456 



Fishery Bulletin 88(3). 1990 



30°- 



25°- 



20°- 



Figure 13 



Summary of available collections of lar- 

 val, young (see text foonotes 1 and 4. 

 and juvenile Polydactylus octonemus 

 from surface waters in the Gulf of Mex- 

 ico. Reference numbers correspond to 

 those in Table 7 where collection infor- 

 mation is given. 



the occurrence of downcoast alongshore coastal cur- 

 rents (towards Mexico) and onshore surface Ekman 

 transport caused by downcoast wind stress along much 

 of the Texas and western Louisiana coast from Aug- 

 ust-September through April-May (Kelly et al. 1981). 

 When alongshore currents are downcoast, an eastward 

 or northeastward counterflow along the shelf break 

 forms a cyclonic gyre on the continental shelf off Texas 

 and western Louisiana (Kelly et al. 1984). The along- 

 shore components of this gyre probably transport, in 

 merry-go-round fashion, eggs, larvae, and juvenile P. 

 octonemus from the outer shelf and slope to their shal- 

 low nurseries. Portions of this current not diverted 

 westward at the Mississippi Delta (Kelly et al. 1983) 

 may carry eggs and larvae east towards the west coast 

 of Florida where older stages of P. octonemus have 

 been collected in the surf and estuaries (Powell et al. 

 1972, Ogren and Brusher 1977) even though, as noted, 

 larvae have not been found. Shaw et al. (1985) sug- 

 gested a similar model for Gulf menhaden Brevoortia 

 pat r onus. 



Onshore surface Ekman transport components of the 

 gyre seemingly could transport young P. octonemus 

 directly inshore in the northwestern Gulf; however, this 

 does not seem to be the case. Unlike P. octonemus, 

 fishes of the brown shrimp community which spawn 

 offshore and have pelagic eggs and larvae do not ap- 

 pear to be transported into the white shrimp commu- 

 nity; they are basically absent there (Chittenden and 

 McEachran 1976). 



Depending on how long the young remain pelagic, 

 current regimes are such that the spawning areas that 

 produce young P. octonemus of the northwestern Gulf 

 can lie almost anywhere in the western or central Gulf. 

 Recent studies (Parker et al. 1979) cited in Rezak et 

 al. (1983) found almost all surface drifters released west 

 of a line from the Mississippi delta to the middle of the 

 Yucatan Straits washed ashore along Texas; those 

 released to the east were almost all found outside the 

 Gulf along the Atlantic coast of Florida. This probably 

 reflects a well-documented picture of Gulf circulation 

 (Nowlin and McLellan 1967, Nowlin 1972, Behringer 

 et al. 1977, Merrell and Morrison 1981, Merrell and 

 Vasquez 1983): The major driving force for nearsur- 

 face circulation in the deep eastern Gulf is the Loop 

 Current which enters the GuJf via the Yucatan Straits, 

 loops toward Alabama, and turns back to exit the Gulf 

 via the Florida Straits. The major driving force in the 

 western Gulf is a large, permanent anticyclonic gyre, 

 centered at 23.5°N, and maintained by consistent 

 pinching off and westward drift of Loop Current rings 

 (Merrell and Morrison 1981) or wind stress curl across 

 the western Gulf (Sturges and Blaha 1976, Merrell and 

 Vasquez 1983). South of this gyre is a cyclonic gyre in 

 the Bay of Campeche from which water joins a pre- 

 dominantly northerly flow along the Mexican shelf 

 (Nowlin 1972, Merrell and Morrison 1981, Rezak et al. 

 1983). Pelagic early stages of P. octoneynus have been 

 collected along much of the western Gulf margin (Fig. 

 13). The northerly flow there could transport pelagic 



