Fine Tuning Reef Fish Surveys through the Incorporation of Hydroacoustic Technology

By Ryan Munnelly and Brett Pittinger

Stereobaited remote underwater video arrays (S-BRUVs) have become a standard gear used to sample fish distributions in aquatic systems around the world.  Over the past decade, the Fisheries-Independent Monitoring (FIM) program of the Florida Fish and Wildlife Conservation Commission (FWC) has used S-BRUVs like the array shown in Fig. 1 to study fish populations associated with natural and artificial reef habitats of the West Florida Shelf (WFS).  This effort has involved thousands of 30-minute deployments in waters 10–180 m deep, from Pensacola to the Florida Keys.

An example of some of the sampled artificial habitat of the West Florida Shelf.

Some advantages of S-BRUVs are that they are minimally invasive to the fish community and habitat, they are less selective than other gears, and they provide behavioral information.  However, despite these advantages, it is difficult to determine the distance from which fishes are attracted to the bait during a deployment.  This complicates fish-habitat relationships observed in the video by adding uncertainty regarding the total area sampled and whether fishes observed were in fact associated with the habitat targeted.  Improving our current understanding of the range of attraction of fishes to an S-BRUV is an important step toward determining absolute species abundances.

Hydroacoustics use sound to detect fish in the water column in the same way that a typical fish finder works.  Hydroacoustics can be used to rapidly survey a large area and are even less invasive than S-BRUVs in that they do not influence fish distributions.  These features make hydroacoustics a complementary method to the S-BRUV surveys conducted by the FIM program.  Figure 2 shows the results from one survey designed to evaluate spatial redistributions of fishes that take place during an S-BRUV deployment relative to before the gear entered the water.  At this site located in 61 m water depth offshore of Panama City, fish abundance increased near the S-BRUV during deployment and decreased to the northwest of the site, where the current was oriented.  This information will be used to improve assessments of commercially targeted fishes, sportfish, and other ecologically valuable species throughout WFS waters.

Fig. 2. Mean volume backscatter in the lower 5 m of the water column from a hydroacoustic survey over several patches of low-relief habitat before (left panel) and during (right panel) deployment of an S-BRUV video array.  The dots represent data points that were interpolated throughout the 375 x 375 m survey grid, hashed areas are patches of previously identified habitat, and brighter colors indicate higher fish abundances.  The S-BRUV was deployed in the center of the survey grid and an arrow in the right panel shows the direction of the prevailing bottom current distributing the odor plume from the bait.