Discussion 



The distribution and abundance of sciaenid larvae 

 differed between the two spawning seasons studied. 

 The estimated numbers of spot and Atlantic croaker 

 larvae present at selected stations (those that were 

 sampled each month and could be compared be- 

 tween the two years) in each of the three sampling 

 areas indicated that during the 1973-74 season the 

 numbers of spot and Atlantic croaker larvae were 34 

 and 87% less, respectively, than during the previous 

 year, and that the combined number of larvae for 

 both species was down 55%. This decline could have 

 resulted from a higher mortality of eggs or early-stage 

 larvae, from stations being missed (other than select- 

 ed stations), from peaks of abundance being missed, 

 or from a true decline in numbers present. 



Nelson et al. (1978) found that inshore and offshore 

 Ekman transport mechanisms affected the abun- 

 dance of Atlantic menhaden larvae in the surface 

 waters of Onslow Bay. Since no comparable studies 6 

 have been conducted for spot and Atlantic croaker 

 larvae in these offshore areas, we must infer move- 

 ment and drift patterns of these species from studies 

 within the estuaries, and by relating egg and larval 

 stages to their likely position in the water column as 

 they move from offshore towards an inlet. 



Eggs and preflexion sciaenid larvae are buoyant 

 and would be near the surface waters where they 

 would respond to the forces of Ekman transport; 

 however, larvae probably become demersal during 

 the flexion and postflexion stages. As this "settling 

 out" phenomenon occurs, they would be less affected 

 by the Ekman transport of surface waters and more 

 affected by other forces (inertia and Coriolis) that oc- 

 cur at greater depths. 



Within the estuary, young spot and Atlantic croaker 

 larvae are abundant in the middle and lower layers of 

 the water column during the day, while at night they 

 tend to accumulate near the surface (Wallace 1941; 

 Haven 1957; Lewis and Wilkens 1971; Weinstein et 

 al. 1980). Most sciaenid larvae, except for the earlier 

 stages, are probably within these layers in Onslow 

 Bay. Stefansson et al. (1971) found that the circula- 

 tion pattern of water in Onslow Bay was progressive- 

 ly changing seasonally, but that there was always an 

 indication of a general counterclockwise eddy. Un- 

 published field and experimental observations at the 

 Southeast Fisheries Center Beaufort Laboratory of 

 the National Marine Fisheries Service indicate that 



small sciaenid larvae are able to swim against the 

 current for brief periods of time. Sciaenid larvae 

 probably are able to move into the estuaries from 

 offshore by a combination of swimming, resting on or 

 near the bottom, and drifting with the current or 

 water mass. 



Even though spot and Atlantic croaker larvae were 

 taken over the same general time period each year, 

 some differences between the two species occurred 

 in spawning time, movement, abundance, and larval 

 size. Atlantic croaker spawned before October of 

 each year, since larvae were captured in relatively 

 large numbers in the ocean in October, about 2 mo 

 earlier than spot larvae (Tables 2,3). Atlantic croaker 

 larvae were first captured in the estuary in either 

 September or October of each year, whereas no spot 

 larva was caught in the estuary until January, about 4 

 mo later than the Atlantic croaker. 



Atlantic croaker larvae caught in October and 

 November at offshore and inshore stations were 

 generally larger than spot larvae caught in November 

 and December (Tables 2, 3). This larger size, 

 however, was not maintained. After the initial 2 mo, 

 mean lengths of the Atlantic croaker were usually less 

 than those of spot in comparable time periods and 

 sampling areas. Also, Atlantic croaker larvae caught 

 in inside waters were noticeably smaller than spot 

 larvae. This size difference between the two species, 

 and the fact that Atlantic croaker larvae were cap- 

 tured in the estuary much earlier in the season than 

 spot, indicate that Atlantic croaker spawn earlier in 

 the season and/or closer to shore than spot, and that 

 their larvae usually move into the estuary at a smaller 

 size and probably at an earlier age. Warlen (1982) 

 and Warlen and Chester 7 generally agreed that larval 

 size and age are related. However, during winter 

 months with water temperatures relatively low, they 

 found more age variations in larvae of similar lengths 

 than with water temperatures relatively high. 



Hildebrand and Cable (1930) believed that spot 

 and Atlantic croaker spawned relatively nearshore in 

 the Beaufort, N.C., area and that the principal spawn- 

 ing months were December-January for spot and 

 October-March for Atlantic croaker. Powles and 

 Stender (1978) reported that spot and Atlantic 

 croaker spawn over the continental shelf in the South 

 Atlantic Bight. We found that spot spawn more 

 heavily offshore than inshore, whereas Atlantic 

 croaker spawn with about equal intensity in both 



6 Cape Fear River Estuary studies in southern Onslow Bay were 

 done under different hydrographic conditions than we encountered 

 in the current study. 



7 Warlen, S. M, and A. J. Chester. 1982. Age, growth and distribu- 

 tion of larval/early juvenile spot, Leiostomus xanthurus, off North 

 Carolina. Unpubl. manuscr., 25 p. Southeast Fisheries Center 

 Beaufort Laboratory, National Marine Fisheries Service, NOAA, 

 Beaufort, N.C. 



410 



