THE DISTRIBUTION, ABUNDANCE, AND TRANSPORT OF 



LARVAL SCIAENIDS COLLECTED DURING WINTER AND EARLY SPRING 



FROM THE CONTINENTAL SHELF WATERS OFF WEST LOUISIANA^ 



James H Cowan, Jr.^ and Richard F. Shaw^ 



ABSTRACT 



The larvae of six species of Sciaenidae were collected in continental shelf waters off west Louisiana 

 on five midmonthly ichthyoplankton cruises from December 1981 to April 1982. Ranked in order of 

 abundance these species were sand seatrout, Cynoscion arenarius; Atlantic croaker, Micropogonias 

 undulatus; spot, Leiostomus xanthurus; black drum, Pogonias cromis; southern kingfish, Menticir- 

 rhus amencanus; and banded drum. Larimus fasciatus. Total larva density was highest in April, and 

 the high densities were associated with the coastal boundary layer, a horizontal density front caused 

 by an intrusion of fresher water onto the inner shelf that probably issued from the Atchafalaya River 

 east of the study area. Spawning by sand seatrout began in January, two months earlier than 

 previously reported, and first occurred offshore of midshelf but moved shoreward as the season 

 progressed. Analysis of length-frequency data suggest that spot probably began to spawn in Novem- 

 ber, one month earlier than once thought. Both sand seatrout and Atlantic croaker larvae were 

 captured at higher rates at night than during the daytime. Sand seatrout larvae appear to be 

 somewhat surface oriented while spot may undergo vertical migration. Interpretation of the sciaenid 

 data support a previously developed transport hypothesis involving gulf menhaden larvae and west- 

 northwest alongshore advection within and just outside of a horizontally stratified coastal boundary 

 layer. 



Members of the perciform family Sciaenidae are 

 an important sport and commercial fishery re- 

 source along the United States coast of the Gulf of 

 Mexico and are perhaps the most prominent 

 group of northern Gulf inshore fishes. Sciaenids 

 exceed all other families in numbers of species 

 (18) and in numbers of individuals or biomass; 

 they are among the top four families with Mugili- 

 dae, Engraulidae, and Clupeidae (Gunter 1938, 

 1945; Moore et al. 1970; Franks et al. 1972; Hoese 

 and Moore 1977). Of the six species of sciaenids 

 captured during this study, only the banded 

 drum, Larimus fasciatus, is not commonly sought 

 by both sport and commercial fishermen. 



Many of Louisiana's sciaenids spawn in coastal 

 or offshore waters. They have pelagic eggs and 

 young which are then transported into estuaries 

 (Johnson 1978 for review). The seasonal impor- 

 tance of Louisiana's estuaries as nursery grounds 



'Louisiana State University Contribution No. LSU-CFI-86- 

 08. 



^Center for Wetland Resources, Louisiana State University, 

 Baton Rouge, LA 70803-7503; present address: Center for Envi- 

 ronmental and Estuarine Studies, University of Maryland, 

 Chesapeake Biological Laboratory, Box 38, Solomons, MD 

 20688-0038. 



^Center for Wetland Resources, Louisiana State University, 

 Baton Rouge, LA 70803-7503. 



for postlarval and juvenile sciaenids is well docu- 

 mented (Cowan 1985 for review), and several 

 summary works are available which contain tax- 

 onomic and biological information on adult 

 sciaenids (Pearson 1929; Suttkus 1955; Guest and 

 Gunter 1958; Hoese and Moore 1977; Johnson 

 1978; Powles and Stender 1978; Barger and John- 

 son 1980; Barger and Williams 1980; Mercer 

 1984a, b). In contrast, there is little information 

 about sciaenid ichthyoplankton assemblages in 

 Gulf continental shelf waters, their offshore and 

 coastal distribution, or the oceanic current sys- 

 tems which influence their estuarine recruit- 

 ment. 



This study provides such early life history in- 

 formation by determining larva distribution, 

 abundance, and length frequency; by document- 

 ing spawning location (depth and distance from 

 shore) of winter and early spring-spawned 

 sciaenids off west Louisiana; and by analyzing 

 larval sciaenid distribution with respect to known 

 water circulation patterns and a larval gulf men- 

 haden, Brevoortia patronus, transport hypothesis 

 in the shelf waters of the northwestern Gulf of 

 Mexico (Shaw et al. 1985b). Recruitment implica- 

 tions of the observed distribution, larva age struc- 

 ture, and transport of sciaenids in Louisiana 

 waters are also discussed. 



Manuscript accepted September 1987. 

 FISHERY BULLETIN: VOL 86. NO. 1, 1988. 



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