NOTE Oliveira: Field validation of annular growth rings irAnguilla rostrata t 



187 



Figure 1 



Map of collection sites in the Annaquatucket River, a tributary of the Narragansett 

 Bay, Rhode Island. The inset shows the location of the river to the Bay as well as 

 the Atlantic Ocean. Yellow-phase American eels, Anguilla rostrata, were 

 electrofished from stations 1-7 in August and September 1990. 



fluorescent light with a compound microscope at 40x 

 magnification. When a fluorescent ring was visible, 

 a preliminary count of the number of rings after the 

 tetracycline mark was made. The sections were then 

 etched for one minute with a 5% ethylenediamin- 

 etetraacetic acid (EDTA) solution and stained for two 

 minutes with 0.01% toluidine blue. This procedure 

 enhanced all otolith rings and enabled the differen- 

 tiation of supernumerary zones. A 3-minute treat- 

 ment with both EDTA and toluidine blue produced 

 greater resolution of rings but decreased the inten- 

 sity of the fluorescent marl The otoliths were also 

 observed under both transmitted light and reflected 

 UV light so that the tetracycline mark and ring for- 

 mations were visible simultaneously. All opaque ring 

 formations (complete and incomplete) described were 

 observed with transmitted light. 



Results 



Nine tetracycline-injected eels were recaptured af- 

 ter the first winter and 11 after the second winter at 

 liberty. Examination under UV-light revealed a fluo- 

 rescent mark on all otoliths. All otoliths showed a 

 distinctive, complete opaque ring aligned with the 

 external edge of the tetracycline mark (Fig. 2). This 

 ring (false complete) was distinguishable from other 

 complete rings because of its association with the 

 tetracycline mark and its atypical spacing compared 



with other complete rings (Fig. 2). The association of 

 these rings and the tetracycline marks was the same 

 for all eels, whether tagged in early August or late Sep- 

 tember; therefore no seasonal effect was apparent. 



Eels were recaptured from all locations except sta- 

 tion 5: 3 from station 1, 3 from station 2, 1 from station 

 3, 4 from station 4, 2 from station 6, and 7 from station 

 7. Nine eels were recaptured more than once (4 twice, 

 3 three times, and 2 four times), but only one false com- 

 plete ring was observed on each otolith. These nine eels 

 were all recaptured twice prior to the first winter. 



After the fluorescent mark and its associated false 

 ring, one complete ring (an opaque zone which com- 

 pletely encircled the otolith, as seen with transmit- 

 ted light) for each winter was visible on all otoliths 

 (Fig. 2). Otoliths from all locations had a similar tet- 

 racycline mark and false ring arrangement as well 

 as only one new ring for each winter. 



Many otoliths had numerous incomplete rings 

 made of narrow opaque zones which did not extend 

 completely around the otolith. These incomplete rings 

 were located throughout the translucent zone 

 bounded by complete rings (Fig. 2). 



Discussion 



All otoliths showed a single complete ring (annulus) 

 for each winter after the tetracycline mark. Two com- 

 plete rings beyond the tetracycline mark were 



