Interestingly the list of marine mammals spontaneously released in the Black Sea includes also other species, like the grey seal, the northern fur seal, the Steller sea lion, the harbour seal, the Caspian seal and, possibly, some other pinnipeds. Such cases of escape/release have been known in the Black Sea since the early 1980s, but occurred also in other regions. For example, the escape of a sea lion from an aquarium to the wild is also known in the Canary Islands. Otherwise three sea lions escaped from the Prague zoo after the severe flood of 2002, they were all recaptured within a few days, but one of them managed to roam for hundreds of kilometers along the Elbe river from Prague to Dresden, before being recaptured.
|Harbour seals at Copenhagen zoo © Photo: Vibe Kjaedegaard|
The number of animals escaped and/or released in the Black Sea is unknown (but is likely around a few tens), and also the actual fate and impact of the relevant species is uncertain. It is likely that the marine mammals escaped from dolphinaria and similar facilities did never lead to established populations, however it is known that species may have a very long lag phase before getting naturalised, or showing any impact. Of course this does not mean that in the meantime they do not affect the hosting ecosystem. This is especially true in the case of long-living organisms, in which case also a single animal can have a major impact on the ecosystem. For example there is some concern that they could be a source of infections circulating in dolphinaria. In any case such introductions show that the extent of the problem can be unexpectedly large, both in terms of size of animals moved from place to place, and in terms of size of ecosystem affected.
On the other hand, a recent paper by Gladilina & colleagues (2013) highlighted some positive aspect related to the introduction of an exotic grey seal in the Black Sea. The presence of this North Atlantic species has been regularly recorded in the north-east Black Sea since 2001. Its introduction is considered the consequence of an escape from captivity. Surprisingly, no major conflicts have been recorded with fisheries, as fishermen seem to tolerate the presence of this mammal despite the little damage to fishing gears. In any case, the seal seems perfectly adapted to the new environment. This led Gladilina and colleagues to assume that the long term survival of the grey seal in the Black Sea might indicate the possibility of successful re-colonization of the area by monk seals, the only extant aboriginal pinniped in the Black Sea, disappeared at the end of the 20th Century. Hopefully this will be compatible with the growing "novel" community of marine mammals.