The Role of Tidal Fronts in Maintaining Patches of Brachyuran Zoeae in Estuarine Waters

Brachyuran larvae were sampled along a 200-m, NE-SW transect crossing a tidal front in lower Delaware Bay. Concentrations of the larval mud crab Neopanope sayi ranged from approximately 30 zoeae m-3 NE of the front to more than 7,000 zoeae m-3 SW of the front with an intermediate concentration in the front. Zoeal stages I and II were most abundant at each sampling site, suggesting that the patch of N. sayi resulted from a spawning event that occurred 4-5 days earlier. Physical concentrating mechanisms associated with the front, combined with vertical movements by the larvae, appeared to be responsible for maintaining the concentrated patch of larvae of N. sayi. Larvae of decapod crustaceans generally occur in assemblages that are nonuniformly distributed in horizontal and vertical space (e.g., Epifanio et al., 1984). This may be due to: (1) the pulsed reproductive behavior of adult populations (Provenzano et al., 1983); (2) behavioral characteristics of the larvae (Sulkin, 1984); or (3) physical concentrating or dispersing mechanisms related to water circulation (Shanks, 1983). In estuarine and coastal waters, the abundance of larval decapod crustaceans has been reported at levels ranging from 1-100 larvae. m-3 (Sandifer, 1975; Dittel and Epifanio, 1982). In the present paper, I describe results of a preliminary study of a dense patch of decapod larvae in Delaware Bay. Larval collections and physical measurements in this investigation were made during a cruise that was otherwise devoted to a survey of fish larvae in Delaware Bay. While this limited the amount of attention that could be devoted to the patch of decapod larvae, the concentration of zoeae in that patch was extraordinarily high and warrants discussion of factors important in maintenance of the patch. MATERIALS AND METHODS Larvae were collected in mid-June 1986 from a station in lower Delaware Bay (39?05'N, 75?05'W) where a visible tidal front had developed. The frontal region was characterized by a line of foam and floating debris separating areas of water of different color. Water SW (inshore) of the front was brownish green, while that NE (offshore) of the front was bluish green. Sampling was conducted in late morning and nearly coincided with the time of maximum (80 cm-s-') flood tidal current in the bay mouth (NOAA Tidal Current Tables, 1986). Visual evidence of the front extended for at least 2 km along a NW-SE axis. Collecting gear consisted of a l-m, 333-,um mesh, closing plankton net fitted with a Kahl Scientific flowmeter. The collecting gear was towed from an 8.5-m vessel equipped with LORAN C for precise location of stations. One tow (depth = 1 m; duration = 1 min) was taken 100 m NE of the front, while similar tows were taken in the frontal region and SW of the front. All tows were parallel to the front. The total volume of water sampled ranged from 34.3-57.7 m3. Salinity and temperature were measured (Beckman RS-5 CST meter) at a depth of 1 m SW of the front and at 1 m and 8.5 m (1 m above the bottom) NE of the front. Density was calculated as sigma-t. After collection, larvae were preserved on board in 4% formaldehyde in sea water. Samples were transferred to 70% ethanol 2 days after return to the laboratory. Larvae were identified according to Sandifer (1972). Fiddler crab larvae were not identified beyond genus, since external morphological characteristics do not allow separation of the two species of Uca that are likely to occur in Delaware Bay (Dittel and Epifanio, 1982).