Age-based stock assessment of the European eel (Anguilla anguilla) is heavily biased by stocking of unmarked farmed eels

Stocking of farmed eels is a commonly used management measure across Europe, and partly Asia, to supplement local stocks that chronically suffer from poor recruitment. During the farming process, increased growth and stress-related annulus-like rings are formed, which have been hypothesised to bias ageing and growth estimation. Alizarin red S (ARS) marked European eels (Anguilla anguilla) from eel farms were used to demonstrate that these stress rings cannot be distinguished from potential true annuli. Two readers overestimated the age on average by approximately 2, and up to 7 years in blind readings. In addition, a significant positive correlation between the estimated age and the number of counted stress rings was observed. The individual number of rings inside the ARS mark (i.e., prior to stocking) was used to correct the estimated age, which led to an increase in the calculated growth rate between 18 +/- 16% and 108 +/- 48%. Furthermore, an age-based cohort model indicated that the stocking-related ageing error strongly affects estimates of total biomass, with potential effects on silver eel escapement, depending on the proportion of stocked recruits. Chemical marking of all farmed recruits in the future is proposed to enable statistically necessary individual age corrections.