The Black Sea is an isolated body of water with an unusual marine environment. Of the entire water volume, 87% of the water is anoxic and it contains high levels of hydrogen sulphide rendering it uninhabitable for marine life. The remaining 13% of the water volume is in the shallow surface water which is under severe environmental stress. Due to its isolation, only three species of marine mammals are thought to inhabit the Black Sea today, the bottlenose dolphin (Tursiops truncatus ponticus), the short-beaked common dolphin (Delphinus delphis ponticus) and the harbour porpoise (Phocoena phocoena relicta).
These three species are endemic to the Black Sea and remain genetically distinct from their counter-parts in the near-by Mediterranean. Cetaceans (including small cetaceans) in the Black Sea have been heavily hunted in past decades for their meat and oils but hunting bans that were enforced in 1966 and 1982 have resulted in a slight recovery. However all species are still either listed as endangered or vulnerable. Overfishing, eutrophication, contaminants such as DDT and invasive predators such as the ctenophore, Mnemiopsis leidyi, have also contributed to severally reduce marine mammal numbers.
Whilst hunting has ceased (although illegal hunting is thought to occur) the Black Sea is still subjected to degradation with eutrophication and overfishing thought to be the biggest threats to these species. Their original abundance was estimated between 1.5-2million cetaceans, however today’s abundance for T.truncatus ponticus is less than 1000 individuals. In the 1990s it was predicted that there was a very real risk of full disappearance of cetaceans from the Black Sea by 2010. Thankfully, all three species are still present, but very little is known about the true abundance or biology of these animals which makes protecting them extremely problematic.
Marine mammal surveys conducted as part of an EU funded FP7 project: CoCoNet (Coast to Coast Networks of Marine Protected Areas) have confirmed that these three species, although genetically distinct, show similar ecological traits to the counterparts in the Mediterranean and Atlantic. Both the harbour porpoise and bottlenose dolphin were found to inhabit near-shore coastal waters, whereas the common dolphin was only observed in deeper, offshore waters. In addition to spatial segregation, there is evidence of temporal segregation which may allow different species to inhabit the same area at different times of the year, potentially to minimise competition on an already depleted food supply. The surveys also suggested that the waters adjacent to the Danube Delta Biosphere Reserve (along the coast of Romania) were also a potentially important feeding area for the harbour porpoise and bottlenose dolphin.
There is still much to learn about these animals, and indeed about the general biology of the Black Sea. What is clear is that we must ensure we protect all our oceans and seas from risk of pollution, overfishing and chemical contamination if we are to have healthy and productive marine life.
Image Source: Whalefish