CLBMON-49. Lower Columbia River Effects on Wintering Great Blue Herons

Author: N.T. Hentze, V.C. Hawkes, D. Robichaud, J. Novoa, D. Terbasket

This monitoring program was initiated in November 2013 and field work is complete. CLBMON-49 changed from a count-based study to a habitat suitability study in the final two years of this project. GBH distribution and abundance, along with physicochemical and other habitat and environmental parameters were monitored at a variety of sites throughout the Kootenays during the winter months. Surveys were completed prior to, during, and after flows related to management of Mountain Whitefish (generally November 1 to February 28) in the Castlegar area, encompassing varied water elevations and flow rates resulting from known dam operations. The information collected throughout the Kootenays will be used to assess GBH habitat and use in the Lower Columbia River.

Several management questions are addressed by this study (Table 6-1). This study has benefited from a broadening of the study area and investigations of herons elsewhere in the Kootenay region, as the Castlegar area is not a microcosm. Rather, a complex of variables related to climatic conditions and heron breeding success will impact how many herons are present in the Castlegar area during the winter months. Nonetheless, severe limitations arose owing to the inability to manipulate flow rates and establish an experimental study. Rather, an observational study was designed which leaves many unanswered questions as to the ultimate reasons for the observed heron trends. In addition, prey data were not collected, and the inter-annual trends in fish numbers, fish species composition, and effects of flow regime on fish habitat utilization are unknown. Based on the data collected, the Waldie Island area hosts a high number of wintering herons, until at least January, and in the
Kootenay region heron numbers were only higher in the Creston area. Furthermore, there is no direct evidence of an impact of flow regime on heron numbers or population trends. Confounding analysis was an inconsistent pattern of flow regime among years, but what is clear is a declining amount of available shoreline habitat with increasing water elevation. The effects of this are most pronounced on smaller and lower land areas, in particular Breakwater Island, which otherwise appears to be an important roosting area for Great Blue Herons in the region. Dam operations should aim to keep water elevations below 420.7 m when possible, as the island is essentially inundated at elevations above 422 m. Breakwater Island could be raised, or additional islands created in the Pass Creek area, to help maintain habitat during periods of high water elevations, but overall heron populations and the temporal trend of heron numbers throughout the winter may be unaffected by such measures.

Several management questions are addressed by this study (Table 6-1). This study has benefited from a broadening of the study area and investigations of herons elsewhere in the Kootenay region, as the Castlegar area is not a microcosm. Rather, a complex of variables related to climatic conditions and heron breeding success will impact how many herons are present in the Castlegar area during the winter months. Nonetheless, severe limitations arose owing to the inability to manipulate flow rates and establish an experimental study. Rather, an observational study was designed which leaves many unanswered questions as to the ultimate reasons for the observed heron trends. In addition, prey data were not collected, and the inter-annual trends in fish numbers, fish species composition, and effects of flow regime on fish habitat utilization are unknown. Based on the data collected, the Waldie Island area hosts a high number of wintering herons, until at least January, and in the
Kootenay region heron numbers were only higher in the Creston area. Furthermore, there is no direct evidence of an impact of flow regime on heron numbers or population trends. Confounding analysis was an inconsistent pattern of flow regime among years, but what is clear is a declining amount of available shoreline habitat with increasing water elevation. The effects of this are most pronounced on smaller and lower land areas, in particular Breakwater Island, which otherwise appears to be an important roosting area for Great Blue Herons in the region. Dam operations should aim to keep water elevations below 420.7 m when possible, as the island is essentially inundated at elevations above 422 m. Breakwater Island could be raised, or additional islands created in the Pass Creek area, to help maintain habitat during periods of high water elevations, but overall heron populations and the temporal trend of heron numbers throughout the winter may be unaffected by such measures.