Recovery Act: Hardbottom mapping and community characterization of the west central Florida Gulf coast

by Ryan Maloney and Sean Keenan

Habitat classification and mapping are a crucial part of fisheries conservation and ecosystem-based management.  Essential fish habitats act as nurseries, feeding, and breeding grounds for many commercially and recreationally important species. Technological advances like side scan sonar and satellite imagery allow researchers to gain a better understanding of where these habitats are located, their aerial extent, and what the habitat and fish community looks like.

The distribution and composition of benthic habitats on the west Florida shelf including submerged aquatic vegetation (SAV) and submerged marine carbonate reef outcroppings (hard bottom) are poorly understood. While these habitats serve as nursery and forage areas for many economically-important reef-fish species (e.g. gag, red snapper, gray snapper and hogfish), benthic mapping and characterization of the benthic communities are nonexistent for a majority of the West Florida continental shelf.

Mapping habitat in such a large area is a daunting task, which requires an extensive amount of time, and can be very costly.  Current methods for characterizing marine habitats include acoustic (side scan sonar) and optical (satellite) imagery.  Side scan sonar provides high resolution (~0.1 m) imagery through processing of acoustic backscatter or echo strength (Figure1). While commercial grade side scan systems have become more cost effective, time and resource requirements for data collection and post-processing still add up. Satellite imagery, in comparison, has a substantially lower cost per unit area and is much more time efficient. Satellite imagery, however, is restricted by water depth and clarity and is more difficult to acquire and has a limited resolution of ~3 m (Figure2).

imagery comparisons
Figure 1. This slide illustrates some or our more commonly used habitat classifications. By utilizing camera ground truthing images, we can better define the corresponding side scan imagery.
satellite imagery
Figure 2. The image to the left represents raw satellite data, while the image to the right shows the delineated habitat defined based on the parameters mentioned.

For this project, the Fisheries Independent Monitoring (FIM) group is compiling past and present side scan sonar surveys contained within two designated study areas.  The areas are divided into a 790 km² area off of Clearwater, FL and 820 km² area just off of Sarasota, FL.  Within each study area, surveys are being scanned and read for the presence of essential fish habitats.  In conjunction with scanning, a drop camera is deployed at each survey site as a ground truthing technique.  All side scan and ground truth data will be compared to preexisting satellite imagery, processed and classified by grant collaborators.  By comparing and contrasting side scan sonar with satellite imagery, this project is exploring different methods to map essential fish habitats at varying scales (Figure 3).

side scan sonar mosaic
Figure 3. A side scan sonar mosaic, with defined habitat outlined, shows that brighter imagery returns, correspond to hard harder substrates. These returns are inversely related to satellite imagery, where we see dark returns representing hard bottom.

With a more economical way of characterizing nearshore marine habitats, scientists and fisheries managers can 1) establish the exact locations and spatial extent of these habitats; 2) monitor any changes in habitats and fish communities; and 3) implement habitat-based fisheries independent surveys.