The ‘twilight zone’ which daylight cannot reach, at depths of 200 to 1000 metres, is estimated to hold a community of fish, squid and crustaceans, including more than a million undescribed species, with a biomass which surpasses by far all of the world’s current fisheries. This accounts for up to 90% of the world’s total fish biomass and, even when squid and krill are excluded, is equivalent to 1.3 tons of fish biomass for every person on Earth.

All of this offers huge potential source of fishmeal and Omega 3 fatty acids needed to feed the world population. However, the species exist in a “no man’s water”, where there is no regulation; furthermore, little is currently known about the biological processes in these waters, making it impossible to estimate accurately the fishing pressure the stocks can sustain, to assess the resilience of the community and, as a result, to develop sustainable management strategies.

Professor Mike Heath, of Strathclyde’s Department of Mathematics, was a partner in the study, led at the Technical University of Denmark and also involving AZTI-Tecnalia in Spain, the University of the Azores, and the Southampton-based National Oceanography Centre. The research has been published in the journal Frontiers in Marine Science.

Professor Heath said: “Fish stocks in many parts of the world have been extensively harvested, often leaving them severely depleted. While careful, sustainable management has been introduced in many cases to help arrest the decline, it remains a widespread problem.

“The deep seas are a vast untapped potential source of new resource for fisheries, but lessons must be learned from the past, and further knowledge gained in the future, if any fishing activity in these waters is to be sustainable. The deep sea is an essential reserve of biodiversity and our study demonstrates that environmental, economic and legal considerations must be addressed before embarking on large scale fishing.”

The new study warns that a better understanding of the role of the deep sea in the preservation of biodiversity, and its influence on climate regulation, is required if the ‘twilight zone’ community, currently one of the most understudied regions in the world oceans, is to be fished in a sustainable manner.

The study concludes that, to define the limits of sustainable fishing in this community, fundamental knowledge is needed on everything from population biology and controls on recruitment success to its role in the food web and for climate regulation.

The ‘twilight zone’ fish and plankton species provide food for other key species, such as tuna, sharks and whales, but their importance in the ocean food web is not yet fully quantified. Furthermore, they play an important role in climate regulation. Many ‘twilight-zone’ fish migrate to the ocean surface at night to feed on plankton, descending back during the day to the depths, where they release carbon dioxide. The result is an additional mechanism for fast transport of carbon from the atmosphere to the ocean`s interior, dampening the effect of CO2 emissions on global warming.

The study finds that, at present, there is no major fishing effort on the ‘twilight zone’ community, although test fisheries are in progress. Existing fishing techniques are marginally economical when the catch is used for fish meal; however, the high essential fatty acid content of some species will make fishing more economically viable.



University of Strathclyde

Frontiers in Marine Science