Mid- and outer-shelf banks in the northwestern Gulf of Mexico represent important naturally occurring aggregation areas for exploited fish species.  Except for the Flower Garden Banks National Marine Sanctuary, most of these banks are unprotected and poorly studied.  Further, high vertical relief and greater depths at these banks makes it difficult to assess fish abundance.  We are studying two of these banks, Sonnier Bank (depths 20 to 60m) and McGrail Bank (depths 45 to 130m), to develop survey approaches with ROV (Plate 1).  Most of our effort to date has concentrated on Sonnier Bank, which appears as a ring of topographic highs (Figure 1).

Plate 1. Remotely operated vehicle (ROV-Seamor) in operation underwater.

Figure 1.   Map and bathymetry of Sonnier Bank. Click here for description of multi-beam survey and data products from USGS.

We are mapping habitats at both banks using high resolution sidescan sonar (Figure 2).  At depths <30m, we are comparing density estimates between visual surveys with SCUBA and video surveys with ROV.  From our initial surveys during 2004, we observed a predictable relationship between SCUBA and ROV for counts of target species (Figure 3). We are conducting additional comparisons in 2005 to better characterize this relationship across a broader range of abundances.

From ancillary otolith samples, we found several exceptionally old individuals of some dominant species (rock hind, vermilion snapper, and tomtate, Figure 4).  The presence of these old individuals not only highlights the significance of Sonnier Bank as an essential habitat, but also emphasizes the need to obtain size or stage related data from the surveys.  We have designed and constructed a laser array for our ROV that gives us ranging and measuring capabilities (see video), and this will aid in the estimation of fish size and area surveyed from video recordings.

Figure 4. Age-distribution of species captured at Sonnier Bank. Ages were determined by annulus counts from thin sections of otolith.

Images from underwater video taken in 2004 at the main peak of Sonnier Bank.

This work is funded through NOAA’s Coral Reef Initiative Program, administered by the Gulf of Mexico Fishery Management Council.