Discussion 



Once daily periodicity in increment formation 

 was validated in sheepshead otoliths through the 

 use of tetracycline marking, it was possible to 

 determine the timing of life history transitions. 

 The time required for transition from larval to 

 juvenile stage has been determined for a number 

 of species (Brothers et al. 1983: Campana 1984). 

 The data presented in Figure 2 indicate that by 

 about 8 mm SL most sheepshead larvae have 

 disappeared from dip net collections. This coin- 

 cides with the size at beginning of transforma- 

 tion as reported by Mook (1977). The disappear- 

 ance of larvae from collections may reflect their 

 ability to avoid capture or their movement out of 

 the pelagic environment, both facilitated by in- 

 creased swimming ability acquired after meta- 

 morphosis. Sheepshead are substrate-oriented 

 fish, and a "settling" of larvae may occur at 

 metamorphosis. Using the data presented in 

 Figure 3, the minimum and ma.ximum age of an 8 

 mm fish can be roughly estimated at about 30 

 and 40 days, respectively. If daily increment for- 

 mation begins at hatching (see Brothers et al. 

 [1983] for a discussion of this assumption) then 

 the pelagic stage of the sheepshead under the 

 environmental conditions of Bayboro Harbor is 

 between about 30 and 40 days. Other species 



possessing similar ages/sizes at transition in- 

 clude the spot, Leiosiomns xanthurus. which 

 transforms at 8 mm after 40 days (Fahay 1983; 

 Warlen and Chester 1985) and the Atlantic 

 croaker, Micropogonias undulatus, which trans- 

 forms at 10 mm after 60 days (Fahay 1983; Joann 

 Lyczkowski-Shultz". The results presented here 

 suggest that sheepshead larvae develop more 

 slowly than the larvae of the closely related sea 

 bream, Archosargus rhomboidalis. Sea bream 

 larvae reared in captivity began transformation 

 at the same size as sheepshead (8 mm) but only 

 15 days after hatching (Houde and Pothoff 1976). 

 It is noteworthy that five sheepshead larvae 

 (7.9-8.1 mm), held in the laboratory during 

 tetracychne treatments, were found to have a 

 mean age of 36.8 days (range = 35-38 days) (Fig. 

 3). These comparisons must be cautiously inter- 

 preted, however, because gi'owth acceleration or 

 retardation may occur in captivity. 



Acknowledgments 



I would like to thank the employees of the 

 Florida Department of Natural Resources, 



^J. Lyczkowski-Shultz, Gulf Coast Research Laboratory, 

 P.O. Bo.x 7000, Ocean Springs, MS 39564, pers. commun. 

 March 1989. 



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OTOLITH AGE (days) 



Figure 3. — A scatter plot of length on age for larval and early juvenile sheepshead between 5 and 10 



mm SL. 



987 



