186 



Field validation of annular growth 

 rings in the American eel, 

 Anguilla rostrata, using 

 tetracycline-marked otoliths 



Kenneth Oliveira 



Department of Zoology. University of Rhode Island 

 Kingston. Rhode Island 0288 1 



Present Address. Department of Biology University of Massachusetts Dartmouth 

 North Dartmouth, Massachusetts 02747 



The American eel, Anguilla ro- 

 strata, is a catadromous species 

 which enters freshwater or estua- 

 rine habitats as a glass eel after 

 metamorphosing from the plank- 

 tonic leptocephalus larval stage. 

 The glass eel phase is followed by 

 the yellow phase, which is the pri- 

 mary feeding and growth stage. 

 The yellow phase is maintained 

 until a critical length is obtained, 

 at which time the eel undergoes 

 another metamorphosis to the sil- 

 ver stage and begins a return mi- 

 gration to the Sargasso Sea (Frost, 

 1945; Helfman et al., 1987). The age 

 at which the eel reaches the size 

 necessary for the silver metamor- 

 phosis is an important factor in 

 understanding the growth of the eel 

 in specific habitats. 



Age determination of anguillid 

 eels is restricted to otolith analysis 

 (Tesch, 1977). Numerous otolith ex- 

 amination techniques exist (sum- 

 marized by Panfili et al., 1990). The 

 common problems with present 

 methods are the subjective nature 

 of interpreting annuli and the dif- 

 ficulty in distinguishing false rings 

 (Deelder, 1976, 1981; Michaud et 

 al., 1988). These problems can be 

 reduced by validating the pei iodic- 

 ity of otolith ring formation Once 

 the presence of true annuli is es- 

 tablished, false (supernumerary) 

 rings can be more easily discerned. 



Several annulus validation meth- 

 ods have been reported. These have 

 used eels of known age {Anguilla 

 rostrata: Liew, 1974), tagged eels in 

 the field (A. anguilla: Berg, 1985), 

 and tetracycline- or calcein-treated 

 eels in captive ponds (A. anguilla: 

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

 1992; Dekker 1 ). Chisnall and Kalish 

 (1993) used tetracycline-treated 

 otoliths to confirm the periodicity 

 of ring formation in wild popula- 

 tions of Anguilla dieffenbachii and 

 A. australis. No validation study 

 has been done for any wild popula- 

 tion of American eels. This study 

 validates annular ring formation 

 and describes supernumerary 

 zones for tetracycline-injected Ameri- 

 can eels from a Rhode Island river. 



Materials and methods 



Eels were collected from the Ann- 

 aquatucket River, North Kings- 

 town, RI (lat. 41°30'N , long. 71° 

 22'W), a tributary of Narragansett 

 Bay. Collections were made from 

 seven locations along a 5.5-km sec- 

 tion of the river (Fig. 1). 



Yellow-phase American eels, 

 Anguilla rostrata, were collected in 

 August 1990 and September 1990. 

 Eels were collected with a Smith- 

 Root model 12 Electrofisher. Cap- 

 tured eels were anesthetized in MS- 



222, measured (total length, TL), 

 and individually marked with liq- 

 uid nitrogen-cooled brands. Four 

 hundred eels less than 30 cm TL 

 were given an intraperitoneal injec- 

 tion of 75 mg tetracycline/kg body 

 weight (after Dekker 1 ). Eels larger 

 than 30 cm were not injected ow- 

 ing to the high probability of mi- 

 gration out of the river in the fall. 

 Most eels (more than 95%) undergo 

 the silver-phase metamorphosis 

 and begin their seaward migration 

 out of the Annaquatucket River at 

 total lengths between 30 and 40 cm 

 (Oliveira, unpubl. data). Upon re- 

 covering from anesthesia, all eels 

 were released at the location from 

 which they were captured. 



All stations were resampled bi- 

 monthly from August 1990 to Sep- 

 tember 1992. Marked, tetracycline- 

 injected eels recaptured from April 

 through September (1991 and 

 1992) were kept for otolith exami- 

 nation. Some marked eels recap- 

 tured prior to the 1992 interval 

 were anesthetized, identified, and 

 measured prior to being released 

 again; nine of these eels were even- 

 tually recaptured and kept for 

 otolith examination. 



Both sagittal otoliths were re- 

 moved from each eel, cleaned of 

 extraneous tissue, and stored dry 

 in glass vials. Otoliths were pre- 

 pared for examination according to 

 Secor et al. ( 1992) with the follow- 

 ing modifications. One of each pair 

 was embedded in epoxy, and a 0.18 

 mm section that transected the 

 nucleus was cut with an Isomet 

 low-speed saw. The sections were 

 mounted on glass slides with Crys- 

 tal Bond 509 thermoplastic adhe- 

 sive and polished with 5-^um and 3- 

 fjm metallurgical lapping film. Pol- 

 ished sections were viewed under 



1 Dekker, W 1986. Age reading of eels 

 using tetracycline marked otoliths. ICES 

 Council Meeting 1986; Copenhagen, Den- 

 mark, 14 p. 



Manuscript accepted 5 July 1995. 

 Fishery Bulletin 94:186-189 (1996). 



