Overwintering behaviour of yellow-stage European eel (Anguilla anguilla) in a natural marine fjord system

Like many animals, northern temperate eel can enter a hibernation-like state and become dormant during the winter. Knowledge of overwintering behaviour in eel is sparse and mainly based on anecdotal observations and a few experimental studies on thermal tolerance. We studied European eel (Anguilla anguilla) overwintering behaviour in a Skagerrak fjord in Southern Norway, during three consecutive years, using an array of acoustic receivers and acoustic tags with depth and temperature sensors. We obtained results from 55 yellow eel, of which 19 were studied for one winter, 35 for two winters and one for three winters. Dormancy was inferred to begin in September for the earliest individuals and lasted until May for the last, with the majority of eel dormant from at least late October–November until mid-April. The timing of dormancy was mainly related to photoperiod and less to temperature. More than 50% of eel became dormant when day length was <9 h and became active when day length was >14 h. Approximately 10% of eel remained active during the winter and 31% of eel changed their pattern between consecutive years. Some dormant individuals exhibited activity periods that interrupted their dormancy. Eel in the outer fjord nearer the open sea became dormant before eel in the inner more freshwater part of the fjord, and were dormant longer.