DENSITY AND DEPTH DISTRIBUTION OF LARVAL 



GULF MENHADEN, BREVOORTIA PATRONUS, ATLANTIC CROAKER, 



MICROPOGONIAS UNDULATUS, AND SPOT, LEIOSTOMUS 



XANTHURUS, IN THE NORTHERN GULF OF MEXICO 



Susan M. Sogard,' Donald E. Hoss,- and John J. Govoni^ 



ABSTRACT 



Densities of larval gulf menhaden, Brevoortia patronus; Atlantic croaker, Micropogonias undulatus;  

 and spot, Leiostomus xanthurus. compared among three transects in the northern Gulf of Mexico, 

 indicated that all three species were more abundant at inshore ( 18 m isobath) than offshore stations 

 (91 and 183 m isobaths). Gulf menhaden and Atlantic croaker were most abundant off Southwest 

 Pass, Louisiana, a major outlet of the Mississippi River into the Gulf of Mexico. Gulf menhaden larvae 

 caught at inshore stations were larger than those collected at offshore stations. Of the three species, 

 only gulf menhaden showed any consistent pattern in vertical distribution. At inshore stations, gulf 

 menhaden were concentrated near the surface at midday, but distributed across sampling depths 

 (1 m, 6 m, and 12 m) at dawn, dusk, and midnight, a pattern opposite to that typically reported for 

 larval fish. At offshore stations (with sampling depths of 1 m, 30 m, and 70 m), gulf menhaden larvae 

 were present at 70 m, but most were caught near the surface. A concentration in surface waters was 

 again most pronounced at midday. 



Gulf menhaden, Brevoortia patronus; spot, Leios- 

 tomus xanthurus; and Atlantic croaker, Micropo- 

 gonias undulatus, are thought to spawn offshore 

 in winter months in the northern Gulf of Mexico 

 (Nelson 1969; Fore 1970; Diaz 1982; Christmas et 

 al. 1982). Larvae of the three species are trans- 

 ported inshore to nursery grounds in marshes and 

 estuaries along the northern coast. One passive 

 mechanism suggested for movement of gulf men- 

 haden includes longshore advective transport, en- 

 trainment into the coastal boundary layer, and 

 eventual transport into the estuary effected by 

 the seasonal rise of sea level in spring (Shaw et al. 

 1985a). The passage of winter cold fronts can also 

 be expected to influence transport. 



Spawning of gulf menhaden occurs in shelf 

 waters out to at least 91 m (Guillory et al. 1983), 

 but is concentrated around the Mississippi River 

 Delta (Fore 1970). Atlantic croaker apparently 

 spawn in waters <54 m in depth (Diaz 1982), 

 while spot spawn in waters >27 m (Dawson 

 1958; Nelson 1969). Gulf menhaden larvae spend 

 3 to 5 weeks at sea before entering estuaries 



'Southeast Fisheries Center Beaufort Laboratory, National 

 Marine Fisheries Service, NOAA, Beaufort, NC 28516; present 

 address: Rutgers Marine Field Station, P.O. Box 41. Tuckerton, 

 NJ 08087. 



^Southeast Fisheries Center Beaufort Laboratorv, National 

 Marine Fisheries Service, NOAA, Beaufort, NC 28516. 



Manuscript accepted March 1987. 



FISHERY BULLETIN: VOL. 85. NO. .3, 1987. 



when they are 12 to 25 mm in length (Reintjes 

 1970; Christmas and Etzold 1977; Guillory et al. 

 1983). 



Fish larvae are nonrandom in their spatial dis- 

 tribution in both the vertical and horizontal di- 

 mensions. One primary influence on the vertical 

 distribution of larvae is their diel vertical move- 

 ment (migration) in the water column; larvae of 

 many species rise to the surface by night and de- 

 scend by day (e.g.. Smith et al. 1978; Kendall 

 and Naplin 1981; Sameoto 1982, 1984). Horizon- 

 tal distribution is also dynamic, with dispersion 

 and aggregation of larvae affected by such factors 

 as adult spawning behavior, water mass move- 

 ments, localized larval mortality, and larval be- 

 havior (Smith 1981; Houde 1982; Jahn and 

 Lavenberg 1986). 



In this study we examined the density and 

 depth distribution of larval gulf menhaden, spot, 

 and Atlantic croaker at three locations in the 

 northern Gulf of Mexico, with emphasis on the 

 area around Southwest Pass, LA, the main dis- 

 charge of the Mississippi River among the delta 

 distributaries. Size distributions of gulf men- 

 haden were compared to determine if inshore lar- 

 vae were older than offshore larvae, the expected 

 pattern if adults are spawning primarily offshore 

 and larvae are moving inshore. 



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