Low-cost bathymetric survey for marine protected areas: Coral reefs and coastal islands

Bathymetry surveys are the first step for every conservation effort on Marine Protected Areas (MPAs). Maps depicting depth and bottom geomorphology are used to generate hillshades, that later become basemaps for other themes, such as fish distribution, bottom types, and water quality. Bathymetry map availability is limited or non-existent at the scales that are necessary for MPA management plans. Survey costs by traditional sounding methods or new technologies, such as LIDAR, are prohibitive for most MPA budgets. Remote Sensing bathymetry is yet unreliable, except for crystal clear waters. A low-cost technique, based on commercially available GPS-echosounder units was applied to obtain detailed bathymetry of two MPAs in coastal Brazil: at the Parque Municipal Marinho da Coroa Alta (PMMCA), located in the coastal reefs of Santa Cruz de Cabralia, State of Bahia, and Monumento Natural das Ilhas Cagarras (MNIC), off Guanabara Bay, Rio de Janeiro. Equipment consisted of a Simrad NSS-7 chartplotter mounted permanently on Iamany, a 7 m-long, fiberglass-hull, twin-engine boat for PMMCA, and a Garmin GPSMap 521 chartplotter mounted on Cação, an inflatable, 6 m-long, single-engine boat hired for MNIC. Survey frequencies for the echosounders were set at 50 kHz. Cross-shelf, East-West lines spaced 100m apart with readings every second (1-3 m, depending on boat speed) were obtained for PMMCA over 30 days of 4-6 hours of navigation around the highest tide (ranging from 1.3 to 2.0 m above mean sea-level). At MNIC, radial lines surrounding each of the islands were placed 50 m apart at their outer limits, and readings were obtained every 10 seconds (~10 m) in 7 days of 3-4 hours of navigation around the highest tides (ranging from 0.8 to 1.2 m above mean sea-level). In the shallow coral reefs of Bahia, 62 km² were surveyed, with depths varying from 0 to 20m, while at the sandy bottom around the granite-gneiss islands of Rio de Janeiro, 6 km² were surveyed, with depths varying from 0 to 40m. Data in the echosounders proprietary format, were saved on Micro-SD or SD memory cards, converted to XYZ format (.csv) and migrated to GIS, where a linear tide correction was applied. Interpolation methods such as Radial Basis Functions and ANUDEM generated maps with 10 m pixels for PMMCA (presented at 1:10.000 and 1:25.000) and 5 m pixels for MNIC (presented at 1:3.000 and 1:10.000). Average survey costs are estimated at R$ 500-650/day or R$ 1.000/ km² (~US$ 250/day or ~US$ 400/km²), depending on boat ownership vs. rental cost. This represents less than 1/10 of the official traditional survey costs, and even less when compared to LIDAR bathymetry. Precision obtained is an order of magnitude higher than previously available, and enough for environmental assessment for conservation management of the MPAs involved. Surveys were conducted at PMMCA under the auspices of Projeto Coral Vivo and at MNIC under Projeto Ilhas do Rio, both Petrobras Ambiental programmes.