Surveying the seabirds of Ka'ula Island using high resolution aerial imagery
Mark Rehfisch1, Simon Warford
1, Frans Juola3, Julia Wilmot2
1APEM Ltd, Stockport, Cheshire, UK, 2Normandeau Inc, Gainesville, FL, USA, 3University of Miami, Miami, FL, USA
A combination of oblique and vertical imagery has been used to undertake detailed surveys of a bird colony located on a remote island in the Hawai'ian Archipelago. Ka`ula Island is a small (0.64 km2), uninhabited crescent shaped islet in the west of the chain of Hawaiian islands and is home to a variety of breeding seabirds including the Red-tailed tropicbird, Great frigatebird and Sooty tern. Since 1952, the U.S. Navy has used the southeastern tip of the island as a range for munitions training, although this has been restricted to inert ordnance in recent years. As such, the US Navy requires bird colony surveys to be undertaken in order to monitor bird population through the years. Historically, these surveys have been undertaken by land based surveyors, but these have now ceased due to health and safety reasons. An alternative survey method was therefore needed and aerial photographic missions are now used as the chief method of survey.
APEM ltd has undertaken these surveys since April 2013 using a specialized vertical and oblique aerial camera system specifically designed for this application. The camera system was housed in a survey modified twin engine aircraft and survey lines flown, such that coverage is obtained of the entire island. The oblique imagery was used to gather data on the steep sided cliff faces that characterise the island and enabled counts of birds occupying these areas to be undertaken, such as the brown noddy. Imagery was collected at ultra-high resolution (at least 2.5cm resolution), allowing almost all birds to be identified to species level. The imagery was georeferenced and all birds geo-tagged providing a map of their location.
Since 2013, five surveys have been undertaken (April 2013, August 2013, January 2014, January 2015, July 2015) and over 40,000 individuals have been identified within them. 13 different species of birds were identified along with one species of seal (Hawai'ian monk seal). The dataset was also made up of threatened species such as the Black-footed albatross and the Laysan albatross.
The survey method employed here shows that seabird colonies can be monitored remotely using ultra-high resolution aerial imaging. The method provides an auditable, repeatable and robust survey strategy that can be used elsewhere. The unique combination of vertical and oblique imagery allow areas with undulating terrain and steep cliffs to be surveyed, which would be extremely difficult using other methods.
Delegates can learn of this new survey technique, which can hopefully aid is surveying of bird colonies where it is required as part of offshore wind baseline monitoring.