Excessive fishing in the Black Sea has triggered major changes in the marine ecosystem, scientists suggest.
Mnemiopsis leidyi thrived in fish-scarce waters
The collapse of fish stocks altered the sea's food chain, triggering a "regime shift" that allowed a species of invasive jellyfish to bloom, they said.
But curbing fishing will not be enough to return the habitat to a stable state unless pollution and biodiversity controls are also adopted, they warn.
The findings appear in the Proceedings of the National Academy of Sciences.
A long-term study by Georgi Daskalov, from the UK's Centre for Environment, Fisheries and Aquaculture Science (Cefas), and colleagues found that excessive fishing had resulted in a series of shifts in the sea's ecosystem over the past 50 years.
"Ecosystems exist in a dynamic balance of predator and prey," explained Dr Daskalov.
"Changing relationships within the food web, such as removing top predators through fishing, can tip the scales and lead to large-scale changes in ecosystem make-up, scientifically referred to as regime shifts."
By analysing the population trends of the sea's fish and plankton, the team identified two major shifts over the 50-year period.
Dolphins were hunted in the Black Sea during the 50s and 60s
They found that overfishing before the 1970s effectively removed top predators - including dolphins, mackerel and bluefin tuna.
Without predation, small plankton-eating fish, such as anchovy and sprat, increased in number. As a result, the fishing industry changed its focus.
"Modern fisheries developed, and during the 1970s and 1980s these stocks were overfished as well," Dr Daskalov told BBC News. "By the early 1990s, there was a collapse of these stocks."
This triggered the second regime shift in the Black Sea's marine ecosystem, he explained.
An invasive jellyfish-like organism, the comb jellyfish (Mnemiopsis leidyi), was able to thrive in the fish-scarce waters.
A European Environment Agency report in 2005 found that comb jelly, which arrived in the Black Sea via ships' ballast water, accounted for 90% of the sea's entire biomass at one point.
"Initially, the jellyfish was blamed for the fish stock collapse," Dr Daskalov recalled.
"Our hypothesis is that overfishing and the collapse of the planktivorous fish was favourable for the Mnemiopsis leidyi to develop in huge quantities."
He said the jellyfish boom was a result of an abundance of zooplankton (microscopic animal organisms), whose numbers increased because they were not being eaten by the fish.
Overfishing of the top predators and the smaller plankton-eating fish changed the relationship between the sea's organisms.
The resulting adjustment among the different levels of the food chain, known as "trophic cascades", helped explain why the regime shifts occurred, Dr Daskalov suggested.
He said the relationship between the planktivore fish and the comb jellyfish was the most important in terms of stabilising the marine ecosystem.
"The control is that the fish are better suited to take advantage of a lower abundance of zooplankton, but jellyfish need high levels in order to reproduce in vast numbers."
As stocks fell, many left the fishing industry. This allowed the populations of plankton-eating fish to stage a recovery and compete with the jellyfish for the zooplankton.
However, Dr Daskalov warned that the strategy of limiting catches was not enough.
"Recognising that fishing influences not only the fish stocks but also the ecosystems is essential for modern-day management.
"Given the current situation worldwide of largely overfished stocks and degraded environments, management must aim to restore ecosystems into balanced states."
He said this would include reducing human impacts, increasing biodiversity and improving the quality of the environment.
"Partial recovery is not a stable objective because further changes, such as unfavourable climate or alien species, may drive the system into the next catastrophic loop."
Mnemiopsis leidyi image copyright of Lyubomir Klissurov