Current Vortices and Fish Aggregations: The Current Field and Associated Fishes Around a Tugboat Wreck in Onslow Bay, North Carolina
Abstract:We placed two Aanderaa Recording Current Meters (ARCMs) 15 m to either side of a 27 m tugboat located 18 km off Carolina Beach, NC. The ARCMs were suspended 5 m above the bottom in 18 m of water and were preset to sample current speed and direction every 10 min during the period 24 September to 11 December 1985. Fishes associated with the tugboat were censused by divers on three dates during the same period using a modification of the stationary survey technique (Bohnsack and Bannerot, 1986). Diver surveys included counts of fishes located upstream, downstream, and normal to the tugboat current field. Results from the ARCM data analysis indicated that coherent tidal, inertial and subinertial frequency motion was interfered with by the tugboat and that the tugboat creates a mechanism whereby momentum is transferred from low frequency or long period motions to high frequency motions with short shedding periods. The high frequency motions appeared as broad elliptical eddies on the downstream side of the tugboat and as very narrow elliptical eddies on the upstream side. The shed eddies appeared in a swath covering 120° on the starboard side of the tug (measured from the bow) and 140° on the port side (also measured from the bow). No shed eddies appeared in a 78° shadow zone to the NE and SE of the stern and a 70° shadow zone to the NW and SW of the bow. Results from the analysis of the diver fish surveys indicated that only the dense schools of semi-pelagic round scad (Decapterus punctatus) were significantly more abundant upstream of the wreck. Benthic reef species (e.g., Haemulon aurolineatum and Diplodus holbrooki) showed no statistically significant abundance differences in relation to the current field.
Document Type: Research Article
Publication date: March 1, 1989
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