Do stocked freshwater eels migrate?: Evidence from the Baltic suggests "yes"

In response to declining catches of eels in the brackish Baltic Sea, the Swedish government stocks eels Anguilla anguilla (L.), both in lakes (mainly glass eels/elvers) and in the sea itself (mainly yellow eels). However, the degree to which these fish contribute to the spawning stock, if at all, was unknown. We collected silver eels at the exit of the Baltic Sea and analyzed indices of their maturity status. In addition, we used electron (WDS) and nuclear (microPIXE) microprobes to map out the strontium and calcium contents of their otoliths, as Sr:Ca correlates with salinity. As a calibration, we analyzed otoliths from eels collected around the Swedish coast and fresh water (0-25 psu) and derived a relationship between salinity and Sr:Ca. Our results show that, of 86 silver eels analyzed, 17 eels had Sr:Ca profiles consistent with having been stocked into fresh water, six showed patterns consistent with stocking directly into the Baltic from marine waters, and 10 showed patterns indicative of natural catadromy. In all, 31.4% of silver eels showed histories of freshwater experience, including 24% of those found outside the Baltic. Silver eels caught exiting the Baltic had higher fat contents (21.1% of body weight) than those collected in the southern Baltic near Denmark (18.6%), but differences were not significant between wild and presumed stocked fish within geographic areas. The conclusions of Tsukamoto et al. (1998), i.e. that freshwater eels are not supported by catadromous individuals, do not appear to hold for the Baltic, although it is clear that noncatadromous fish composed the majority of our silver eel samples.