Baltic Eel Recruitment and Escapement; Quantitative Estimates from Survey Data

Two scientific surveys where eel is a by-catch - the Baltic International Trawl Survey and herring larvae monitoring in Kattegat – allows estimates of time series of silver eel abundance in the Baltic from 1992 and glass eel abundance in the Sound from 1981. With the assumption that the integrated number of juvenile eels and silver eels represent an accumulation of eels migrating into and out from the Baltic respectively the total natural recruitment of new eels and the escapement of migrating eels can be estimated.

The result shows a varying recruitment, declining from more than 600 to less than 100 million eels/year. This is large compared to the stocking in the Baltic area. The proportion of stocked to naturally recruited eels is approximately 10%. The escapement shows no significant trend after 1992 and averages 3 million silver eels/year. Without anthropogenic mortality the escapement predicted from the recruitment estimate, applying a 12 year age at silvering, should have been approximately 10 times higher.

This analysis is based on several uncertain assumptions. The main advantage is that the data are fishery independent and give quantitative estimates not available in other ways.