Entering the Twilight Zone: Global moratorium needed on mesopelagic fishing

Leading scientists have today called for a global moratorium on fishing in the “Twilight Zone” – the area of ocean between 200m and 1000m deep – also known as the mesopelagic. In a paper published today, scientists highlighted the crucial role played by mesopelagic fish – small, often strange-looking fish that make up more than 90 per cent of all fish in the sea by weight – in carbon sequestration and the regulation of a stable global climate. The paper also warned that more research is needed to fully understand the role and importance of mesopelagic life to ocean functioning before any damage is caused by fishing.

The paper, written by world-famous marine biologist Professor Callum Roberts and other scientists from the Universities of Exeter and York, describes how mesopelagic fish undertake the largest daily migration on Earth, by migrating vertically at night to feed in shallow waters above 200m in the safety of darkness and then retreating to the depths by day. It is this migration which makes these fish so important, helping the ocean’s uptake and sequestration of carbon, slowing global warming, and acting as critical food for dolphins and large predatory fish like tuna.

Bethan O’Leary, Research Fellow at the University of Exeter and an Associate Researcher at the University of York, said: “Who can forget the ten thousand strong pod of dolphins in the Netflix series Our Planet, demolishing a titanic shoal of mesopelagic lanternfish trapped near the surface? Given their vast abundance, wide distribution and vertical migration, the collective influence of mesopelagic fishes on the structure and function of ocean ecosystems is certainly profound.”

However, the large abundance of mesopelagic fish, combined with the fact many fish stocks in shallower waters are now depleted, is attracting growing interest from the fishing industry. In particular, these small fish are seen as new sources of feed for farm animals and aquaculture, as global demand for fish and meat increases.

Callum Roberts, Professor of Marine Conservation at the University of Exeter and one of the authors of the report, said: “Large-scale commercial fishing of mesopelagic fishes could have catastrophic consequences for marine life and the global climate. The ecological and environmental value of these fishes far exceed their extractive value and, for this reason, we are calling for a full global moratorium on fishing in the Twilight Zone.”

To date, even where targeted, mesopelagic fishes have been protected to some degree by their ability to avoid trawling gear and the high operating costs of commercial fisheries. However, as innovation of new fishing methods is likely, these features are unlikely to protect them for long. With the prospect of emerging mesopelagic exploitation, close regulatory oversight taking a highly precautionary approach is urgently required for both national and international waters.

Please click here to read the full report.