Aerial survey of upper trophic level predators on PLatts Bank, Gulf of Maine

The study area is located 50 km from shore in the western Gulf of Maine and covers 1672 km2, including Platts Bank, Three Dory Ridge and surrounding deep water. Platts Bank (43°10’N, 069°40’W) is a glacial deposit composed primarily of sand and gravel. When defined by the 100 m isobath, the bank is approximately 15 km in its longest dimension and has an area <140 km2.

Aerial surveys were flown on ten days from July 11 to 29, 2005 to record the distribution and relative abundance of marine mammals, birds and large fish. Surveys were typically conducted in the morning or early afternoon and consisted of six transects, each 46 km long oriented on an East-West axis to minimize interference from reflected sunlight. Survey legs were flown at 185 km/hr and an altitude of 230 m using a high-wing, twin-engine aircraft. Observation effort (two observers) was concentrated from both sides of the plane perpendicular to the flight path. To estimate the distances of sightings of mammals and fish from the plane’s flight path, sightings were binned into five groupings corresponding to 15 degrees of arc from 15° (the area directly beneath the plane was not visible) to 90°. When species identification or number of individuals was uncertain, search effort was interrupted while the plane circled to confirm identifications and number of individuals and to obtain a more precise location. Birds were recorded only within a 170 m strip on each side of the aircraft (15° to 45° of arc) during the survey legs. Sightings of birds continued when the plane circled for closer inspection of mammals and fish, but these data were not used in analyses since this would bias bird sightings towards areas where cetaceans were concentrated. Data were recorded by a dedicated data recorder directly onto a computer using software that recorded the time and location from the GPS navigation system aboard the plane at regular intervals throughout the flight and for each recorded sighting.