SOGARD ET AL : LARVAL GULF MENHADEN, ATLANTIC CROAKER. AND SPOT 



Spot 



1 



6 



12 



J L. 



J I 1 1 



B- 



I I I I 



Croaker 



_i I I I 



13- 



j I I I 



Total Larvae 



1 

 6E 



12 



J L 



I I 



-I J I 



.25 .75 .25 .75 .25 .75 .25 .75 

 0600 1200 1800 2400 



TIME 



Figure 3. — Mean percentage of Atlantic croaker, spot, and total larvae collected at 

 three discrete depths at inshore stations (18 m isobath). Error bars are standard 

 errors. 



mean densities were greater in surface waters at 

 1200 and 1800 h and in deeper waters at 2400 h 

 (Table 3). The high mean densities of spot in sur- 

 face waters at 1800 h and at 12 m at 2400 h (Table 

 3) were related to the encounter with the previ- 

 ously mentioned patch of larvae at Cape San 

 Bias. When mean relative proportions were con- 

 sidered, however, these trends were moderated 

 (Fig. 3). Although mean densities suggested a 

 propensity for Atlantic croaker larvae to occur in 

 deeper waters (Table 3), this trend also weakened 

 when relative proportions were considered (Fig. 

 3>. Gulf menhaden larvae in all three length 

 groups were highly concentrated at the surface at 

 1200 h, but showed inconsistent patterns at other 

 times (Fig. 4). 



At the offshore stations (91 and 183 m), where 

 there was a broader scale for vertical distribution, 

 total larvae were generally less abundant at the 

 deepest sampling depth (70 m, Table 3), but mean 

 relative distributions indicated only slight trends 

 (Fig. 5). Gulf menhaden larvae at offshore sta- 

 tions had gi'eater densities at the surface at all 

 times, with few larvae present at 70 m (Table 3). 

 They again occurred almost exclusively in surface 



samples at 1200 h (Fig. 5). (Spot and Atlantic 

 croaker were too rare at the offshore stations to 

 allow examination of vertical distribution.) 



Comparison of MOCNESS casts in thermally 

 stratified versus isothermal water columns indi- 

 cated that the presence of a weak thermocline did 

 not inhibit vertical movement by any of the three 

 target species or total fish larvae. Depth distribu- 

 tions were similar regardless of the thermal 

 structure of the water. In most cases where a ther- 

 mocline occurred, it was reversed, with colder 

 water overlying warmer water, and a tempera- 

 ture difference of <5 C, the result of the Missis- 

 sippi River plume. 



DISCUSSION 



High densities of gulf menhaden larvae at the 

 Southwest Pass stations support the conclusions 

 of Fore (1970) and Christmas and Waller (1975^) 

 that spawning is concentrated around the Missis- 



sChristmas, J. Y., and R. S. Waller. 1975. Location and 

 time of menhaden spawning in the Gulf of Mexico. Unpubl. 

 manuscr. Gulf Coast Laboratory, Ocean Springs, MS 39564. 



605 



