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Fishery Bulletin 97(3), 1999 



males; 218 females) for each year. Because our study 

 and that by Thompson et al.^ were conducted con- 

 currently, we are unable to explain this discrepancy, 

 except to suggest differential segregation or a higher 

 mortality for males east of the delta. 



Sagittal otoliths were determined to be valid age- 

 ing structures for 7?. canadum, and alternating 

 opaque and translucent bands were most conspicu- 

 ous in the ventral lobe of otolith thin-sections. An- 

 nuli were not uniformly visible in thin-sections for 

 some fish and were occasionally obscured along the 

 ventral sulcal ridge, particularly for fish age 5 and 

 older. Marginal-increment analysis indicated that 

 annuli formed once per year during April-August. 

 Therefore, age in years for cobia was presumed equal 

 to the number of opaque bands observed in sectioned 

 sagittae, findings that agree with those of Thomp- 

 son et al.'^ off Louisiana and Smith (1995) off North 

 Carolina. Because cobia are infrequently caught in 

 northeastern Gulf waters during the winter, the scar- 

 city of otolith samples from November through March 

 precluded us from making an unequivocal assertion 

 on the annual nature of opaque band formation. 

 However, thin-sectioned sagittae from seven cobia 



caught in the Florida Keys during January 1991 and 

 sampled dockside by us showed a substantial zone 

 of translucent material extending from the distal 

 edge of the last opaque band to the otolith margin. 

 This finding suggests that winter annulus formation 

 does not occur in the otoliths of cobia from south 

 Florida waters (cobia that may migrate into north- 

 ern Gulf waters in spring). 



Although the timing of annulus formation coincides 

 with the cobia's spawning season in the northern Gulf 

 (Biesiot et al., 1994; Lotz et al.. 1996), annulus depo- 

 sition may be more related to cobia migration into 

 the northern Gulf in spring. We found that sagittae 

 of several sexually mature cobia sampled in April 

 (early part of the spawning season) already showed 

 opaque bands, as did sexually immature fish in 

 spring. The relationship of annulus formation to 



