Sanctuary Integrated Monitoring Network
Monitoring Project

Davidson Seamount: 2010 Marine Mammal Aerial Surveys

Principal Investigator(s)

  • Chad King
    Monterey Bay National Marine Sanctuary
  • Andrew DeVogelaere
    Monterey Bay National Marine Sanctuary


  • SIMoN
Start Date: January 14, 2010

In 2010 Monterey Bay National Marine Sanctuary (MBNMS) staff and partners conducted two aerial surveys of the surface waters surrounding the Davidson Seamount, an area typically regarded as having a higher abundance and diversity of marine mammals and seabirds than immediately adjacent areas absent of significant bathymetric features. Aerial surveys occurred during a single day in January and April, and covered 542 km and 400 km, respectively, of linear “on-effort” transects. Marine mammal encounter rates (marine mammals per linear km) were calculated for each survey: 0.042 for the January aerial survey, and 0.788 for the April aerial survey. Differences in encounter rates are the result of multiple factors including survey methods, inter-seasonal and inter-annual variation in population dynamics, and community-level processes. To differentiate natural vs. human-induced changes requires multiple years of surveys collecting both biotic and abiotic data.

Summary to Date

On January 14, 2010, approximately 542 kilometers of “on effort” transects were flown in five zig zag formations to the north, south, east, west and over the Davidson Seamount. Species identified during the survey included: California sea lion (Zalophus californianus), Cuvier’s beaked whale (Ziphius cavirostris), Grey whale (Eschrichtius robustus), Killer whale (Orcinus orca), Humpback whale (Megaptera novaeangliae), Sperm whale (Physeter macrocephalus), Laysan Albatros (Phoebastria immutabilis), Nothern Fulmar (Fulmarus glacialis), Storm petrel (family Hydrobatidae), and the Ocean sunfish (Mola mola). Four groups of sightings included unidentified dolphins, unidentified bird, unidentified alcid, and unidentified whale, for a total of 13 distinctly recorded species groups. 139 individual animals for an individual encounter rate of 0.26 animals per kilometer, 0.20 seabirds per kilometer, and only 0.04 marine mammals per kilometer.

On April 19, 2010, five transects were flown perpendicular to the main axis of the Davidson Seamount, totaling approximately 400 kilometers of “on effort” transects. Species identified during the survey included: Blue whale (Balaenoptera musculus), California sea lion (Zalophus californianus), Humpback whale (Megaptera novaeangliae), Sperm whale (Physeter macrocephalus), and the Ocean sunfish (Mola mola). Two groups of sightings included unidentified dolphins and unidentified whale, for a total of eight distinctly recorded species groups. Seventy-eight (78) individual sightings were recorded, totaling 336 individual animals for an individual encounter rate of 0.84 animals per kilometer and 0.79 marine mammals per kilometer.


The two surveys conducted in 2010 serve as a baseline for future surveys. Although one 2010 ship-based survey yielded more marine mammals encountered per kilometer than either of the aerial surveys, correlating non-temporal variables (seasonality, inter-annual, etc.) with encounter rates is not yet possible. Seasonality most certainly plays a large role in the relative density and species diversity of marine mammals; not just in terms of oceanographic patterns and prey availability, but also marine mammal annual migration patterns and feeding behaviors. Another factor to consider is the relative survey footprint each survey design covers. It is estimated that every kilometer of linear aerial transect length corresponds to a visual footprint of 2.02 square kilometers. Footprint estimates have yet been made for ship-based surveys. However, a temporal encounter rate reveals that aerial surveys may be more efficient to observe marine mammals in terms of man-hours. Hours of effort are not cumulative of total observer hours; they reflect hours the plane or ship was on transect. Please see full draft report (including ship-based survey methods) at

Study Parameters

  • Range/Biogeography
  • Diversity
  • Abundance
  • Maps
  • Habitat association
  • Distribution
  • Upwelling/downwelling

Study Methods

In consultation with experts from NOAA’s National Marine Fisheries Service, the following aerial survey methods were designed in early 2010 and will be used for all future aerial surveys of marine mammals and turtles.

A NOAA DeHavilland Twin Otter aircraft was used as the aerial observation platform for both the January 14 and April 19, 2010 surveys (Figure 3). The survey method takes into account the Twin Otter’s ability to fly at low altitudes and consistent speeds, but due to the remote location of the Davidson Seamount, also takes into account a maximum safe flight distance of 500 nautical miles. However, the designs differed considerably between the January and April surveys to evaluate two potential methods for conducting aerial surveys.

The January survey consisted of a zig-zag (NW to SE) flight pattern of three lines in five distinct boxes over and around the Davidson Seamount. One box was over the seamount itself (yellow lines), while the other four boxes (red lines) were to the north, east, south, and west of the seamount (Figure 3).

As project management transitioned after the January 2010 survey, the design changed. To maximize the area surveyed, above and away from or surrounding the seamount, while optimizing the number of transects and maintaining flight efficiency, transects were selected perpendicular to the main axis of the seamount (Figure 4).

The maximum survey footprint was designed to cover the entire axial length of the seamount (42.2 kilometer) and to include 44.4 kilometers of surface water perpendicular to both sides of the axis of the seamount for a total potential area of 3,747 square kilometers (Figure 2). The aerial observer "footprint" was 2.02 km wide (1.01 km on each side of the plane). This was based on a maximum observer angle of 12 degrees and a flying altitude of 210 meters.

Given the approximate width of the observer footprint, and to prevent overlap of parallel survey lines, 22 origination points fit within the 42.2 km axial length of the study area. These 22 origination points were placed at an equal distance from each other along the northwestern (red dots) and southeastern (yellow dots) boundaries of the study area (Figure 3).

For both aerial survey dates, six random numbers with a value between 0 and 21 were selected without replication. Additionally, three were even numbers (transects starting from the northwest) and 3 were odd numbers (transects starting from the southeast). These points were used to create the flight plan. When the Twin Otter was in the study area, it flew transects in descending order of point value, starting from the northwest at a heading of 126 degrees, then back at 306 degrees. This process was repeated for all coordinates selected or until fuel was low.

Counts and identifications of marine mammals were made by trained and qualified observers. There was at least one observer on each side of the plane positioned in the most forward portion of the main cabin to use a large observation “bubble” (convex viewing port made of plexiglass) that extended away from the body of the aircraft, providing an unobstructed view of marine mammals directly below and out to 12 degrees (Figure 5). A third observer was positioned above a plexiglass port located in the rear belly of the plane, which allowed viewing directly below the plane. Each observer also used a Suunto Tandem (inclinator and compass) to approximate the angle and distance of the marine mammal observed. All observers verbally relayed their counts and identifications to a fourth person who entered the data into custom-written software program on a laptop. This laptop was connected to the plane’s GPS system, enabling all observations to have a latitude and longitude automatically incorporated into the record. Variables recorded for each flight included: event code, effort, time, latitude, longitude, observer, viewing conditions, altitude, speed, weather, mammal sighting (marine mammal species code, see Table 1), re-sights, and turtle sightings. After the flight, this digital tabular file was imported into a GIS for spatial analysis and cartographic products.

Figures and Images

Three dimensional perspective looking north into MBNMS. The Davidson Seamount can be seen in the lower left, Monterey Bay in the middle of the figure.

Monterey Bay National Marine Sanctuary staff and Moss Landing Marine Laboratories graduate students prior to an aerial survey for marine mammals near the Davidson Seamount aboard the NOAA DeHavilland Twin Otter N48RF on April 19, 2010.

Aerial survey design for January 2010.

Aerial survey design for April 2010.

View of the Twin Otter’s main cabin, looking forward. Observers 1 and 2 sit near the front of the cabin and view marine mammals through “bubble” widows. A data entry person sits near the rear of the plane and enters observations into a laptop. A third observer (position not seen here) lays down in the rear of the plane to view marine mammals through a plexiglass port in the belly of the plane.

Marine mammal and seabird observations from the January 14, 2010 aerial survey of the Davidson Seamount.

Map of aerial survey flight line and marine mammal sightings over the Davidson Seamount on April 19, 2010.