SOGARD ET AL : LARVAL OLU.F MENHADEN. ATLANTIC CROAKER. AND SPOT 



the samples and counted. Gulf menhaden were 

 measured (standard or notochord length) to the 

 nearest 0.01 mm with an ocular micrometer. 

 When more than 30 menhaden occurred in a sam- 

 ple, 30 individuals were randomly selected and 

 measured (length measurements were not cor- 

 rected for shrinkage). Due to the scarcity of lar- 

 vae at outer stations, menhaden from both the 91 

 and 183 m Southwest Pass stations were com- 

 bined in the comparison of offshore size distribu- 

 tion with inshore (restricted to the Southwest 

 Pass station to allow valid comparison). Spot and 

 Atlantic croaker were too rare at all offshore sta- 

 tions to allow size comparisons. 



Analysis of vertical movement was based on 

 the mean percentage of larvae caught at each of 

 three discrete depths on each MOCNESS cast, 

 allowing comparison among casts with widely 

 varying densities of larvae. Gulf menhaden 

 caught at the inshore stations were divided into 

 three size classes to determine if vertical distribu- 

 tion varied with age. Because of the low number 

 of larvae at the 183 m stations (see above), analy- 

 sis of offshore vertical distribution was based on 

 MOCNESS casts from both the 91 and 183 m sta- 

 tions. Mean densities at each depth were also cal- 

 culated for each time of sampling. 



RESULTS 



Overall densities (number x 100 m •^) of gulf 

 menhaden, spot, Atlantic croaker, and the total of 

 all species (these three species plus all others, 

 including damaged and unidentifiable clupeids 

 that may have been gulf menhaden) varied 

 widely among cruises, stations, times, and 

 depths. The majority of the 529 net tows did not 

 catch any gulf menhaden (67%), spot (83%), or 

 Atlantic croaker (82%). Smaller individuals of all 

 three species, however, were probably not re- 

 tained by the 505 jjim mesh nets. In all but four 

 cases, gulf menhaden were more abundant than 

 spot or Atlantic croaker (Table 1). The density of 

 all three species was generally greatest at the 

 inshore (18 m) stations and declined offshore, 

 with low or zero densities common at both off- 

 shore stations of Cape San Bias and Galveston 

 (Table 1). 



Gulf menhaden were most abundant at the 

 Southwest Pass stations, except on the December 

 1980 cruise, when they were most abundant at 

 the 18 m Cape San Bias station (Table 1). Atlantic 

 croaker larvae were most abundant at the inshore 

 Southwest Pass station in December 1980 and 

 February 1981, but not in February 1980. Spot 



Table 1 — Mean densities (SD in parentheses) of ichthyoplankton (larvae  100 m 3) 

 collected at three stations at three sites in the northern Gulf of IVIexico. Densities are 

 averaged over three discrete depths and four times of day. Station 1 was over the 18 

 m isobath, station 2 over the 91 m isobath, and station 3 over the 183 m iso- 

 bath, n = number of net tows. "Total larvae" includes the three target species and 

 all others. 



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