Crabtree et al.: Age and growth of Megalops atlanticus 



627 



thus, we expected at least this percentage of otoliths 

 from captive fish to be unreadable. Indeed, it is likely 

 that captive conditions and nonseasonal food avail- 

 ability diminished the seasonal nature of otolith 

 growth in captive fish and thereby increased the dif- 

 ficulties in otolith interpretation. Our use of heaters 

 at SERF during the winter reduced the seasonal 

 change in water temperature; five of the six fish with 

 problematic otoliths were held at this facility. We 

 used the heaters at SERF during winter cold fronts 

 when water temperatures might have reached low lev- 

 els lethal to tarpon. Otoliths from all six of the tarpon 

 held in the flow-through facility at Keys Marine Labo- 

 ratory, where water at ambient temperature was con- 

 tinuously pumped from Florida Bay, showed the ex- 

 pected pattern of one annulus formed per year. 



Marginal-increment analyses also supported our 

 hypothesis that the marks we counted formed once 

 per year. The consistent marginal-increment minima 

 observed for YOY and 1-year-old tarpon suggest that 

 the marks present on otoliths of these fish were an- 

 nual marks formed during winter or spring. 



Additional support for the validity of our age esti- 

 mates comes from the life span of a captive tarpon 

 placed in the John G. Shedd Aquarium in Chicago, 

 Illinois, in November 1935. This tarpon was still alive 

 in April 1994 and was at least 59 years old 1 confirm- 

 ing that tarpon can reach ages of more than 50 years 

 as our data suggest. 



Tarpon otoliths had annuli on both the dorsal and 

 ventral sulcal ridges that were similar in appearance 

 to validated annuli in other species. Typically, an- 

 nuli on the ventral ridge were more easily distin- 

 guished, and annulus counts from this ridge were 

 usually higher than annulus counts from the dorsal 

 ridge. Some otoliths had regions where bands were 

 distorted, unclear, or confluent, making counts diffi- 

 cult or impossible. In other otoliths, portions of the 

 sulcal ridge were dark in color and annuli were ob- 

 scured. Our rejection of many otoliths as unreadable 

 could have biased our growth-parameter estimates, 

 but the size distribution of tarpon with otoliths 

 judged to be unreadable was not significantly differ- 

 ent from that of all tarpon examined for age and 

 growth. Thus, we did not systematically reject a 

 higher proportion of larger and presumably older fish 

 than smaller and presumably younger tarpon. We 

 do not know if rejected otoliths tended to come from 

 faster- or slower-growing tarpon; this is a potential 

 source of bias in our growth-parameter estimates. 



We could not sex most of the 0-, 1-, and 2-year-old 

 tarpon examined and this is another potential source 



1 Anderson. James A. 1994. Assistant Curator of Fishes, John 

 G. Shedd Aquarium, Chicago, IL. Personal commun. 



of bias in our growth models. Observed lengths at 

 age for the males and females we could sex at ages 

 0-2 were larger than the observed lengths of unsexed 

 fish (Table 4). It is likely that the 0-, 1-, and 2-year- 

 old fish we could sex were precocious and thus larger 

 than comparably aged fish that we could not sex. 

 Consequently, our sex-specific growth models were 

 biased towards larger fish at these young ages; how- 

 ever, the predicted lengths at age from both sex-spe- 

 cific growth models at ages 0, 1, and 2 were between 

 the observed lengths of sexed and unsexed fish, thus 

 this bias was probably small. In addition, because 

 both growth models included over 40 year classes, 

 this bias probably had little effect on our growth- 

 parameter estimates. 



Tarpon scales do not appear to be suitable for age 

 estimation. Scale-derived estimates of tarpon longev- 

 ity by Breder ( 1944 ) and de Menezes and Paiva ( 1966 ) 

 suggested a maximum age of only 15 years, much 

 lower than our otolith-derived estimate of 55 years 

 and the known age of captive tarpon. De Menezes 

 and Paiva ( 1966) presented scale-derived estimates 

 of von Bertalanffy growth parameters for tarpon and 

 estimated that for males, L m = 2,062 mm, #=0.084, 

 andt =0.20 and for females,! =2,633, #=0.065, and 

 t =0.17. These estimates are probably biased by a 

 consistent underestimation of ages and are consid- 

 erably different from our otolith-derived estimates. 

 Scale-derived estimates of L x are unrealistically 

 high and are much larger than the maximum size 

 documented for any tarpon. 



Acknowledgments 



We thank the Don Hawley Foundation and the Pate 

 Foundation, and in particular Capt. Mike Collins and 

 Billy Pate for their support of this project. We also 

 thank Ike Shaw Taxidermy, Pflueger Taxidermy, the 

 Boca Grande Fishing Guides Association, the Florida 

 Keys Fishing Guides Association, the Gold Cup Tar- 

 pon Tournament, the Suncoast Tarpon Roundup, and 

 Millers Marina for providing the specimens we ex- 

 amined. Many people participated in portions of this 

 study including Renee Bishop, Laura Crabtree, Chris 

 Harnden, Victor Neugebauer, Derke Snodgrass, 

 Connie Stevens, and Fred Cross and other employ- 

 ees of the Florida Game and Freshwater Fish Com- 

 mission. Jim Colvocoresses, Stu Kennedy, Judy Leiby, 

 Mike Murphy, Kevin Peters, Jim Quinn, and Ken 

 Sulak made helpful comments that improved the 

 manuscript. Llyn French assisted in the preparation 

 of figures. Grant Gilmore provided support for field 

 collections. Buck Dennis, Bill Gibbs, Bill Halstead, 

 Victor Neugebauer, and John Swanson participated 



