Swirling currents called mesoscale eddies are hot spots for biodiversity in the sea's bustling twilight zone and may shelter unique fish communities in the western North Atlantic Ocean, new research indicates.

Scientists found more and rarer fish species dwelling within eddies off the Canadian coast than outside them and reported the findings May 3 in Deep Sea Research Part I: Oceanographic Research Papers. Little is known about how eddies influence adult fish in the twilight zone, but previous research suggests that these features may serve as nurseries and transport species to different parts of the ocean.

"I personally found the high number of species observed only inside the eddies particularly striking," said Brynn Devine, a postdoctoral research fellow in integrative biology at the University of Windsor, in Ontario, Canada, and first author of the study. "Most fishing sets only yielded a few kilograms, but these often included dozens of species!"

The ocean's twilight, or mesopelagic, zone is found at depths between 200 and 1,000 meters (about 656 and 3,280 feet). Despite being nearly devoid of sunlight, Devine says, this zone has a substantial population of organisms ranging in size from tiny zooplankton to large predators, such as giant squid. Mesopelagic fish are believed to be the most abundant vertebrates on Earth, with estimates of their total biomass ranging from 1 billion to over 15 billion metric tons (about 1.1 billion to 16.5 billion U.S. tons).

Many of the fish and other animals that inhabit this layer journey to the sea surface every night to feed, and then return during the day to avoid predators. The twilight zone's inhabitants play important roles in global food webs and provide important ecosystem services, such as transporting carbon from the surface to the deep sea, Devine said. 

She and her colleagues were interested in how fish at these depths were affected by the presence of mesoscale eddies. These transient vortices typically range in size from 50 to 150 kilometers (31 to 93 miles) and are shed by swift ocean currents, such as the Gulf Stream. Mesoscale eddies mix water masses and transport heat, salt and nutrients around the ocean. They can have warm-water or cold-water cores, which cause the eddies to rotate clockwise or counterclockwise, respectively, in the Northern Hemisphere. 

In the northwest Atlantic Ocean, the warm waters of the Gulf Stream mix with cold subarctic waters. The Gulf Stream produces around 50 to 60 mesoscale eddies every year in this region, Devine says, making it a great location to study how eddies influence fish communities. 

She and her colleagues identified warm-core eddies off the coast of Newfoundland, using satellite estimates of sea-level height and measurements of temperature and current speed. In April of 2015 and 2016, the researchers used trawl nets to collect 23 sets of mesopelagic fish from different areas inside and outside the eddies. They examined 6,087 fish representing at least 109 different species from 38 families. 

On average, the researchers captured nearly twice as many fish per set inside the eddies as outside the eddies. The eddies also had more diverse populations, with at least 63 species gathered exclusively from within eddies, and only nine species found exclusively outside them. The researchers also observed more juvenile fish and rare species inside eddy waters, including some species generally found farther south that may have been carried up by the Gulf Stream.

The findings indicate that mesoscale eddies shape fish communities in the northwest Atlantic Ocean, act as hot spots to attract both prey and predatory species in the open ocean, and provide nursery habitats for young fish, Devine and her team concluded. Eddies may also contribute to regional differences in commercial fisheries and ecosystem services carried out by twilight-zone organisms, she said. 

The researchers observed fish communities from only a handful of locations during a limited window of time, Devine acknowledged.

"So, we really have just a snapshot in that particular season," she said. 

Individual eddies can change over time, perhaps leading to seasonal or other variations in the fish communities that use them, Devine said. More research is needed to understand how mesoscale eddies shape the twilight-zone ecosystem throughout the year. 

Another important question is how climate change might impact the location, frequency and duration of eddies across the world's oceans and how changes to mesoscale eddies might in turn affect the twilight zone's residents.

"Our study was really just a first step in exploring how these dynamic features influence mesopelagic fishes in the Northwest Atlantic," Devine said. "There is still so much we do not know about this incredibly important but understudied ecosystem."

The study, "Influence of anticyclonic, warm-core eddies on mesopelagic fish assemblages in the Northwest Atlantic Ocean," published May 3 in Deep Sea Research Part I: Oceanographic Research Papers, was authored by Brynn Devine, University of Windsor; Sheena Fennell, National University of Ireland; Daphne Themelis, Fisheries and Oceans Canada; and Jonathan A.D. Fisher, Fisheries and Marine Institute of Memorial University of Newfoundland.