FISHERY BULLETIN: VOL. 86, NO. 1 



densities were higher offshore in the early part of 

 the study and then greater inshore during Febru- 

 ary and March. Depth group 4 had the highest 

 mean density (1.3/100 m-'^) followed by depth 

 groups 1 and 3 (0.4/100 m'^ each) and depth 

 group 2 (0.1/100 m^). Day-night comparisons 

 proved nonsignificant as a main effect for spot 

 larvae. The average catch rates for all day and 

 night tows were identical (0.5/100 m^). However, 

 the interaction between day-night and horizontal 

 tow type was statistically significant (P < 0.05). 

 In this case, vertical migration and stratification 

 may be indicated. Average catch rate of spot lar- 

 vae during the day at the surface was 0.1/100 m^, 

 while near the bottom it averaged 1.6. Con- 

 versely, nighttime average catch rate at the sur- 

 face was 1.0/100 m-^ while the near-bottom rate 

 averaged 0.04. These are very low densities but 

 the vertical differences are an order of magnitude 

 and their reversing pattern suggests that spot 

 larvae were stratified and undergoing diel verti- 

 cal migration. Daytime bottom and nighttime 

 surface average catch rates were higher than for 

 oblique (O) tows (day, = 0.45/100 m^; night, 

 O = 0.49/100 m^). Average catch rates for surface 

 and near-bottom tows, regardless of time of day, 

 were 0.5 and 0.9/100 m\ respectively. 



As previously mentioned, other larval sciaenid 

 species (i.e., black drum, banded drum, southern 

 kingfish) were collected during these cruises. 

 However, relatively few individuals were cap- 

 tured (Table 1), making information on their dis- 

 tribution inconclusive. 



TRANSPORT ANALYSIS 



Alongshore advection within and just outside 

 the coastal boundary layer in the northwestern 

 Gulf of Mexico has been hypothesized as the 

 major mechanism transporting gulf menhaden 

 larvae to the estuaries in western Louisiana, 

 rather than across-shelf transport from directly 

 offshore. In contrast, such direct across-shelf 

 transport has been demonstrated for sciaenids 

 and other species along the U.S. mid-Atlantic 

 coast (Nelson et al. 1976; Norcross and Austin 

 1981; Miller et al. 1984). The data collected for 

 sciaenid larvae (all species combined) were exam- 

 ined in light of this Gulf hypothesis. Larval 

 sciaenid densities were less than those for gulf 

 menhaden but similarities in distribution were 

 evident. Both larval sciaenid and gulf menhaden 

 densities were highest at midshelf early in the 

 study. By March and April the highest densities 



were found towards the east and inshore and were 

 associated with a horizontal density front (coastal 

 boundary layer) caused by an intrusion of fresher 

 water onto the shelf. 



The along-transect length-frequency patterns 

 exhibited by larval sciaenids and gulf menhaden 

 were also similar. No apparent increase in size 

 was seen until gulf menhaden larvae were on the 

 inner shelf or sciaenids were on the mid- to inner 

 shelf. The expected pattern of a gradual increase 

 in larva size from offshore to inshore, which 

 would result if there were significant across-shelf 

 (south to north) transport, was not evident in ei- 

 ther data set. Off the North Carolina coast, War- 

 len (1981) and Miller et at. (1984) showed that 

 ages and lengths of both spot and Atlantic 

 croaker larvae increased systematically toward 

 shore in an area where winter water currents fa- 

 vored across-shelf (west to east) transport. 



During the winter of 1981-82, moored current 

 meter data from sites H and S (Fig. 1) indicated 

 that flow was directed primarily alongshore in 

 the west-northwest direction. Several researchers 

 have reviewed the circulation in the northwest- 

 ern Gulf (Nowlin 1971; Kelly et al. 1982; Crout 

 1983). It was not until Cochrane and Kelly (1986) 

 developed their comprehensive circulation model 

 for the Louisiana-Texas continental shelf, how- 

 ever, that the ocean current patterns, which led to 

 the hypothesized larva transport model, were 

 fully documented. Flow in nearshore coastal 

 waters is westward all year except in summer 

 when it usually reverses, while farther offshore 

 flow is eastward all year (Cochrane and Kelly 

 1986). 



To quantify transport, larval sciaenid densities 

 were combined with the vertically averaged, in- 

 stantaneous current measurements. The resul- 

 tant curves present the number of larvae trans- 

 ported per unit time at each station (Fig. 5). Early 

 in the winter, highest sciaenid larva transport 

 (mostly Atlantic croaker and spot) was located 

 midshelf. Later (March and April), transport val- 

 ues were higher inshore and reflected the in- 

 crease in larval sciaenid density (primarily sand 

 seatrout). Overall, larva transport was primarily 

 westward and ranged from about 0.05 to 4.0 lar- 

 vae/meter per second. 



Although the oceanographic data collected 

 were insufficient to precisely quantify onshore 

 transport rates, an estimate was obtained by 

 using the mean current vectors from the near sur- 

 face meter at site H (Fig. 1) from 24 January to 12 

 May 1982 (14.33 cm/second alongshore westward 



138 



