Methods

Study Area

         The primary study area included Sabine Bank and adjacent open-bottom areas. Sabine Bank (Figure 1) is located 28 km South of the mouth of Sabine Pass, at the Texas/Louisiana border, and approximately 74 km East of the mouth of Galveston Pass. The main body of the bank is 33 km long, extending to the WSW of the dredged channel leading to Sabine Pass.  At its widest point, the bank is 7 km wide. Another part of Sabine Bank, about 17 km long by 4 km wide, extends ENE from the Sabine Pass channel.

Figure 1.  Sabine Bank, excerpted from NOAA Chart No. 11330. Patterned
blue areas indicate the extent of the Disposal Areas.  The grid is composed
of 1235 meter squares, used to choose locations of random trawl samples.

Sampling Protocol

         Station Selection – A virtual grid was projected over Sabine Bank with a cell size of 1,235 m² (Figure 1). This cell size roughly corresponds to the distance covered by a 20 minute trawl at 2 knots. Surrounding off-bank areas were also included up to 2,470 m from the edge of the bank. Cells which contained submerged pipelines or other obstructions, noted on the NOAA Nautical Chart No. 11330 (Figure 1), which would prevent trawling success, were excluded. There are areas on the bank which are designated as "Disposal Areas" on the nautical chart (Figure 1). Samples from this area were kept separate to estimate if there are any differences between disposal and non-disposal areas. It is possible that sediment differences may exist which leads to differential faunal use. Cells on the bank were then classified as to either "bank interior" or "bank edge". Edge cells were determined to be cells that contained any combination of on and off-bank areas, while interior cells were exclusively on the bank. After classification, ten random bank interior, bank edge, and off-bank cells were selected as sampling stations.

         Reconnaissance Trawl Sampling – At each sampling station, one trawl was completed. After preliminary sampling, it was decided, due to low benthic fish abundance*, that all trawls would be fifteen minutes in duration at a towing speed of 2.0 to 2.5 knots. A 6 m otter trawl was used with 2 cm mesh, 0.62 cm mesh liner, and a 0.6 cm tickler chain (Figure 2).

Figure 2. The 6 m otter trawl used for fish sampling.

         The position of the ship was recorded at all times using a mounted Garmin GPS unit and Blue Marble Geographic's program GeoTracker. GeoTracker, when used with an ArcView extension, will allow for the visualization (and recording) of the ship's position relative to the bank. A Garmin GPSMAP 76S was attached to the cod end of the trawl by using a 5-float cradle (Figure 3). The use of the secondary GPS unit allowed us to determine the actual track of the trawl over the bottom, as compared to that obtained from the GeoTracker positioning system. The GPS unit was attached to the cradle using a waterproof Otter Box which provided a dry environment and also allowed for satellite reception. *(see comment on hypoxia below)

Figure 3. The float used to attach the GPS unit to the trawl.

         All demersal and non-schooling pelagic fish (and shrimp over 100 mm total length) caught in the trawl were recorded to species and either a standard, fork, or total lengths were taken to the nearest millimeter (Figure 4). The abundance of all pelagic schooling fish (e.g., Anchoa mitchilli, Anchoa hepsetus, Chloroscombrus chrysurus, Opisthonema oglinum) was recorded, and lengths were taken from thirty randomly selected individuals of each species. All fish collected were identified to species under the supervision of Dr. John Caruso from the University of New Orleans. Dr. Caruso has over twenty years of experience with Gulf of Mexico fish taxonomy. Once a fish was identified to species (and a length measurement taken) it was discarded. Any fish which could not be positively identified to species was preserved in 10% formalin and are currently archived at the University of New Orleans for further taxonomic clarification.

         Angling – To supplement trawl sampling, angling was performed to potentially catch species or larger individuals which may avoid the trawl. Specifically, demersal fish species were targeted using hook and line gear (six foot fiberglass poles with Penn 320GT reels and 40 lb main line). Cut bait, either squid or fresh caught fish, was weighted for bottom fishing.

Figure 4. USGS scientists recording individual
fish species abundance and length.

         Water Parameter Sampling – A SEABIRD SBE-19Plus self-recording CTD profiler was used to measure water parameters (Figure 5). The CTD records water temperature (0.000 ^oC), conductivity (0.00000 S/m), pressure (depth, 0.00 m), and oxygen concentration (0.0000 mg/L). With further processing of the CTD data, salinity (0.000 PSU), density (0.000 kg/m³), and oxygen saturation (0.000 %) can be calculated. After the completion of each trawl sample, the boat returned back to the midpoint of each trawl track, and a water column profile was taken.

Figure 5. Seabird CTD Deployment, in a custom
protective cage for water parameter measurements.

         Sediment Collection – After the completion of each trawl sample, the boat returned back to the midpoint of each trawl track. The box grab was deployed to collect a relatively undisturbed plug of sediment (Figure 6). A subsample was taken from each successful grab using a 2.54 cm diameter corer to a depth of 5-8 cm. The subsample will be used to supplement and ground-truth the information obtained from Texas A&M's side-scan sonar mapping.

Figure 6. The custom box grab (K. Sulak design)
used for sediment collection.

         Research Vessel - The R/V Eugenie, a 17.6 m research vessel, from the Louisiana Universities Marine Consortium (LUMCON), Cocodrie, Louisiana, was used as the sampling platform for this cruise (Figure 7). The vessel accommodates four scientific crew and twelve hours of sampling per day. The Eugenie has a two day sea-keeping limit before having to return to port.

Figure 7. The R/V Eugenie based out of LUMCON used for all sampling.

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