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Entering the Twilight Zone: the Ecological Role and Importance of Mesopelagic Fishes

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Entering the Twilight Zone: the Ecological Role and Importance of Mesopelagic Fishes DECEMBER 2020 Entering the Twilight Zone: The ecological role and importance of mesopelagic fishes Callum M. Roberts1, Julie P. Hawkins1, Katie Hindle2, Rod W. Wilson3 and Bethan C. O’Leary1 1Centre for Ecology and Conservation, University of Exeter, Penryn Campus, Cornwall, TR10 9FE, UK Email: [email protected] 2Department of Environment and Geography, University of York, York, YO10 5NG, UK 3Biosciences, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4QD, UK © 2019 Danté Fenolio – www.anotheca.com, courtesy of the DEEPEND Consortium 1 BLUE MARINE FOUNDATION ENTERING THE TWILIGHT ZONE: THE ECOLOGICAL ROLE AND IMPORTANCE OF MESOPELAGIC FISHES 2 1. EXECUTIVE SUMMARY Ocean waters between 200 and 1000m “Not everything that meets the eye is as it appears” deep – the Twilight Zone – sustain immense - Rod Serling, The Twilight Zone: Complete Stories quantities of fish, believed to be greater than The above sentiment is certainly true of the ocean. Powerful and complex, the ocean dominates the Earth’s global processes and supports life from the majestic the combined mass of all fish living closer to to the bizarre. Most people, however, from their land-bound perspective, see the sea only as a playground backed by a vast expanse of featureless water. But the ocean the surface, that the fishing industry is keen has three-dimensions and holds 97% of the liveable space on the planet. What lies to exploit. But their value to the planet’s life beneath deserves greater recognition and respect. support system and in climate mitigation is The ocean’s three dimensions are structured. While some creatures criss-cross different depth zones, particularly deep-diving predators like whales and tuna, most likely far greater than their value as food. life is specialised to live in particular layers. The twilight zone, between 200 m and 1,000 m, is a place where little sunlight penetrates. It is one of the least understood places on the planet because it is so vast and difficult to study. The creatures glimpsed there stretch the bounds between reality and fantasy. Nonetheless, it We must urgently protect them from fishing could be one of the most important parts of the global ocean for life on Earth and while we undertake research to determine human wellbeing. their importance in global ocean processes. 1.1 WHY THE TWILIGHT include the north Atlantic, Arabian Sea and Mediterranean Sea, while the polar seas and ZONE MATTERS south Atlantic generally appear to contain low mesopelagic biomass and diversity The twilight zone, referred to by scientists compared with other ocean basins (Figure 1). as the mesopelagic, constitutes about 20% of the global ocean volume and contains Many mesopelagic fishes undertake a daily an extraordinary biomass of invertebrates commute, migrating vertically at night to and fish, such as squid and lanternfishes. feed in shallow waters above 200 m in the Scientists have found tantalising evidence safety of darkness and then retreating that by weight, mesopelagic fishes could to the depths by day. This unseen mass make up more than 90% of all fish in the movement is thought to be the largest daily sea. Mesopelagic fishes are small, quirky migration on Earth. By eating, being eaten, looking and live throughout the world breathing and defecating across ocean ocean. Areas of particularly high biomass depths, they present a key mechanism for 3 BLUE MARINE FOUNDATION ENTERING THE TWILIGHT ZONE: THE ECOLOGICAL ROLE AND IMPORTANCE OF MESOPELAGIC FISHES 4 ocean uptake and sequestration of carbon, management and conservation, however of the lanternfish Diaphus danae in the degrees centigrade higher. thereby helping to slow the rate of global mesopelagic fishes have been largely Coral Sea. A seasonal tuna fishery in the warming, and are an important driver for ignored in these negotiations to date. That equatorial Atlantic Ocean has been linked 1.3.3 Fisheries interest and exploitation other biogeochemical cycles. They are omission needs urgent rectification. to the presence of large schools of the potetial also a crucial food source for predators, mesopelagic fish, Vinciguerria nimbaria particularly ocean-going megafauna 1.3 THE STUDY (Photichtyidae). And off the coast of Costa We searched peer-reviewed and grey like tuna and dolphins. Given their vast Rica, vast ‘superpods’ of dolphins up to ten literature for records of commercial abundance, wide distribution and vertical To inform negotiations at the UN to protect thousand strong have been filmed feeding or exploratory fisheries and levels of migration, the collective influence of biodiversity in international waters, to better on lanternfish they have trapped at the vulnerability of mesopelagic fishes. While mesopelagic fishes on the structure and manage emerging fishing activities and to surface. there is considerable and relatively function of ocean ecosystems is likely to be better assess the wisdom of exploitation, long-standing interest in industrial- great, but details of their ecological role are this report considers the global importance 1.3.2 The role of mesopelagic fishes in carbon scale exploitation of mesopelagic fish, still to be fully established. of mesopelagic fishes in ocean ecosystems. cycling we found few records of commercial We examine their roles as food for higher and exploratory fisheries activity so far. 1.2 THREATS TO ocean predators and in the cycling of carbon Determining the influence of marine life on Challenges faced in developing fisheries environmental CO levels has important include patchy distribution of mesopelagic MESOPELAGIC FISHES and nutrients, and assemble information 2 on fisheries activity to determine current consequences for understanding of global fish concentrations, trawl avoidance carbon cycling, particularly the uptake and behaviour, difficulties in processing catches The vast abundance of mesopelagic fishes exploitation threats. Finally, we identify key uncertainties in our understanding that it storage of carbon from the atmosphere to and high operating costs. We found very is attracting growing interest from the the deep ocean. We identified three potential limited evidence of the likely vulnerability fishing industry, particularly to plug the is essential that we fill to ensure effective conservation not just of the mesopelagic mechanisms by which mesopelagic fishes of mesopelagic fishes to exploitation, expected deficit in feed for farm animals and may contribute to carbon cycling (Box 1). which represents a fundamental gap in our aquaculture as the human population grows. zone, but of its broader connections with the marine environment globally. Although many understanding and ability to sustainably Some nations, like Norway and members Estimates of the contribution of mesopelagic manage future fisheries for them. The of the EU, are funding substantial research animal groups inhabit mesopelagic depths, we focus on mesopelagic fishes given their fishes to organic carbon export from the prevalence of overexploitation in existing and trial commercial fisheries. Others are surface to the deep sea vary between more and past fisheries signal the need for great worried about the consequences of large- large biomass, daily vertical migrations, and rapidly increasing commercial interest. than 10% of the total carbon export from caution in developing new fisheries in what scale exploitation. The United States, for surface to deeper waters, to greater than is evidently a highly sensitive environment. example, has proactively prohibited directed 1.3.1 Mesopelagic fishes are important as 40%. It has been estimated that mesopelagic Catches to date have been highly variable commercial fisheries for mesopelagic fishes fishes play such an important role in carbon with larger catches often followed by rapid in its Pacific waters for the time being due to food for surface living predators sequestration in the deep sea, that without declines (Figure 3), underlining the need for a concerns over potential adverse ecosystem them, atmospheric CO2 levels would be 50% highly precautionary approach. consequences. We analysed the importance of mesopelagic greater and global temperatures several fish in the diets of tuna and sharks based on Much of the mesopelagic is at great research published between 1996 and 2017. Box 1: Mechanisms by which mesopelagic fishes contribute to carbon cycling risk today. Beyond the limits of national Mesopelagic fishes varied substantially in jurisdiction, there is a vast pool of water importance for different predator species, Mechanism 1: Mesopelagic fishes consume organic matter from surface waters which is then where no regulations exist to protect contributing the greatest proportion metabolised at c.500-700m deep, and faeces produced at this depth transfers carbon to deep twilight zone fish. The high seas, as these of prey, in terms of weight, for deeper water. international waters are colloquially known, diving (e.g. bigeye tuna, 26%) and deeper constitute 61% of the world’s ocean and benthopelagic (e.g. longnose velvet dogfish, Mechanism 2: Mesopelagic fishes precipitate and excrete calcium carbonate within their 43% of the Earth’s surface (Figure 1). They 83%) or pelagic-oceanic (e.g. bigeye thresher intestine as a by-product of drinking seawater for osmoregulation. Dense inorganic carbonate precipitates within faecal pellets may then facilitate sequestration of surface CO2 to ocean are owned by everyone and no-one, being shark, 22%) species (Figure 2). There is depths by
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