Lyczkowski-Shultz and Steen: Diel vertical distribution of Sciaenops ocellatus in northcentral Gulf of Mexico 



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waters also suggest that red drum larvae, at least 

 during daytime and under flooding conditions, do not 

 occur in large numbers in nearbottom waters of the 

 study area (Lyczkowski-Shultz and Richardson, unpubl. 

 data). Monthly daytime plankton collections were taken 

 from November 1979 to October 1980 at 16 sites in- 

 side Mississippi Sound (mean depth 3.9m), in three tidal 

 passes (mean depth 7.2m), and at two sites outside the 

 Sound located 7.4km south of Horn and Petit Bois 

 Islands (mean depth 15.6m). At each location two depth 

 strata, surface to midwater and midwater to within 

 0.5m of the bottom, were sampled separately with 

 stepped oblique hauls of an opening/closing meter net 

 (Lyczkowski-Shultz et al. 1990). Red drum larvae rang- 

 ing in size from 2.0 to 8.5 mm from 88 collections at 

 all sampling sites combined were more than twice as 

 abundant in the upper half of the water column as in 

 the lower half. This difference was even more pro- 

 nounced at the two sites outside the Sound where the 

 mean density of larvae (number/100m 3 ) was 12.3 in 

 the surface stratum and 4.0 in the bottom stratum. 

 Unlike red drum larvae, the larvae of Atlantic croaker 

 Micropogonias undulatus and spotted seatrout Cyno- 

 scion nebulosus from this same series of collections 

 were more than twice as abundant in the lower half 

 of the water column as in the upper half. 



Although the Mississippi Sound and adjacent waters 

 survey was not designed to investigate the role of tide 

 on the distribution of red drum larvae, data from the 

 three barrier island pass stations did tend to support 

 the Aransas Pass inlet observations. Ebbing flow 

 prevailed during sampling at the pass stations only 

 twice during the months when red drum larvae were 

 collected (September and October), and the only in- 

 stance when drum larvae were more abundant in near- 

 bottom (9 larvae/100 m 3 ) than in surface waters (0 lar- 

 vae) occurred during one of those ebb tide collections 

 in Petit Bois Pass. The density of red drum larvae at 

 the offshore station directly south of this pass and 

 under the same ebbing tide, however, was almost three 

 times higher in the surface stratum than in the near- 

 bottom stratum. Our observations from coastal and 

 inner shelf waters during 1984 and 1985 also indicate 

 that tidal stage has little influence on vertical position 

 of red drum in offshore waters. 



The vertical distribution of larvae found on cruise 

 84-10-1 was the only data set that deviated from this 

 generalized pattern. Larvae were concentrated at 12 m 

 during both afternoon and night sampling periods. On 

 the morning of the second day, mean density at 1 and 

 16 m was the same, suggesting that only a portion of 

 the larval red drum population had moved upward or 

 that the population was still in the process of moving 

 upward. Collections during this cruise contained many 

 large larvae (>4.0mm) and the night-to-day catch 



ratio indicated that twice as many larvae were caught 

 during nightime as in daytime collections. Gear avoid- 

 ance alone does not explain the absence of larvae at 

 1 and/or 5 m early in the cruise. The catch ratio for 

 cruise 85-9-1, when larvae >4.0mm were numerous in 

 collections (Table 3), was 0.2, indicating that more 

 larvae were captured in the daytime. 



Local conditions in biological and physical environ- 

 ments can modify diel vertical migration patterns 

 among fish larvae (Neilson and Perry 1990). The 

 "anomalous," deeper daytime distribution of red drum 

 larvae early in cruise 84-10-1 may have been related 

 to local environmental conditions. There are no data 

 on which to assess the influence of predator distribu- 

 tions during this study, and there was no apparent 

 coupling between red drum larvae and their prey. 

 Variations in tidal phase or times of sunrise and sunset 

 do not account for the different pattern observed dur- 

 ing cruise 84-10-1. Tidal phase was about the same 

 during all three cruises in 1984 (Fig. 2), and the dif- 

 ferences in time of sunrise and sunset between sam- 

 pling dates in September and October were small, only 

 17 and 36 minutes, respectively. Temperature and 

 salinity profiles during this cruise were indicative of 

 a homogeneous water column. 



Meterological conditions may have been in part 

 responsible for the distribution pattern observed dur- 

 ing cruise 84-10-1. On both the day preceding and the 

 first day of this cruise, skies were overcast and rainy 

 with winds in excess of 15 knots; whereas on the 

 morning of the second day, and during all other cruises 

 of our study, skies were clear and winds generally were 

 light. Heath et al. (1988) investigated the effects of sea- 

 surface light intensity and wind stress on the vertical 

 distribution of herring larvae. He found larval aggrega- 

 tion at depth to vary at about the same frequency as 

 light intensity, i.e., diurnally; whereas mean population 

 depth (center of density) was most influenced by wind- 

 induced mixing, with larvae being found deeper in the 

 water column at times of higher wind stress. Weather 

 conditions may also have affected the vertical distribu- 

 tion pattern observed during cruise 84-9-2. Winds dur- 

 ing this cruise were light, but for prior days winds had 

 been greater than 10 knots. Although larvae were more 

 abundant higher in the water column in the daytime 

 than at night, the diel changes in maximum abundance 

 extended deeper than in the previous cruise, occurring 

 between 5 and 12 m, instead of 1 and 5 m (Fig. 2B). 



Characterization of any dynamic biological process 

 such as vertical migration is constrained by sampling 

 design and gear (Pearre 1979). Plankton nets do not, 

 for example, yield information on whether movement 

 within a population is synchronous or asynchronous, 

 or whether rates of ascent and descent differ among 

 segments of the population. Contradictory observations 



