Age and growth of American Eel in lower St. Johns River in northeast Florida

Abstract Objective Little is known about American Eel Anguilla rostrata populations across much of the species' large geographic range. This study aimed to collect important information on the life history and population biology of American Eel in a northeast Florida river. Methods During two 12-week periods, from August to November 2021 and September to December 2022, American Eel were collected from a ~90-km stretch of the lower St. Johns River via boat electrofishing. Data on length, weight, age, Anguillicoloides crassus parasite infection, and swim bladder condition were recorded for American Eel collected in both years, and additional data on sex, sexual maturity, and fin and eye morphology were recorded for American Eel collected in 2022. Result Overall, biologists spent more than 63.5 h of effort over 42 sampling days to collect 297 American Eel, ranging in size from 150 to 705 mm total length (TL) with a continental age ranging from 0 to 8 years. American Eel catches were low and varied by section of river and over time; however, no differences in the length–weight relationship or size structure were observed between river sections. Adult parasites were observed in swim bladders of 36% of American Eel, and 57% of eel exhibited some level of swim bladder damage. Although length–weight relationships were similar regardless of swim bladder condition, relative swim bladder length was lower for fish with severe damage. Histological examination of gonads from 160 American Eel in 2022 revealed that the male : female sex ratio was 0.9:1 and nearly all eel were sexually immature. The majority of fish were 2–4 years of age; average growth was modeled with a von Bertalanffy curve for American Eel aged 1–8: TL = 740 × [1 – e−0.12(age + 1.98)]. Conclusion Given the paucity of information about American Eel in this region, the data provided in this study will be critical to informing future decisions on the management of the American Eel stock.