RETENTION OF THI^E TAXA OF POSTLARVAL FISHES IN 



AN INTENSIVELY FLUSHED TIDAL ESTUARY, 



CAPE FEAR RIVER, NORTH CAROLINA 



Michael P. Weinstein,^ Sidney L. Weiss,^ Ronald G. Hodson,^ and Lawrence R. Gerry^ 



ABSTRACT 



Fixed nets were used to sample postlarvae of spot, Leiostomus xanthurus, Atlantic croaker, Mi- 

 cropogonias undulatus, and flounders, Paralichthys spp., over several 24-hour periods in the Cape Fear 

 River, near Wilmington, North Carolina. Results of analyses of variance on abundance data collected 

 at two locations indicate that these taxa exhibit distinct behavioral responses, primarily to photo- 

 period and tide, which allow them to maintain a selected position in the estuary and avoid being flushed 

 seaward. The level of response to these variables dictates ultimate residence in at least two primary 

 nurseries, the river main stem in the vicinity of the salt boundary and the tidal salt marshes. By 

 migrating to the surface at night, both spot and floimders make apparent use of tides to augment lateral 

 migration into marshes. Conversely, by tending to remain more bottom oriented at all times, Atlantic 

 croaker accumulate in greater numbers in deep water at the head of the estuary. 



Mechanisms by which larval fishes are recruited 

 to, and concentrated in, estuaries are poorly un- 

 derstood. Attempts to elucidate these mechanisms 

 suffered from the generally high degree of vari- 

 ability associated with sampling larval fish popula- 

 tions. Recognizing this, Graham (1972) employed 

 fixed nets to collect larval Atlantic herring, 

 Clupea harengus, in the Sheepscot estuary of 

 Maine. His gear offered the advantage of obtain- 

 ing synoptic samples over the entire water col- 

 umn, and because much greater volumes were 

 filtered, the variability of the data was also re- 

 duced. Consequently, he was able to infer a 

 mechanism used by Atlantic herring larvae to 

 select a specific reach of the estuary, i.e., a behav- 

 ioral response manifested by interactions between 

 depth, or location, and tidal direction. 



The importance of such interactions has not al- 

 ways been fully appreciated; e.g., Pearcy and 

 Richards ( 1962) postulated that larval fish trans- 

 port in the Mystic estuary occurred mainly in the 

 lower layer by net nontidal flows. Similarly, 

 Haven ( 1957) and Sandifer ( 1975) described utili- 

 zation of net nontidal transport in the lower layers 



•Lawler, Matusky & Skelly Engineers, One Blue Hill Plaza, 

 Pearl River, NY.; present address: Department of Biology, Vir- 

 ginia Commonwealth University, Richmond, VA 23284. 



^Lawler, Matusky & Skelly Engineers, One Blue Hill Plaza, 

 Pearl River, N.Y.; present address: Sandoz, Inc., East Hanover, 

 NJ 07936. 



'Department of Zoology, North Carolina State University, 

 Raleigh, NC 28603. 



Manuscript accepted November 1979. 

 nSHERY BULLETIN: VOL. 78, NO. 2, 1980.. 



for fishes and invertebrates in the Chesapeake 

 Bay. However, these investigators collected lar- 

 vae during the daytime only and did not consider 

 diel migrations which may bring many larvae to 

 the surface at night (Pacheco and Grant 1968; 

 Lewis and Wilkens 1971; Williams and Porter 

 1971). Moreover, certain larval fishes (e.g., 

 menhaden ) may also frequent the upper layers to a 

 considerable extent during the day (Thayer et al. 

 in press). Thus, it is probable that the retention 

 mechanism is species-specific and involves several 

 elements as described by Bousfield (1955): 1) diel 

 changes in vertical distribution; 2) utilization of 

 the residual, or nontidal, drift seaward in the 

 upper layer and landward along the bottom; and 3) 

 changing behavioral parameters with respect to 

 tidal direction (e.g., see also Hughes 1969a, b, 

 1972; Turgeon 1976). Individual species may 

 utilize one or more of these mechanisms to reach 

 and stay within a preferred zone of the estuary, 

 from its mouth (Carriker 1951) to the headwaters 

 (Haven 1957; Turgeon 1976). 



Here we describe distributions of postlarval 

 fishes in two locations within the Cape Fear River 

 estuary, near Southport, N.C. Both sampling 

 areas were situated upriver, in an area believed to 

 constitute a primary nursery zone for several fish 

 species. Sampling was stratified by location, 

 depth, photoperiod, and tidal direction, and an 

 attempt was made to depict postlarval fish behav- 

 ior with respect to these strata. A hypothesis is 



419 



