Survival and progression rates of large European silver eel Anguilla anguilla in late freshwater and early marine phases

Abstract

The population of European silver eel Anguilla anguilla has declined tremendously in the last decades. The cause of this decline is unknown, and it is necessary to investigate the migra- tory behaviour and survival rates of silver eels during the reproductive migration in order to under- stand if the decline is related to factors acting during that migration. We estimated survival and pro- gression rates of European silver eel migrating in the lower part of the River Gudenaa and during the first phase of the marine migration in the Randers Fjord in Denmark. Fifty migrating silver eel (total body length: 56 to 84 cm) were captured, and each was equipped with an acoustic transmitter. Their migration was subsequently monitored using an array of automatic listening stations, and progres- sion rate and mortality in the river, inner part of the fjord and outer part of the fjord were estimated. Survival was high in fresh water. However, 60% of eels were lost in the inner and outer fjord, sup- porting the hypothesis that mortality is large in the early phase of the marine migration and that fish- ing may be a major cause of mortality of silver eels. There was no indication that the slowest-migrat- ing individuals were more prone to fishing mortality than the faster-migrating individuals. Progression rate increased as the eels proceeded downriver and out of the fjord. The migration was predominantly nocturnal, both in the river and fjord. Based on the available evidence, a considerable increase in eel survival in the river-fjord system will be needed in order to fulfil the goals in the Euro- pean Union recovery plan for eels.