1-cm Resolution Seafloor Surveys of Octopus Garden, an Octopus Brooding Site Near Davidson Seamount, Offshore Central California

The Monterey Bay Aquarium Research Institute (MBARI) Low Altitude Survey System (LASS) was used in April 2021 to survey Octopus Garden, where aggregations of Muusoctopus robustus are brooding in warm fluids emanating from the seafloor. The site is on a volcanic ridge at the base of Davidson Seamount, offshore of Central California, and was discovered in 2018 by Monterey Bay National Marine Sanctuary and Ocean Exploration Trust researchers. Two surveys conducted from ROV Doc Ricketts on the R/V Western Flyer covered a 95 m by 245 m area including the Octopus Garden and Octopus Spa brooding sites. The LASS combines a 90° field of view (FOV) lidar laser scanner, 400 kHz 135° FOV multibeam sonar, and stereo 80° FOV color still cameras illuminated by strobe lights. Operated at a 3-m standoff, the lidar maps a 6 m wide swath at 1 cm resolution, the sonar maps a 10 m wide swath at 5 cm lateral resolution, and the cameras collect 4 m wide images at 2.5 mm resolution. Position and attitude data are provided by an inertial navigation system (INS) integrated with a Doppler velocity log (DVL) and pressure sensor. The LASS package is mounted on an articulating frame that rotates automatically to stay oriented perpendicular to the seafloor, enabling surveys in steep and complex terrain. Survey lines were run autonomously with 3 m spacing with the LASS oriented uphill. Automated feature recognition processing of the photomosaic identifies more than 7200 brooding octopuses. In addition to a few dense congregations, brooding octopuses are found scattered over the entire ridge, and there are dozens of free-ranging males and females. The Octopus Garden site is a depression in the slope with an overhang; from there sub-linear aggregations extend north along the ridge and southeast obliquely downslope. At Octopus Spa brooding females blanket the southern end of a shallow depression on top of the ridge. Three aggregations occur midway down the east flank, and others lie along the bases of low scarps that may be lava flow contacts. The areas supporting the densest groupings of octopuses are generally darker in the photomosaics, possibly a result of fluid flow or octopus activity removing surficial sediment. Rock samples from the site are dense basalt, suggesting it is an outcrop of the spreading ridge that predates the fractionated lavas of Davidson Seamount.