ICHTHYOPLANKTON IN NERITIC WATERS OF 



THE NORTHERN GULF OF MEXICO OFF LOUISIANA: 



COMPOSITION, RELATIVE ABUNDANCE, AND SEASONALITY 



James G. Ditty 1 



ABSTRACT 



Ichthyoplankton samples were collected monthly between November 1981 and October 1982 in neritic 

 continental shelf waters off Louisiana. The survey provided the first quantitative data on the abundance 

 and seasonal occurrence of larval fishes from open coastal waters of this area. At least 48 families of 

 fishes were represented in samples that included 107 taxa, 54 of which were identified to species. Larval 

 densities were lowest during the winter and highest during the summer with a mean monthly density 

 of 208/100 m 3 . Five families accounted for about 90% of total larvae: Engraulidae, Sciaenidae, Clupeidae, 

 Carangidae, and Bothidae. The five most abundant taxa overall, in order of decreasing abundance, were 

 anchovies (Engraulidae); Atlantic croaker, Micropogonias undulatus; Atlantic thread herring, Opistho- 

 nema oglinum; gulf menhaden, Brevoortia patronus; and Atlantic bumper, Chloroscombrus chrysurus. 

 These taxa accounted for 82% of all larvae collected. Comparison of ichthyoplankton surveys throughout 

 the Gulf of Mexico showed that the 10 most abundant families contributed over 90% of total larval abun- 

 dance in coastal surveys but less than 70% in offshore surveys. Likewise, the five most abundant taxa 

 contributed over 80% of total larval abundance in all but one of the coastal surveys but less than 40% 

 in the offshore surveys. These data suggest that compared with offshore waters, there are relatively 

 fewer dominant taxa among the ichthyoplankton in neritic waters of the Gulf of Mexico. 



The northern Gulf has traditionally been one of the 

 most productive fishery areas in North America 

 (Gunter 1967), yet seasonality and abundance of 

 larval fishes from open waters are poorly known. 

 Previous studies of early life history stages in this 

 area have mainly been focused either on select taxa 

 (Turner 1969; Fore 1970, 1971; Christmas and 

 Waller 1975; Fruge 1977; Ditty 1984; Cowan 1985; 

 Shaw et al. 1985) or to surveys limited in temporal 

 and areal coverage (Walker 1978; Ditty and Trues- 

 dale 1984). Stuck and Perry (1982) surveyed the 

 ichthyoplankton community adjacent to Mississippi 

 Sound, while Marley (1983) conducted an egg survey 

 and Williams (1983) a larval fish survey of lower 

 Mobile Bay, AL. The most comprehensive studies 

 of the offshore larval ichthyofauna in the Gulf of 

 Mexico and adjacent areas were those of Finucane 

 et al. (1977) from the south Texas outer continen- 

 tal shelf; Houde et al. (1979) from the eastern Gulf 

 of Mexico off Florida; Richards (1984) from the 

 Caribbean Sea; and Powles and Stender (1976) from 

 the South Atlantic Bight area off the east coast of 

 the United States. The objective of this paper is to 

 provide quantitative data on the abundance and 

 seasonal occurrence of larval fishes from open 



Louisiana Department of Wildlife and Fisheries, Seafood Divi- 

 sion, P.O. Box 15570, Baton Rouge, LA 70895. 



coastal waters of the northern Gulf of Mexico off 

 Louisiana. 



MATERIALS AND METHODS 



Plankton samples were collected monthly between 

 November 1981 and October 1982 (except March 

 1982) in neritic continental shelf waters off Louisi- 

 ana. Samples were collected at six stations in a 3.2 

 km 2 area located about 12.9 km south-southwest of 

 Caminada Pass, in depths of 10-12 m (Fig. 1). Col- 

 lections were made with a 60 cm paired-net, open- 

 ing and closing bongo-type BNF-1 sampler 2 , each 

 net was of 0.363 mm Nitex 3 mesh. Nets were 

 lowered to depth, opened, and towed simultaneous- 

 ly, in series, at discrete depths (surface, middepth, 

 and near-bottom) for about 3 min, at a ship speed 

 of approximately 1.5 kn; all samples were collected 

 during the day. A General Oceanics (Model 2030) 

 flowmeter was placed in the mouth of each net to 

 estimate volume filtered. Samples were preserved 

 in seawater with buffered Formalin and returned 

 to the laboratory for sorting. Fish larvae were 

 removed from each net and identified to the lowest 



Manuscript accepted July 1986. 



FISHERY BULLETIN: VOL. 84, NO. 4, 1986. 



2 Tareq and Co., 8460 S.W. 68th Street, Miami, FL 33143. 

 3 Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



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