Vertical distribution and the role of physical processes in the feeding dynamics of two larval sciaenids Sciaenops ocellatus and Cynoscion nebulosus

Red drum Sciaenops ocellatus and spotted seatrout Cynoscion nebulosus larvae were collected using plankton and benthic-sled lows every 2 h for 26 h on 4 separate dates in late August to early October 1990 in Aransas Bay, Texas, USA. Gut contents and gut fullness were evaluated to determine if feeding was randomly distributed over depth and time, and to compare feeding between the 2 species. Calanoid copepods were the dominant prey over all size- classes of red drum larvae. Copepod nauplii, bivalve and barnacle larvae were important for small red drum, while calanoid copepods, dinoflagellates and soft-bodied organisms were important in the diets of large red drum larvae. Calanoid copepods and bivalve larvae were the most important food items for spotted seatrout; gastropod veligers and copepod nauplii were also important prey. Gape size was positively correlated with standard length in both species. Mean prey size increased with gape width but there was wide variation in prey size at any given gape width. Small (<3.0 mm) and medium (3.0 to 4.5 mm) fish of both species fed on similar prey but large (>4.5 mm) fish had distinct diets. Similar numbers of larvae were collected in surface and bottom waters (6 m) during the night, but during the day more larvae were collected in bottom waters. Larvae of both species fed primarily during daylight hours. Feeding extended 2 h later in the evening for spotted seatrout than for red drum. There was no significant relationship between current speed and gut fullness in spotted seatrout or in red drum. Larvae were very successful at feeding under all conditions with little indication that vertical distribution was associated with feeding success. Transit periods through well-mixed tidal inlets may provide excellent feeding opportunities, as well as a route to transport larvae to essential nursery habitats.