674 



Fishery Bulletin 90(4), 1992 



501 



IS <6.5 □ 6.5-10 ■tO.1-15 a 15.1-20 



24 2S 2a 30 32 



Temperature (C) 



4C^ 



24 28 32 



Salinity (ppt) 



<25 25-50 51-100 >100 



Depth Zone (m) 



Figure 3 



Distribution of cobia Rachycentron canadum larvae in the Gulf of Mexico with respect to hydrography. Larval stage/length class was 

 assigned as follows: preflexion, <6.5mmSL; flexion/early postflexion, 6.5-lO.OmmSL; late postflexion, 10.1-15.0mmSL; and transi- 

 tion larvae, 15.1-20.0 mm SL. 



Our data on egg and oil globule diameter agree with 

 historical data (Ryder 1887, Joseph et al. 1964, Rich- 

 ards 1967, Hassler and Rainville 1975) except that our 

 mean oil globule diameter (0.45 mm) is greater than 

 that found for eggs from the Chesapeake Bay area 

 (0.37mm, Richards 1967; 0.38mm, Joseph et al. 1964). 

 Only two cobia eggs are previously illustrated, one in 

 early- and the other in midstage development (Ryder 

 1887). The diameter of the early-stage egg, however, 

 is considerably smaller than that of the midstage egg, 

 and the specific identification of the early egg is 

 unclear. 



Cobia spawn during the day, since all embryos ex- 

 amined from the Gulf are at similar stages of develop- 

 ment (i.e., late stage after Ahlstrom and Moser 1980) 

 when collected during midmorning, except for one col- 

 lection of late-stage eggs near midnight. Furthermore, 

 daytime spawning cobia have been reported about 48 

 km southwest of Panama City, Florida (see Shaffer and 

 Nakamura 1989 for details) in waters we estimate at 

 82-165 m deep. Our data also show that cobia larvae 

 occur in both estuarine and shelf waters of the Gulf 

 (Figs. 1,3), primarily during May-September. The only 

 confirmed cobia eggs and yolksac larvae collected 

 together in the Gulf are from the Crystal River estuary 

 at station depths of 3-6 m. Early larvae (<6.8mm) are 

 also collected at stations within the 65-134 m isobath 

 range off Texas during September (Finucane et al. 



1978b). The location of these collections suggests that 

 some spawning also occurs on the shelf 50-90 km from 

 the coast. Offshore waters beyond the edge of the con- 

 tinental shelf are relatively well sampled during May 

 (SEAMAP 1983-87) when histological analyses in- 

 dicated adult cobia are ripe (Thompson et al. 1991), but 

 no cobia larvae were identified. Seven cobia larvae (all 

 >9.5mm) were identified from beyond the 180m depth 

 contour during this study and all were collected off the 

 Mississippi River delta. Distribution of larvae centered 

 around the Mississippi River delta, however, may 

 reflect the intensity of neuston net sampling in this area 

 rather than actual distribution of spawning adults. Only 

 two larvae were collected off Florida during a com- 

 prehensive multiyear survey of eastern Gulf waters 

 >10m, both during August (Houde et al. 1979). 



Seasonal occurrence and ecological data from along 

 the Atlantic coast of the United States support our find- 

 ings from the Gulf. Cobia eggs occur primarily between 

 May and August at surface water temperatures >20°C 

 (Joseph et al. 1964, Hassler and Rainville 1975, 

 Eldridge et al. 1977; W.F. Hettler and L. Settle, NMFS 

 Southeast Fish. Sci. Cent., Beaufort NC, pers. com- 

 mun.; P. Berrien, NMFS Northeast Fish. Sci. Cent., 

 Sandy Hook NJ, pers. commun.; D. Hammond, S.C. 

 Dep. Wildl. Mar. Resour., Charleston SC, pers. com- 

 mun.). Eggs are collected in lower Chesapeake Bay 

 (Joseph et al. 1964), inlets to North Carolina estuaries 



