Abstract. -The diel pattern in 

 vertical distribution of red drum 

 Sciaenops ocellatus larvae was de- 

 scribed from plankton collections 

 taken at three depths and three time 

 periods over a 24-hour period during 

 five cruises in inner shelf waters of 

 the northcentral Gulf of Mexico (east 

 Louisiana-Mississippi- Alabama 

 region). Larvae ranging in mean size 

 from 1.7 to 5.0mm were vertically 

 stratified at both offshore and near- 

 shore locations over bottom depths 

 <25m. Diel periodicity in vertical 

 stratification was evident in four 

 cruises, with larvae being concen- 

 trated higher in the water column 

 during daylight hours than at night. 

 There was no clear relationship be- 

 tween vertical aggregation of red 

 drum larvae and temperature or 

 salinity profiles or prey microzoo- 

 plankton distribution. 



Diel Vertical Distribution of Red 

 Drum Sciaenops ocellatus Larvae 

 in the Northcentral Gulf of Mexico 



Joanne Lyczkowski-Shultz 

 John P. Steen Jr. 



Gulf Coast Research Laboratory 



P O Box 7000. Ocean Springs. Mississippi 39564 



Manuscript accepted 26 June 1991. 

 Fishery Bulletin, U.S. 89:631-641 (1991). 



Diel vertical migration has been ob- 

 served among the larvae of many 

 taxa of marine fishes in diverse en- 

 vironments ranging from estuaries to 

 the open sea (see Neilson and Perry 

 1990 for recent review), and under 

 both stratified and well-mixed hydro- 

 graphic conditions (Ahlstrom 1959, 

 Smith et al. 1978, Kendall and Nap- 

 lin 1981, Brewer and Kleppel 1986, 

 Perry and Neilson 1988). The most 

 frequently observed pattern of larval 

 movement has been daytime concen- 

 tration at lower depths with subse- 

 quent movement towards the surface 

 at night (Smith et al. 1978, Ken- 

 dall and Naplin 1981, Boehlert et al. 

 1985), but the reverse tendency has 

 also been noted (Hempel and Weikert 

 1972, Boehlert et al. 1985, Yamashita 

 et al. 1985, Sogard et al. 1987, Neil- 

 son and Perry 1990). Another pattern 

 is one where larvae are aggregated 

 at depth during daytime but become 

 more dispersed at night (Brewer and 

 Kleppel 1986, Heath et al. 1988). On- 

 togenetic differences in vertical dis- 

 tribution and migration patterns have 

 frequently been observed (Brewer 

 and Kleppel 1986, Castonguay and 

 McCleave 1987, Stephenson and 

 Power 1988). The adaptive signifi- 

 cance of vertical migration remains 

 unknown (Pearre 1979), but some in- 

 ferred or proposed advantages to fish 

 larvae include maintenance of posi- 

 tion relative to prey (Hunter and San- 

 chez 1976); predator avoidance (Zaret 

 and Suffern 1976); energy conserva- 

 tion through swimbladder inflation at 

 the surface (Hunter and Sanchez 



1976); maximization of energy intake 

 through depth-mediated thermoreg- 

 ulation (Wurtsbaugh and Neverman 

 1988); and enhancement of transport 

 to nursery grounds (Miller et al. 

 1984, Norcross and Shaw 1984, Boeh- 

 lert and Mundy 1988). Knowledge of 

 vertical distribution patterns among 

 early life stages is necessary, not 

 only to better understand early ecol- 

 ogy but also to improve the design 

 of broadscale surveys for resource 

 and population assessments using 

 the abundance of fish eggs and lar- 

 vae (Ahlstrom 1959, Stephenson 

 and Power 1988, Perry and Neilson 

 1988). 



Data presented here on diel 

 changes in vertical distribution of red 

 drum Sciaenops ocellatus larvae 

 came from an ongoing, comprehen- 

 sive investigation begun in 1983 in- 

 to the early ecology (Lyczkowski- 

 Shultz et al. 1988), age and growth 

 (Comyns et al. 1989), and spawning 

 seasonality and biomass (Comyns et 

 al. 1991) of this commercially and 

 recreationally valuable sciaenid in 

 coastal and shelf waters of the north- 

 ern Gulf of Mexico. 



Materials and methods 



Ichthyoplankton collections were 

 taken during five, 24-hour cruises in 

 September and October 1984 and 

 1985 in the general area east of the 

 Mississippi River delta and south of 

 the Mississippi barrier islands over 

 the east Louisiana-Mississippi- Ala- 

 bama shelf (Fig. 1). Discrete-depth 



63! 



