NOTES 



COMPARISON OF AMERICAN EEL 



GROWTH RATES FROM TAG RETURNS AND 



LENGTH-AGE ANALYSES 



Estimates of growth rates of American eel, 

 Anguilla rostrata, have been largely indirect, 

 based on projections from length-age regressions 

 or comparisons of mean lengths at particular ages 

 (Smith and Saunders 1955; Boetius and Boetius 

 1967; Ogden 1970; Bieder 1971; Gray and Andrews 

 1971; Hurley 1972; Harrell and Loyacano 1980; 

 Kolenosky and Hendry 1982). Although valid for 

 many fish species, these two approaches may be 

 questionable in eel studies because of high vari- 

 ability in lengths at given ages and because of 

 considerable overlap in lengths among ages (e.g., 

 Bertin 1956; Fahay 1978; Facey and LaBar 1981; 

 Moriarty 1983). Testing the accuracy of a length- 

 age regression as an estimator of growth rate 

 requires a simultaneous mark-recapture study. 

 Our objective was to mark and recapture eels in a 

 Georgia estuary and to compare growth data from 

 recaptures with growth estimates derived from 

 length-age regressions and mean-length-at-age 

 calculations for eels from the same population 

 captured at the same time. We also sought infor- 

 mation on seasonal growth patterns and differ- 

 ences in growth rates among size classes. 



Materials and Methods 



All American eels were captured in tidal Friday- 

 cap Creek (lat. 31°21'N, long. 81°24'W) which 

 enters the South Altamaha River, Ga., about 11 

 km from the river mouth (see Helfman et al. 1983). 

 Salinities and water temperatures ranged from 

 to 22/., and 5.5° to 31°C, respectively. Baited eel 

 traps were set at or before sunset and pulled 

 shortly after sunrise the next day. Animals were 

 anesthetized in an ice slurry or in tricaine meth- 

 anesulfonate, measured (total length), weighed, 

 tagged with 25 mm long Floy 1 FD-68B anchor 

 tags, and released where captured. We tagged 659 

 animals on eight occasions between October 1980 

 and December 1982. Growth data from eels at 

 large < 20 d were not used because of possible 

 confusion with measurement error, which aver- 



aged ± 1 mm (range = 0-5 mm, N = 35 measure- 

 ments of seven eels). 



Age determinations are based on sagittal oto- 

 lith analyses from 305 eels captured concurrently 

 with tagged animals. Most otoliths had distinct 

 opaque and transparent zones, with few apparent 

 supernumerary zones. Seasonal analysis of otolith 

 margins indicated that presumed annuli were 

 deposited on an annual basis and were a reason- 

 able chronicle of age (Helfman et al. in press). 

 Fish used in the mark-recapture study of growth 

 were not collected for histological examination 

 of gonads, and we therefore could not determine 

 if sex-related differences in growth occurred 

 (Tesch 1977). 



Results 



We recaptured 101 individuals, for an overall 

 recapture rate of 15% . Time at large ranged from 

 8 to 493 d. Recapture frequencies were 84 fish 

 recaptured once, 14 recaptured twice, 2 recaptured 

 three times, and 1 recaptured four times. 



Growth rates of recaptured eels were variable 

 but fell into two apparent seasonal categories 

 (Table 1): 1) Slow growth from November through 

 February (0.0-0.08 mm/d,x = 0.026, SD = 0.024, 

 N = 13 recaptures) and 2) fast growth during 

 spring, summer, and fall (0.01-0.63 mm/d, x = 

 0.221, SD = 0.152, N = 78 recaptures); fast period 

 growth was significantly greater U-test, P < 

 0.001). Combining averages, and assuming a slow 

 period of 4 mo, yield an average annual growth 



TABLE 1. — Growth rates of recaptured American eels as a 

 function of season and year. Values in the body of the table are 

 numbers of animals with particular growth rates. Intervals for 

 fastest and slowest rates are subdivided by 0.05 mm/d; other 

 intervals are 0.10 mm/d. 



Slow growth period 

 (Nov.-Feb.) 1 



Fast growth period 

 (Mar -Nov.) 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



'An additional 26 eels at large from late November 1982 to early May 1983, 

 i.e., encompassing primarily the slow growth period, grew an average of 0.054 

 mm/d (SD = 0.034 mm/d). 



FISHERY BULLETIN: VOL. 82, NO. 3, 1984. 



519 



