Franks et aL. Age and growth of Rachycentron canadum 



469 



4 r 



0) 

 CT 



^ 2 



A 



y= 5.826 -0.750X 

 r= = 0.917 



-*- 



+ — ^ - '^ - ■^ 



0123456789 10 11 

 Age (years) 



Figure 9 



Length-converted catch curve for cobia taken in the 

 northeastern Gulf of Mexico recreational fishery. The 

 solid line described by the equation ( Y=a+6X) indicates 

 the age range used in regression estimates of instanta- 

 neous total mortality (Z). Z is equal to the absolute 

 value of the slope (6) of the regression line. 



use in assessment studies of cobia from the north- 

 eastern Gulf. 



Cobia were fully recruited to the recreational fish- 

 ery in the northeastern Gulf at age 4. Catch curve 

 analysis predicted a Z of 0.75. A fairly broad age struc- 

 ture and a low value for Z suggest that the north- 

 eastern Gulf population of cobia is reasonably 

 healthy. We believe our estimate of Z is reliable, al- 

 though several authors (Rounsefell and Everhart, 

 1953; Johnson, et al., 1983; and Manooch et al., 1987 ) 

 caution against using catch curves to predict mor- 

 tality for migratory pelagic species because, in part, 

 such predictions are subject to a variety of assump- 

 tions, including a constant recruitment and mortal- 

 ity for each year and year class comprising a pooled 

 data set. The popularity of cobia warrants contin- 

 ued monitoring of population age structure and 

 growth parameters of this valuable gamefish in the 

 northern Gulf. 



Acknowledgments 



We are indebted to the anglers who allowed us to 

 sample their catch of cobia. We thank Barbara Palko, 

 formerly of the National Marine Fisheries Service 

 (NMFS), Panama Citv (Florida) Laboratory, for mak- 



ing numerous specimens available to us. We express 

 our appreciation to Thomas Mcllwain of the NMFS 

 and Richard Leard of the Gulf of Mexico Fishery 

 Management Council for their encouragement and 

 advocacy of our work, particularly in the initial stages 

 of the study. Many thanks to our colleagues in the 

 Marine Fisheries Division of the Mississippi Depart- 

 ment of Marine Resources. Chuck Wilson, Bruce 

 Thompson, and Louise Stanley of Louisiana State 

 University, Coastal Fisheries Institute, provided 

 valuable advice and great inspiration. We express 

 our sincere gratitude to Mike Allen and Dyan Wil- 

 son for their diligence at the otolith saw and their 

 assistance in reading otoliths. We acknowledge the 

 many contributions of T. J. Becker, the archetypal 

 tournament sampler. We thank Robin Overstreet for 

 photographing the otoliths shown in this manuscript 

 and for sharing with us his interest in the biology of 

 cobia over many years. Many other individuals par- 

 ticipated in portions of this study including the fol- 

 lowing Gulf Coast Research Laboratory personnel: 

 Don Barnes, Lisa Engel, Dale Fremin, Nikola Garber, 

 Nate Jordan, David Lee, Jeffery Lotz, Terry McBee, 

 Casey Nicholson, Steve Vanderkooy, and Mike Zuber. 

 We also offer thanks to Michael Murphy and Roy 

 Crabtree of the Florida Marine Research Institute 

 and to James Duffey of the Alabama Department of 

 Conservation and Natural Resources for their val- 

 ued advice and assistance on statistical treatment 

 of the data. We recognize colleagues Patricia Biesiot 

 of the University of Southern Mississippi, and Joe 

 Smith and John Merriner of the NMFS Beaufort 

 (North Carolina) Laboratory who share with us a 

 deep appreciation for this great fish. We thank three 

 anonymous reviewers and the scientific editor for their 

 extremely helpful comments and suggestions. This 

 work was supported in part by funding fi'om Federal 

 Aid in Sport Fish Restoration, Department of the Inte- 

 rior, U S. Fish and Wildlife Service, Atlanta, GA, Project 

 No. F-91, and the Mississippi Department of Marine 

 Resources, Biloxi, Mississippi. 



Literature cited 



Beamish, R. J., and G. A. MacFarlane 



1983. The forgotten requirement for age validation in fish- 

 eries biolog\'. Trans. Am. Fish. Soc. 112:73.5-743. 

 Beckman, D. W., A. L. Stanley, J. H. Render, and 

 C. A. Wilson. 



1990. Age and growth of black drum in Louisiana waters 

 of the Gulf of Mexico. Trans. Am. Fish. Soc. 119:.537-544. 

 Berkeley, S. A., and E. D. Houde. 



1983. Age determination of the broadbillswordfish.X(p/i;as 

 gladius. from the Straits of Florida, using anal fin spine 

 sections. U. S. Dep. Commer, NOAA Tech. Rep. NMFS 

 8:137-143. 



