NAS scientist Viktor Demchenko spoke about the state of the Black Sea, the consequences of the war, and the risks to biodiversity

As a result of the full-scale war in the Black Sea, complex ecological changes have been recorded: from oil pollution and hypoxia in the marine environment to osmotic shocks and the loss of part of biodiversity after the explosion of the Kakhov dam. The marine ecosystem, already weakened by 2022, is now suffering from the consequences of the explosion of the Kakhovskaya HPP, tanker accidents in the Kerch Strait, local oil slicks, a decrease in salinity and numerous explosions. The fact that the war takes place not only on land, but also in water is no longer a hypothesis. The change in the composition of species, the death of molluscs and dolphins, the dominance of alien organisms — all this indicates a systemic imbalance. And although some ecosystems show signs of recovery, the threat of extinction of a number of species remains. About the state of the Black Sea, the consequences of war and risks to biodiversity shared Viktor Demchenko, doctor of biological sciences, deputy director for scientific work of the Institute of Marine Biology of the National Academy of Sciences of Ukraine, with his thoughts.

He emphasized that the first year of the full-scale invasion was accompanied by local oil pollution. It was not only about oil products falling into the sea during airstrikes or sinking of ships, but also about a whole array of events that created a layer of oil on the surface. This, as the scientist emphasized, led to hypoxia: oxygen did not enter the water, which had a fatal effect on marine mammals, birds and invertebrates. Subsequently, after the reduction of military activity, it would seem that the oil threat decreased, but in 2023, two events occurred that had lasting consequences: the explosion of the Kakhovskaya HPP and the Kerch tanker accident.

After the destruction of the dam, a large amount of fresh water entered the sea, which brought with it the remains of vegetation and freshwater species that are not typical for marine conditions. Demchenko pointed out that many local species died due to a decrease in salinity, in particular, in the northwestern part of the water area. Molluscs and crustaceans that did not have time to migrate could not withstand osmotic shock. In addition, the detonation of the dam led to the flooding of warehouses with petroleum products, fertilizers, pesticides and herbicides, which also entered the sea. As a result, the quality of water deteriorates, the concentration of biogens increases, and the rapid “blooming” of water caused by an excess of phytoplankton.

As for mussels, Demchenko noted that they died by 65-70% in the part of the water area where they lived on hard substrates. By the summer of 2025, most of those populations, he said, had almost fully recovered. At the same time, the composition of crustaceans underwent a restructuring: some species decreased in number, while others, which feed on dead remains, on the contrary, began to dominate. This is due to the mass death of fish and invertebrates after the ingress of fresh water, which caused a short-term enrichment of the environment with organic remains. Thus, according to the scientist, the current processes in the Black Sea are an echo of the Kakhov tragedy, even if visually the picture seems to be stabilized.

Demchenko noted that today marine biologists can conduct research only in certain zones. One of the few such training grounds is in the Maly Fontan area near Odesa, where observations have been conducted for over 40 years. All other areas of the sea remain closed to scientists due to martial law. Therefore, as the scientist admits, it is difficult to unequivocally assess which species have disappeared and which have survived. For example, there is a version that the sea pike, which lived only in the Dnieper-Buzka estuary, could completely disappear: after the Kakhovka tragedy, it was carried out to sea, and due to the change in salinity, it did not survive. This species was already listed in the Red Book.

The fate of dolphins is of particular concern. The scientist emphasized that they are extremely sensitive to noise, explosions, and oil products. Poaching, which was the main threat before the war, was reduced by the ban on shipping, but the threat of war appeared instead. Dead dolphins are found every year, but the scale is difficult to estimate. There were cases of mass death after the Kerch accident, when dead animals were thrown onto the coast of Crimea. As for fish, no mass diseases or catastrophic deaths have been recorded in the Black Sea in recent years. Problems with suffocation of fish in the estuaries, in particular in the Khadzhibey and Tyligul estuaries, are chronic. They are associated with a combination of high temperature, lack of oxygen, poor water exchange. According to the biologist, these processes are not new, but they can be mitigated due to engineering measures and ecological modernization of the water exchange.

Regarding food risks, Demchenko emphasized that from February 2022, fishing in the Black Sea has actually stopped. Some areas allow amateur fishing, but commercial fishing is carried out only in the lower Danube, Dniester and Tyligul estuaries. According to him, products entering the market usually pass the necessary sanitary and epidemiological checks.

The scientist believes that underwater explosions of mines or shells have a direct destructive effect on the habitats of fish, molluscs, and crustaceans. He urges vacationers to follow the instructions and swim only in authorized areas, because mines are not an abstract threat. At the same time, according to him, the water in the coastal zones, as a rule, meets sanitary standards and special bodies check it every year.

Demchenko admits: international mechanisms for the protection of the Black Sea are blocked. The Commission of the Black Sea Basin countries does not actually work because of Russia’s veto power. Ukraine has repeatedly raised the issue of Russia’s exclusion from interstate bodies, but, as the scientist noted, this is not stipulated in the conventions themselves. He recalled that even after the accident with 8,000 tons of fuel oil in the sea, Russia insisted that it was “not a catastrophe” and blocked any decisions.

According to Demchenko, in order to achieve compensation in the future, it is necessary to record all the environmental consequences of the war: pollution, extinction of species, reduction of water quality. These data should become the basis for lawsuits. But in many regions, marine water areas remain inaccessible, and scientists are deprived of the opportunity to conduct full-scale research. He believes that without a systematic collection of evidence, it will not be possible to bring Russia to justice even after victory.

Demchenko also drew attention to the fact that currently scientists cannot confirm the purposeful dumping of toxic substances by Russia. This cannot be verified without physical access to most water areas. At the same time, the Institute of Marine Biology analyzed satellite images and thereby fixed oil slicks. But these data, as the scientist admitted, do not have sufficient legal weight for international processes.

Several years ago, hydrologists were able to mathematically model a possible rise in the level of the Black Sea. Viktor Demchenko says that according to their estimates, the changes can be quite large-scale: the sea can rise by 30-50 cm, and this will depend on the mathematical model used by scientists. After all, the tendency to increase the water level has been observed for a long time – several millimeters are added every year. And although it is practically impossible to influence this at the regional level, globally humanity should focus on combating climate change, reducing greenhouse gas emissions, and transitioning to decarbonization. But, as Demchenko emphasizes, there is no unity in the world on this issue: some countries even withdrew from international climate agreements, declaring that joint approaches are ineffective.

However, at the local level, there are tools for adaptation to new climatic conditions. This applies to the agricultural sector, fish farming, and mariculture. In addition, as the scientist emphasizes, in the conditions of climate change, it is necessary to control the spread of invasive (alien) species much more seriously, which, adapting to the new climatic conditions, can gain a foothold in the region and displace the local fauna.

Demchenko explains that this trend is already observed: with warming, especially in winter, when the number of frosty days decreases and the ice cover on the estuaries actually disappears, the survival of alien species increases. In recent years, scientists have recorded the appearance of Japanese river shrimp in the Danube. This species was first artificially introduced into the Dniester estuary, from where it entered reservoirs with fish stocking material, and later – into the Danube. Last year, dozens of tons of such shrimp were caught in this region. They were able to adapt to the conditions of Bessarabia and are now actively spreading, making the region a potential corridor for the penetration of Mediterranean invasive species into the Black Sea.

As part of the international IASON project, the Institute registered more than 100 alien species, both animals and plants. In the Black Sea itself, Mediterranean fish and invertebrates that have adapted to local winters are already being recorded. For example, there were several finds of blue crab, individuals reaching 15 cm. Varieties of sea bass from the Mediterranean also began to appear.

As for threats, invasive species can actually crowd out native fauna. They become competitors for food, territories, breeding and wintering places. Demchenko emphasizes: when an invasive species enters a new ecosystem, it has no natural enemies, is not caught, and therefore rapidly reproduces. This results in a short-term burst of population that can harm native species. For example, the Japanese shrimp feeds on fish roe, which is deposited on plants, which can lead to a decrease in the number of certain species. However, Japanese and Black Sea shrimp are not direct competitors, because they live in different environments. Black Sea – in salt water, Japanese – in fresh. Although the latter can survive for some time in seawater, it is not able to form stable populations there.

As for another invasive species – rapana, which is often considered local, as Demchenko emphasizes, it appeared in the Black Sea only in the 1960s and was introduced artificially. Later, rapana inhabited natural reservoirs, began to actively feed on mussels and, as a result, significantly reduced their number. Currently, the mussel population in the open sea is in a depressed state. Before the start of the full-scale war, rapana was caught at 12 thousand tons a year, while fish – only 5-6 thousand tons. Thus, rapana became the main industrial resource of the Black Sea. The scientist hopes that the decrease in its number due to the war will give the mussel a chance to recover.

At the same time, the scientist refrains from quantitative assessments of the general state of the Black Sea, explaining that it is too subjective and depends on who is doing the assessment. As a specialist in ichthyology, he emphasizes that the absence of industrial fishing and the reduction of human presence have a positive effect on the condition of fish. Demchenko gives the example of sturgeons: this fish reaches sexual maturity only at the 12th year of life. In recent years, young sturgeons were able to grow up in the sea and began to return to the Danube and even to Zaporozhye to spawn. In 2025, several individuals of Russian sturgeon reached Zaporizhzhia, which was previously impossible due to the Kakhovskaya hydroelectric power plant. Active migration of these fish is also recorded in the Danube.

In addition, the absence of vacationers on the coast gives marine ecosystems a chance to recover. Millions of tourists who visited Odesa in peacetime created a colossal load on the sewage and treatment facilities. All this water entered the sea or estuary, polluting them. The construction of hotels and restaurants also led to the destruction of coastal nature. Now these objects do not function, the sea breathes more freely.

Also, as Viktor Demchenko notes, it is important to mention the diminishing role of shipping, which is almost non-existent now. According to him, all these ships to some extent lead to the pollution of sea water areas or ports – both due to technical emissions and due to the activity of the port infrastructure. On the other hand, now, in the conditions of war, when shipping is practically stopped, the absence of anthropogenic influence has allowed the marine ecosystems to begin recovery. It is the reduction of human pressure on the sea, the scientist emphasizes, that creates a chance for the natural restoration of biocenoses.