The spawning migration of European silver eel ( Anguilla anguilla L.) with particular reference to stocked eel in the Baltic

Abstract

Tagging investigations have been performed in the Baltic with previously stocked eels, anosmic eels, partly anosmic eels and natural eels as a control group. There were great differences between natural eels and previously stocked eels. The anosmic eels differed from natural eels in speed, direction and hibernation and agreed in these aspects more with the previously stocked eels. Among the partly anosmic eels, the group with left nostril blocked behaved like the anosmic eels while that with right nostril blocked behaved like the controls. This suggests that olfaction is essential for orientation. The stocked eels, which have never been in the Baltic before have lacked opportunities to imprint this orientation cue. In spite of that, the previously stocked eels succeeded in finding their way to the southernmost parts of the Baltic but usually only to areas very different from those of the migrating natural eels. This is tentatively explained by the occurrence of a second cue, temperature. The migration and orientation back to the spawning area in the Sargasso Sea may be a reversal of thermal and olfactory prehistory of the young stages during their period of active dispersal. The orientation capacity of eels in shelf areas like the Baltic is not a point to point event, but rather too much swimming with limited progress. Eels stocked in the Baltic drainage area have problems in finding their way out. As a consequence, the contribution to the spawning stock in the Sargasso Sea has to be questioned.