Abstract
The abyssal seafloor makes up three quarters of the ocean floor, and it is generally characterized as a food-limited habitat with low numbers of megafauna, particularly fishes. Baited camera observations from three abyssal seamount summits in the equatorial Pacific challenge this idea. On each of two deployments at the southernmost seamount, over 100 synaphobranchid eels (Ilyophis arx) were recorded feeding on standard bait (1 kg mackerel). This is the highest number of fishes per kg of bait ever recorded below 1000 m, including observations from large organic falls such as cetacean and shark carcasses. It is also the highest number that has ever been recorded at carrion of any kind or size at abyssal depths. We suggest an abyssal ‘seamount effect’ may be responsible, highlighting the potential importance of seamounts in structuring abyssal communities.
Highlights
About 75% of the seafloor is at abyssal depths (3000–6000 m), making it the largest benthic habitat globally (Harris et al, 2014; Priede, 2017)
We report unprecedented observations of large aggregations of fishes at baited cameras on abyssal seamounts in the western Clarion Clipperton Zone (CCZ) in the central Pacific Ocean, which challenge the paradigm of universally low abyssal fish density and show that abyssal seamount summits are capable of supporting high abundances of top predators and scavengers
We compare the findings at this seamount to two further abyssal seamount summits in the western CCZ region. We review these findings in the broader context of global baited camera experiments and food falls to show that our observations represent the highest number of fishes ever observed at one time at abyssal depths, and the largest number of fishes ever observed per kg of carrion below 1000 m
Summary
About 75% of the seafloor is at abyssal depths (3000–6000 m), making it the largest benthic habitat globally (Harris et al, 2014; Priede, 2017). Mean densities are consistently low globally, ranging from 100s to a few 1000s of in dividuals per square kilometer across ocean basins as well as across topographic and oceanographic gradients (Bailey et al, 2006; Cousins et al, 2013; Drazen et al, 2019; Fleury and Drazen, 2013; Leitner et al, 2017; Linley et al, 2017; Merrett et al, 1991; Milligan et al, 2016; Pearcy et al, 1982; Priede and Merrett, 1996; Yeh and Drazen, 2011, see Fig. 3c) Despite their low numbers, fishes play important roles in the abyssal ecosystem as scavengers and top predators (Drazen and Sutton, 2017). They exert top-down pressures that can control prey populations across wide scales, and as scavengers they redistribute organic carbon across the seafloor and influence energy flow and nutrient cycling in the deep sea (Drazen et al, 2008; Drazen and Sutton, 2017)
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