Deep sea fishing in the north east Atlantic could be harming fish stocks below the reach of the deepest nets, a long-term study has revealed.
It showed a decline in fish numbers as deep as 2,500 metres, almost a kilometre below the reach of the deepest commercial nets.
Researchers from Glasgow University said it could be that young fish living in shallower water were being caught.
This could be having a knock-on affect on adult populations, they said.
Dr David Bailey, who led the study, said: “Commercial fishing may have wider effects than anyone previously thought, affecting fish which we assumed were safely beyond the range of fishing boats.
“We were extremely surprised by this result and believe that it has important implications for how we manage the oceans.”
The team began surveying deep sea fish numbers on the slopes off the west coast of Ireland in 1977.
They continued making recordings until 1989 before any fishery was established in the region.
They then mapped the slopes again, from 1997 to 2002, using the same ships used for the earlier study.
The scientists were surprised to discover that fish numbers had declined as deep as 2,500 metres when commercial trawling usually extends to only 1,600 metres.
Team member Dr John Gordon of the Scottish Association of Marine Science said: “Each deep-water species has a defined depth range and very often the juveniles live at depths shallower than the adults.
“Removal of fish by commercial trawling down to 1,600 metres is likely to affect populations in deeper waters.”
Both target species – such as roundnose grenadier and orange roughy – and unwanted fish, sometimes making up to 50% of the catch, were affected.
These would not survive being thrown back into the sea because of the changes of pressure and temperature.
The findings are published in the journal Proceedings of the Royal Society B.