smaller secondary no-boating area at the south end. iii TABLE OF CONTENTS SUMMARY....................................................................................................................................................I LIST OF FIGURES ....................................................................................................................................IV LIST OF TABLES ......................................................................................................................................IV INTRODUCTION......................................................................................................................................... 1 STUDY SITES............................................................................................................................................... 3 METHODS.................................................................................................................................................... 3 CENSUSES ................................................................................................................................................... 3 WATER LEVELS .......................................................................................................................................... 7 NEST SURVEYS............................................................................................................................................ 7 DISTURBANCE STUDIES ............................................................................................................................... 7 PREY AVAILABILITY .................................................................................................................................... 8 HABITAT ..................................................................................................................................................... 8 FORAGING BEHAVIOUR ............................................................................................................................... 9 RESULTS ...................................................................................................................................................... 9 CENSUSES ................................................................................................................................................... 9 WATER LEVELS ........................................................................................................................................ 10 NEST SURVEYS ......................................................................................................................................... 10 Swan Lake ........................................................................................................................................... 10 Okanagan Lake ................................................................................................................................... 15 Salmon Arm......................................................................................................................................... 17 DISTURBANCE........................................................................................................................................... 17 PREY AVAILABILITY .................................................................................................................................. 18 HABITAT ................................................................................................................................................... 22 Swan Lake ........................................................................................................................................... 22 Okanagan Lake ................................................................................................................................... 22 Salmon Arm......................................................................................................................................... 23 FORAGING BEHAVIOUR ............................................................................................................................. 23 POTENTIAL NEST PREDATORS.................................................................................................................... 24 DISCUSSION .............................................................................................................................................. 30 ACKNOWLEDGEMENTS........................................................................................................................ 32 REFERENCES............................................................................................................................................ 33 iv LIST OF FIGURES 1. Swan Lake.................................................................................................................................................. 4 2. Okanagan Lake, north end ......................................................................................................................... 5 3. Salmon Arm............................................................................................................................................... 6 4. Western Grebe censuses........................................................................................................................... 12 5. Red-necked Grebe censuses..................................................................................................................... 13 6. Lake Levels.............................................................................................................................................. 14 7. Boat Activity............................................................................................................................................ 19 8. Disturbance Rates on weekends vs. weekdays......................................................................................... 21 9. Foraging rates .......................................................................................................................................... 26 10. Foraging rates at half-hour intervals ...................................................................................................... 27 11. Western Grebe nest, Okanagan Lake ..................................................................................................... 28 12. Western Grebe nest (predated), Okanagan Lake.................................................................................... 28 13. Nest-like structure built by Western Grebes in milfoil mat, Okanagan Lake......................................... 29 14. Red-necked Grebe nest (eggs covered) in clump of bulrush, Swan Lake .............................................. 29 LIST OF TABLES 1. Distribution of disturbance studies at each site. ......................................................................................... 8 2. Red-necked Grebe nests at Swan Lake. ..................................................................................................
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