SOGAHl) KT AL : LARVAL GULF MENHADKN. ATLANTIC CROAKER, AND SPOT 



sippi River Delta. In addition, Atlantic croaker 

 larvae were rarely caught except at the inshore 

 Southwest Pass station. High levels of nutrients 

 (Riley 1937) and the resultant high plankton 

 biomass in this region (Bogdanov et al. 1968) may 

 make conditions exceptionally favorable for fish 

 larvae. 



Densities of all three target species showed a 

 clear decline from inshore to offshore waters 

 (Table 1). Shaw et al. 11985b) found a similar 

 pattern for gulf menhaden larvae farther west 

 along the Louisiana coast; densities were greatest 

 in waters between the 14 and 24 m isobaths, with 

 a shift in concentration to very nearshore waters 

 by the end of the spawning season. The major 

 spawning efforts of gulf menhaden, spot, and At- 

 lantic croaker appear to occur in a relatively nar- 

 row band along the coast. 



Size-frequency distributions of gulf menhaden 

 larvae along the Southwest Pass transect showed 

 that offshore stations were populated with 

 smaller larvae on two of three cruises (Fig. 2, 

 Table 2), but off western Louisiana, Shaw et al. 

 ( 1985a I detected no difference in the size distribu- 

 tion of gulf menhaden from the 183 m isobath to 

 inshore waters, except at stations immediately 

 adjacent to shore (approximately 9 m in depth). 

 Our observed pattern of decreasing size with dis- 

 tance from shore could arise either by adults 

 spawning offshore and larvae growing as they 

 move toward estuarine nursery grounds, or from 

 serial spawning as adults move offshore during 

 the protracted spawning season. The latter pat- 

 tern is corroborated by Roithmayr and Waller 

 (1963) and Fore (1970). 



Only gulf menhaden showed clear evidence of a 

 diel pattern in vertical distribution; they were 

 concentrated almost exclusively at the surface at 

 midday, but were more vertically dispersed at 

 night at inshore stations. Size did not determine 

 which larvae descended by dusk, because the ver- 

 tical distribution was similar across all three size 

 groups. In contrast, vertical migration of yellow- 

 tail flounder, Limanda ferruginea, and Atlantic 

 herring, Clupea harengus, larvae varies with size, 

 with smaller individuals remaining closer to the 

 water surface (Smith et al. 1978; Wood 1971). 

 Depth distributions of northern anchovy, En- 

 graulis mordax, and white croaker, Genyonemus 

 lineatus. also vary with age, with older larvae 

 concentrating in deeper waters (Brewer and 

 Kleppel 1986). 



Gulf menhaden larvae >12 mm SL have de- 

 flated swimbladders bv dav and inflated swim- 



bladders at night, achieved by swallowing air at 

 the surface (Hoss and Phonlor 1984). This behav- 

 ior, common among clupeoids (Hunter and 

 Sanchez 1976; Uotani 1973), is thought to allow 

 passive depth maintenance during nonfeeding 

 hours at night (Hunter and Sanchez 1976). The 

 observed depth distribution of gulf menhaden in- 

 dicates that larvae must actively swim to stay at 

 the surface during daylight hours. Apparently, 

 the larvae slowly sink at night despite having gas 

 in their swimbladders, and are, therefore, dis- 

 tributed at various depths. Data from offshore 

 stations (Fig. 4) suggests, however, that most lar- 

 vae are able to maintain their position within the 

 upper 30 m of the water column. 



The pattern of vertical distribution of gulf men- 

 haden is opposite of that reported for numerous 

 other species, in which larvae rise toward the sur- 

 face at night and descend by day (e.g., Seliverstov 

 1974; Smith et al. 1978; Kendall and Naplin 

 1981; Sameoto 1982, 1984). A reversed pattern 

 has also been observed for Gadus macrocephalus 

 (Boehlert et al. 1985) and Ammodytes personatus 

 (Yamashita et al. 1985). Yamashita et al. (1985) 

 suggested that diurnal feeding requirements and 

 nocturnal avoidance of upwardly migrating 

 predators influence the vertical migration of Am- 

 modytes. The behavior of Atlantic menhaden, 

 Brevoortia tyrannus, is probably similar to gulf 

 menhaden, as they are also reported to be more 

 concentrated in surface waters by day than night 

 (Thayer et al. 1983). 



The presence of a weak thermocline with a gra- 

 dient of <5°C did not appear to influence the ver- 

 tical movement offish larvae in this study. Other 

 studies have reached conflicting conclusions. 

 Ahlstrom (1959) and Loeb (1980) found thermal 

 stratification with a temperature difference of 8' 

 to 10 C very important in determining vertical 

 distribution. Smith et al. (1978), Kendall and 

 Naplin (1981), and Sameoto (1982), however, 

 found that thermal gradients of 8 to 14 C did not 

 inhibit vertical migration. The depth of the water 

 column, the intensity of temperature change at 

 the thermocline, and behavior of the species in 

 question likely influence migration patterns. In 

 relatively shallow water (Smith et al. 1978; 

 Kendall and Naplin 1981; this study), thermal 

 stratification appears less of a barrier than in 

 deeper water. In this study larvae of gulf men- 

 haden, spot, and Atlantic croaker largely re- 

 mained within the upper 30 m, even when the 

 water column was well-mixed to a depth of over 

 100 m. As we found for gulf menhaden. Brewer 



607 



