The closure of the Clyde fishery has led to the recovery of marine species – but not the same species that once lived there, according to a report.
Published in the journal Current Biology, the paper found that the marine ecosystem of the Clyde Sea in western Scotland is showing signs of recovery after reduced fishing pressure, but with sprat now the dominant species instead of herring.
Scientists say this is an example of how simply reducing fishing does not necessarily mean that commercial fish stocks will return to the same pre-exploitation levels, and that restoration actions can have unintended consequences.
The authors of the article describe the Firth of Clyde as “one of the most anthropized marine environments in the world”. For hundreds of years it was famous for its abundance of herring. But an intense fishing industry from the middle of the 19th century depleted fish stocks and was eventually shut down. Since 2008, langoustine (Nephrops norvegicus) was the main commercial fishery.
Without any of the pressures of commercial fishing, researchers have found that the biomass of pelagic forage fish – such as herring and sprat, which prey on predators including marine mammals and large fish – is now four times greater than what It was in the 1980s. But where herring was the dominant species then, sprat is now.
The paper’s lead author, Dr Joshua Lawrence, said: “We have seen no recovery of the herring stock, as one would normally expect following a reduction in fishing pressure. Instead, we saw a huge increase in sprat biomass in the area. »
It is possible that sprat populations increased due to lack of competition from herring, to the point that herring could not recover even when fishing ceased. The authors suggest other factors could include warming seas or the fact that herring need undisturbed gravel beds to spawn, unlike sprat.
Meanwhile, fish that have been less well protected – such as groundfish, or groundfish, such as cod and haddock – have not experienced the same increase in biomass as pelagic species. Professor Anthony Gallagher, who chairs the Clyde Marine Planning Partnership, said: “These are still heavily caught as bycatch in the langoustine fishery and almost entirely discarded at sea.”
Another example of a fishing moratorium that led to the recovery of an unexpected species is the North Atlantic cod. In general, stocks have not recovered despite fishing closures – but on Georges Bank in the northeastern United States, an attempt to rebuild cod has resulted in a (very lucrative) 14-fold increase in scallop biomass.
Additionally, in Europe, reduced fishing pressure to replenish hake stocks has led to a massive increase in the species, which has spread to the North Sea, where it has been mostly absent since. 50 years. This change may affect the future of mixed demersal fisheries, which have low quotas for hake.
Lawrence said, “Sometimes management interventions can have unintended consequences, most likely due to unforeseen interactions and processes in the ecosystem. These can be difficult to predict and can vary greatly from one system, and even one species, to another.
Because reducing fishing pressure isn’t always successful, the most important thing, Lawrence said, was “to make sure stocks don’t become overfished in the first place.”
There is no sprat fishing around the Clyde, but the authors suggest a more sustainable industry could be ecotourism, particularly whale watching.
Professor Joshua Abbott, an environmental economist at Arizona State University, said that while ecotourism was a viable option, income and job opportunities might not match those offered by fishing, and he highlighted the seasonal nature of ecotourism as a possible restriction.
If sustainable fishing could work alongside tourism, he added, no-take zones would help avoid conflicts between the two industries. “Those considering alternative economic futures in a region must consider these complex realities,” he said.