The Black Sea, a large inland sea bordered by Eastern Europe and Western Asia, supports a variety of fish species. Their distribution is heavily influenced by the sea’s distinct oceanographic conditions. Understanding these conditions provides insight into where and how fish thrive in this remarkable body of water.
The Black Sea’s Anoxic Layer
The Black Sea is distinguished by its permanent anoxic deep layer, making it the world’s largest meromictic basin where water layers do not readily mix. This condition arises from its semi-enclosed nature and significant freshwater inflow from rivers like the Danube, Dniester, and Don. This freshwater creates a less dense surface layer, while denser, saltier Mediterranean water flows in through the Bosphorus Strait and settles at the bottom.
This stratification forms a strong density barrier, a pycnocline, preventing upper, oxygen-rich waters from mixing with deeper layers. Organic matter sinks into these deep, isolated waters. Bacteria decompose this material, consuming all available dissolved oxygen. Once oxygen is depleted, anaerobic bacteria utilize sulfate, accumulating toxic hydrogen sulfide (H2S).
This extensive anoxic zone, beginning at 150 to 200 meters and extending to the seabed, accounts for over 90% of the Black Sea’s total water volume. Hydrogen sulfide makes these deep waters inhospitable to most aerobic marine organisms, restricting complex life, including fish, to the oxygenated upper layers. Recent studies indicate a decrease in oxygen penetration depth in the basin, signaling ongoing changes.
Fish Life in the Black Sea’s Upper Waters
Fish life in the Black Sea is concentrated in the oxygen-rich surface waters, extending to about 150-200 meters. This upper layer, influenced by significant freshwater input from rivers, has a lower salinity, around 17 to 18 parts per thousand, compared to the open ocean. While this reduced salinity contributes to lower species diversity compared to the Mediterranean, the abundance and productivity of species in the oxygenated zone are higher.
The Black Sea supports a diverse array of fish, including 180 to 247 species and subspecies, encompassing marine, freshwater, and brackish water inhabitants. Many species are well-adapted to the upper water column’s conditions. Some freshwater species tolerate varying salinity, allowing them to utilize food resources in coastal marine areas. This adaptability enables them to thrive above the anoxic depths.
Common Fish Species and Fishing Activities
The Black Sea is home to a variety of fish species, many of which hold commercial significance for the surrounding countries. Notable species include:
- Sprat
- Anchovy
- Horse mackerel
- Various types of mullet
- Black Sea shad
- Bluefish
- Red mullet
- Turbot
- Spiny dogfish
- Sturgeons (beluga, Russian, stellate), though critically endangered
Fishing activities in the Black Sea involve both commercial and recreational sectors. Commercial fishing targets species like anchovy, horse mackerel, and whiting, which are particularly abundant and form a significant portion of the total catch. Turkey accounts for a substantial share of total landings. Small-scale coastal fisheries are prevalent, with over 90% of the European Union’s Black Sea fleet consisting of such vessels, providing considerable social and economic benefits to local communities. Recreational fishing also has a high participation rate, particularly in the Turkish Black Sea region, contributing to local economies.
Protecting Black Sea Fish
Black Sea fish populations face numerous pressures, including pollution, overfishing, and invasive species. Land-based pollution, including industrial waste and marine litter, degrades water quality. Overfishing has led to a concerning decline, with 74% to 85% of fish stocks overexploited. Invasive species disrupt the ecological balance, stressing native fish populations.
Protecting Black Sea fish involves conservation measures and international cooperation. Marine Protected Areas (MPAs) limit harmful activities and allow ecosystems to regenerate. Controls on illegal fishing, including coastal surveillance and ship monitoring systems, promote sustainable practices. International bodies, such as the General Fisheries Commission for the Mediterranean (GFCM) and the European Union, work with riparian countries to develop and enforce regulations. These include catch limits and seasonal bans, ensuring the long-term health of fish stocks and the marine environment.