636 



Fishery Bulletin 89(4). 1991 



and night sampling periods, there was an increase in 

 abundance at 1 m in the morning hours with mean den- 

 sities at 1 and 16m being identical. 



Size composition of red drum larvae within cruises 

 remained relatively constant except during cruise 

 85-9-1 when mean size in the morning was much 

 smaller than during two previous sampling periods 

 (Table 3). Red drum larvae did not appear to be depth- 

 stratified by size. 



Vertical profiles of temperature and salinity taken 

 just prior to sampling during each of the five cruises 

 consistently showed the water column to be well-mixed 

 (Fig. 3). There was no evidence of a well-defined or per- 

 sistent thermocline to which red drum larvae might 

 orient. Mean temperature averaged over sampling 

 depths was 3°C higher during cruises 84-9-1&2 and 

 85-9-1 than during cruises 84-10-1 and 85-10. Salinity 

 varied somewhat more throughout the water column 

 than did temperature but usually differed no more than 

 2-3 ppt between September and October observations. 

 Isohaline conditions generally prevailed within the up- 

 per 12 m of the water column during all cruises except 

 84-9-1, when a difference in salinity of 5 ppt between 

 the surface and 12 m was observed throughout all three 

 sampling periods (Fig. 3A). Yet the same general pat- 

 tern in vertical stratification of red drum larvae was 

 evident within that salinity gradient as was observed 

 under more isohaline conditions. There was also no con- 

 sistent relationship between tidal stage and vertical 

 position of larvae (Fig. 2). This was especially evident 

 during cruises 84-9-1 and 85-9-1 when red drum larvae 

 showed the same general pattern in vertical distribu- 

 tion under the opposite stage of the tide. 



Moonlight intensity as influenced by moon phase, 

 time of moonrise, and presence/absence of cloud cover 

 did not appear to explain similarities or differences 

 among cruises in nighttime vertical distribution of red 

 drum larvae. Cruises 84-9-2, 85-9-1,, and 85-10 were all 

 conducted within 2 to 3 days of the new moon, i.e., 



when moonlight was at a minimum. Cruises 84-9-1 and 

 84-10-1 were both conducted 2 to 3 days after the full 

 moon, on cloudless nights with moonrise occurring 

 early in the evening, i.e., when moonlight was near 

 maximum. 



The vertical distribution of prey microzooplankton, 

 based on a single collection per depth, was compared 

 with the depth of maximum abundance of red drum lar- 

 vae at the offshore location (Table 4). Microzooplankton 

 were grouped into three categories based on examina- 

 tion of larval gut contents, net-caught zooplankton, and 

 statistical comparisons of diet and prey availability 



