A recent study looks at the Black Sea not as a series of separate problems — overfishing here, pollution there — but as one system that has changed its state. Its conclusion is that over four decades the sea passed through four clearly distinct regimes, and that the transitions between them were abrupt rather than gradual.
What was actually done
The author builds a dynamic model of the Black Sea food web with 22 functional groups — from phytoplankton to dolphins — and follows it from 1960 to 1999. The modelled area is 150,000 km²: the inner part of the basin, where fishing is most intense.
It matters what kind of study this is. It is not a series of observations but a reconstruction: the model is assembled from historical data on catches, plankton and fish stocks, and then tested for how well it reproduces what actually happened. It succeeds in capturing the sea’s best-known collapse — the crash of the anchovy and the outbreak of the sea walnut (Mnemiopsis leidyi) in 1989.
To find the transitions, a statistical method was used that searches for abrupt changes in time series, rather than leaving the researcher to point them out by eye.
The four states
The 1960s are the starting point — a state close to the intact one. At the top of the food web stand predatory fish: Atlantic mackerel, bluefish, bonito. They exert top-down control, which is to say that the numbers at the lower levels depend on them. Total throughput of the system is 200.7 gC m⁻² per year, and the probability that the fishery is sustainable is 0.89.
The 1970s begin with the overfishing of those predators. From here the loss of sustainability sets in. The mean trophic level of the catch falls significantly — the catch moves down the food web, which means that the large predators are gone and what is left is what is taken. Eutrophication intensifies.
The 1980s are the peak of the pressure. The nutrients carried in by the rivers establish opportunistic species and divert the flow of energy away from the upper levels of the web. Catches reach 750 thousand tonnes — and then collapse to 200 thousand tonnes in 1989.
The 1990s bring a partial stabilisation, once eutrophication weakens.
The statistics place the transitions in 1977, 1981 and 1994.
The figures for a bearable catch
The study calculates what catch the ecosystem could have borne in the different periods:
| Period | Estimate |
|---|---|
| The 1970s | 504 ± 179 thousand tonnes |
| The 1980s | 1108 ± 262 thousand tonnes |
| The 1990s | 702 ± 121 thousand tonnes |
The value for the 1980s looks encouraging, but it means something else: a eutrophic system produces a great deal of biomass at the lower levels of the web. This is not a rich sea but an unbalanced one.
What the study does not claim
The author lists the limitations himself, and they are worth repeating.
The quality of the input data is rated at an index of 0.49 — medium. A large part of the fish-stock estimates after the 1960s were reconstructed backwards rather than measured at the time. Primary production is known only at decadal resolution. Standardised basin-wide surveys are missing.
And most importantly: the analysis is at the level of the ecosystem. From it nothing can be concluded about the state of an individual stock — neither then nor today. The period ends in 1999, so the study says nothing directly about the condition of the sea now.
Why it concerns us
The author’s conclusion is about management: what is needed is effort across the whole basin and across borders, particularly on pollution and fishing. In his words, such coordination could have prevented the unwanted changes and should be applied without delay.
That is also why we follow studies of this kind. The Black Sea is divided among six states, while fish, nutrients and predators take no notice of territorial waters. Monitoring in one place gives part of the picture; decisions are taken over the whole of it.
Source: Akoglu, E. (2023). Ecological indicators reveal historical regime shifts in the Black Sea ecosystem. PeerJ 11:e15649. DOI: 10.7717/peerj.15649. Institute of Marine Sciences, Middle East Technical University, Erdemli, Mersin, Türkiye. The article is open access under a Creative Commons Attribution (CC BY) licence.