Estimating Somatic Weight From Total Weight in the European Eel ( <i>Anguilla anguilla</i> )

ABSTRACT Data on the size of an animal is essential when analysing population dynamics, growth rates or the body condition of individuals for size‐structured populations. Two common weight measurements are total body weight and somatic weight. Compared to total weight, somatic weight needs lethal sampling and is more time‐consuming to assess, but can provide a more standardised measure. Hence, the choice between measuring total or somatic weight should be carefully adapted to data needs for each specific species and monitoring program. In this study, we assessed the need to collect somatic weight compared to total weight for the European eel ( Anguilla anguilla ) from a monitoring perspective. Total weight was compared to two somatic measurements: removal of digestive tract and stomach or removal of digestive tract, stomach and gonads. Based on data from 6999 female eels and 20 males collected along the Swedish coasts, we show that for females, life stage influenced the somatic to total weight ratio, with yellow eels showing the lowest ratios. The weight ratios increased slightly with increasing length and weight. For the male yellow eel, neither length nor weight influenced the weight ratio. With the conversion factors from the linear relationship between somatic and total weight for each life stage presented here, we show that it is possible to estimate somatic weight from total weight. We also demonstrate that using the conversion factors to estimate somatic weight does not distort condition estimates—such as relative condition (Rc), Fulton's K , and Le Cren's K n —compared to calculations based on observed somatic weight. We conclude that lethal sampling of eels can be avoided while still retaining the ability to use somatic weight estimates for various end‐user purposes.