Fishery Bulletin 94(1), 1996 



Figure 2 



Transverse section from a tetracycline-marked sagittal otolith taken from an 

 American eel, Anguilla rostrata, at liberty for two winters after marking. The 

 section was etched with 59c EDTAand stained with 0.01% toluidine blue. Photo- 

 graphed with both transmitted light and reflected UV light. A = tetracycline mark; 

 B = false complete ring; and C = complete ring (annulusl. The first complete ring 

 after the tetracycline mark was not fully enhanced owing to the dissolution of 

 the tetracycline mark with extensive EDTA treatment. 



present only in otoliths from eels recaptured after 

 the second winter at liberty, indicating that normal 

 complete ring formation takes place during winter 

 months. For this population of Anguilla rostrata, it 

 may be concluded that complete rings are produced 

 annually. This is in agreement with Liew ( 1974) and 

 with reports for A. anguilla (Frost, 1945; Berg, 1985; 

 Panfili et al., 1991; Mounaix, 1992). 



Two forms of false annuli were present: complete 

 and incomplete. The false rings most similar to true 

 annuli (false complete) were found only adjacent to 

 the fluorescent mark and resulted from one or a com- 

 bination of several stressful events: anesthetizing, 

 tagging, and tetracycline injection. Tagging has been 

 reported to cause growth discontinuities in the 

 otoliths of Anguilla anguilla (Berg, 1985). In the 

 present study, eels recaptured more than once, but 

 receiving no further branding or injection, did not 

 produce additional false complete rings. The lack of 

 additional false complete rings after the tetracycline 

 mark in these eels suggests that the stimulus for their 

 production was absent. Natural stimuli necessary for 

 formation of false complete rings appear to be missing. 

 Therefore, ages of eels reported for this river are not 

 overestimated by misconstrued false complete rings. 



Incomplete rings may result from environmental 

 stresses associated with a freshwater habitat ( Mounaix, 

 1992). Deelder ( 1981 ) and Liew ( 1974) suggested that 

 temperature extremes were a source of supernumer- 

 ary zone formation. The Annaquatucket River is a shal- 

 low freshwater system subject to extreme temperature 

 fluctuations in the summer months (Oliveira, unpubl. 

 data). The presence of incomplete rings throughout the 

 interval between annuli indicates that incomplete rings 

 form throughout the year. The similarity in ring for- 

 mation among otoliths collected from different locations 

 indicates that the factors influencing ring formation 

 are homogeneous in the Annaquatucket River. 



The validation of the formation of true annuli and 

 the clarification of false rings in this study facilitate 

 the use of otoliths for ageing Annaquatucket River 

 eels. Extension of these conclusions to eels from other 

 locations requires further study 



Acknowledgments 



I would like to thank William Krueger for his assis- 

 tance in all phases of this research and K. E. Hyland 

 for providing access to the UV apparatus. I am grate- 



