FISHERY BULLETIN: VOL 86. NO 1 



METHODS AND MATERIALS 



Detailed sampling methodology has been pre- 

 sented elsewhere (Shaw et al. 1985 a, b). Briefly, 

 larval sciaenids were collected off west Louisiana 

 on a sampling grid consisting of 37 stations on 5 

 transects (Fig. 1) during 5 midmonthly cruises 

 from December 1981 to April 1982. Ichthyoplank- 

 ton samples were analyzed from the 335 |xm mesh 

 net side of an opening and closing, 60 cm, paired 

 "bongo type" plankton sampler fitted with Gen- 

 eral Oceanics' flowmeters (model no. 2030). Most 

 plankton collections (125 of 187 total) consisted of 

 10-min stepped oblique tows from near bottom to 

 surface. Nets were set closed and opened just 

 prior to the stepped ascent. Each tow had five 

 steps with a retrieval rate between steps of 20 

 m/minute; towing speed was about 1 m/second 

 (2 knots). The object of the 10-min tow was to 

 filter approximately 100 m*^ of water. This process 

 increased the water volume filtered per unit 

 depth at the shallow stations relative to deeper 

 stations. This discrepancy is acceptable since the 

 alternative would be to compare 17-s shallow- 

 station oblique tows with 9-min deep-station tows 

 at a uniform retrieval rate (Houde 1977). At se- 

 lected stations (A-3, 6, 9; B-1; C-6; D-1; E-3, 6, 9; 

 Fig. 1), only 10-min simultaneous surface and 

 near-bottom horizontal tows (31 surface and 31 

 near-bottom) were made to determine if sciaenid 

 larvae were vertically stratified. Larva total 

 length (TL) was measured to the nearest 0.1 mm. 

 Larva densities are reported as standardized 

 catch rates at a station (density = larvae/100 m"^). 



A four-way analysis of variance (ANOVA) was 

 performed on logio transformed [(no. larvae/100 

 m^) + 1] data to determine the spatial (vertical 

 and horizontal), temporal, and diel patterns of 

 species density and distribution. The four main 

 effects tested were month (January-April); sta- 

 tion depth group (d.g.) (d.g. 1 < 10 m, 10 m < d.g. 

 2 < 14 m, 14 m < d.g. 3 < 24 m and d.g. 4 > 24 m); 

 day-night (2000 hours < night < 0500 hours); 

 and horizontal tow type (surface vs. near-bottom). 

 Data from the December cruise were not included 

 as only the A transect was completed due to ad- 

 verse weather conditions. 



Two methods of current estimates were utilized 

 (following Shaw et al. 1985b): 1) instantaneous 

 current profiles taken at each station and 

 2) continuous surface and near-bottom current 



■♦Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Services, NOAA. 



meter measurements at two sites (H and S; Fig. 

 1). The instantaneous number of larvae trans- 

 ported on each transect was calculated by using 

 the equation D x U x M = number of larvae per 

 meter per second where D = larva density 

 darvae/m') from either oblique tows or from the 

 mean of the horizontal tows (i.e., average of sur- 

 face and near-bottom catch rates), U = depth- 

 averaged water velocity (m/s) determined from 

 instantaneous current meter profiles at each sta- 

 tion, and M = water depth (m) at each station. 



Distribution diagrams and length-frequency 

 histograms were generated for each cruise for the 

 three most abundant sciaenid species. Inspection 

 of these data along with current measurements 

 allowed a comparison with the previously men- 

 tioned transport hypothesis. 



RESULTS AND DISCUSSION 

 Total Sciaenids 



A total of 5,225 larval sciaenids accounted for 

 9.1% of the fish larvae collected. In December 

 through February, samples were dominated by 

 Atlantic croaker, Micropogonias undulatus, and 

 spot, Leiostomus xanthuriis. In March and April 

 samples contained mostly sand seatrout, 

 Cynoscion arenarius. In all, six species of sciaenid 

 larvae were collected: sand seatrout (N = 4,100); 

 Atlantic croaker {N = 567); spot (A^ = 264); black 

 drum, Pogonias cromis {N = 68); southern king- 

 fish, Menticirrhus americanus (N = 53); and 

 banded drum (A'^ = 13). Additional Menticirrhus, 

 not identifiable to species, accounted for 160 more 

 specimens (Table 1). A more detailed examina- 

 tion of the data on the three most abundant 

 sciaenid species follows. 



Sand seatrout, 

 Cynoscion arenarius 



A total of 4,100 sand seatrout larvae was col- 

 lected making it the most abundant sciaenid 

 taken during the study. Larval sand seatrout den- 

 sities were highest in April (Table 1) with a mean 

 of 46.1 larvae/100 m"^; mean density in February 

 and March was 0.3 and 2.9/100 m'^, respectively, 

 and 1 larva was collected in January. Larvae 

 were distributed mostly over the midshelf in 

 February but highest concentrations were later 

 found inshore and towards the east (Fig. 1). Over 

 the course of study, larvae were found in temper- 

 atures and salinities ranging from 14° to 21°C and 



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