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deep sea

There are plenty more fish in the deep sea!

­Reviewing: Leitner, A. B., Durden, J. M., Smith, C. R., Klingberg, E. D. & Drazen, J. C. Synaphobranchid eel swarms on abyssal seamounts: Largest aggregation of fishes ever observed at abyssal depths. Deep. Res. Part I Oceanogr. Res. Pap. 103423 (2020). doi:10.1016/j.dsr.2020.103423

Where is life on Earth?

When you think of all of Earth’s biomes, what do you think of – rainforest, deserts, prairies, tundra? Would it surprise you that all of these terrestrial habitats combined actually make up less than 30% of Earth’s surface? In fact, the largest habitat on Earth is the deep sea, occurring at depths below ~200m where light does not penetrate. To get a better idea of how expansive the deep sea is, check out this interactive infographic. The deep-sea is mostly made up of the flat areas of the seafloor between continents referred to as the abyssal plains. Dotting this expansive seascape are seamounts, or underwater mountains formed by volcanic activity, usually extending 100’s – 1000’s of km above the seafloor.

A profile of Earth’s seafloor features. Each feature is shown at an example depth, but each may occur at considerable ranges. Figure courtesy of NOAA Office of Education.

Although very little of the abyssal seafloor has been mapped or explored (less than 1%), scientists have characterized this habitat by low abundances of animals caused by low food availability. Particularly rare across the abyssal seafloor are large-bodied animals like fish, whales, or sharks. For example, only 100-1000 fish are typically found in the area of about 190 football fields.

Looking for a meal 

Phytoplankton in sunlit waters grow quickly forming blooms that tend to stick together forming clumps (POC, particulate organic carbon). This material is an abundant food source in most of the upper ocean (see zooplankton grazing). However, as this material gets degraded by microbes (turning into DOC, dissolved organic carbon), or chemically dissolved (remineralization), less and less is available deeper in the ocean. Figure courtesy of Wikipedia

Why is there so little food available at the deep seafloor? The deep sea is void from sunlight and lacks primary producers, so food ultimately originates from microbes living in the sunlit regions of the upper ocean (phytoplankton). Along its 3000-6000 m descent to the seafloor, this material is eaten or dissolved to the point that only ~ 1% of the nutrients produced in the upper ocean makes it down to abyssal depths. With so few nutrients available to go around, few large-bodied animals can be sustained. Instead, many rely on scavenging for large meals from food falls, such as dead whales, sharks or fish.

Seamounts are thought to be have enhanced food availability because food doesn’t have to travel as far to settle on their summits, and they are surrounded by strong currents that may trap and retain food particles on the summit. As a result, seamounts are considered biological hotspots, hosting many more animals. But do seamounts receive enough food to support significantly greater numbers of large-bodied animals like fish, sharks, and whales than the surrounding abyss?

Dr. Leitner and fellow colleagues set out to understand if seamounts host more fish than neighboring abyssal plains. They targeted a region of the western Pacific sampling at 3 seamounts and their adjacent abyssal plains. Fish are challenging to sample, even in shallower water where they are found in high densities. They are highly mobile and will avoid unnatural noise and sound such as from nets or moving cameras. To overcome this challenge, the scientists mimicked a food fall to attract animals in high density by sinking bait and making observations from an attached, extended camera. From the captured video footage, they identified the species, counted abundance, and even estimated the biomass and density of the scavengers.

A new o-fish-al record

Dr. Leitner and fellow colleagues observed large aggregations of fishes, specifically synaphobrachid eels, more commonly known as cutthroat eels, at their baited cameras deployed on seamounts. In fact, they observed the largest number of fish ever observed deeper than 1000 m:115 fish at one time! This value is almost double the previous record. The high number of fish observed challenges our perception of the deep sea as desolate and unable to support dense populations of predators. Interestingly, these eels were only observed on the tops of seamounts, suggesting they may be seamount specialists supported by the higher food availability in this habitat.

Large aggregations of synaphobrachid eels swarming at the bait. A maximum of 115 individuals were counted at this site. Check out a short video of the baited trap footage here. Photo courtesy of Leitner et al. 2020

Further questions reside from these interesting results including: how are so many animals sustained on a small seamount? Are these eels permanent residents or was this just a spawning event? Finally, are these observations comparable to other seamounts?  These questions highlight just how much there is still left to discover in the deep sea, and how much information we may lose should these unknown habitats be poorly managed or severely impacted by humans.


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