Abstract
Southern Ocean eddies shape the foraging ecology of marine apex
predators such as marine mammals and seabirds. A growing number of
animal tracking studies show that predators alter their swimming,
diving, and foraging behavior in mesoscale eddies. However, little is
known about how Southern Ocean eddies influence the distribution of
mesopelagic micronekton (fish, squid, and crustaceans), which are major
prey items of megafauna. Studies in other parts of the world have found
that eddies can impact the abundance and community composition of
micronekton. Here, we analyze acoustic observations from a 14-day survey
of a mesoscale eddy, its surrounding waters, and the Sub-Antarctic
frontal waters where the eddy originated. We report and interpret
spatial patterns of acoustic backscattering at 18 kHz, a proxy
indicating combined changes in species, size, and abundance of
micronekton. We find that the vertical distribution of Deep Scattering
Layers matched the underwater light conditions characteristic of the
eddy core, periphery, and surrounding waters, at scales smaller than 10
km. Furthermore, the average water-column integrated acoustic
backscattering values in the eddy core were only half of the values
measured in the Sub-Antarctic Zone waters surrounding the eddy. By
contrast, the acoustic properties of the eddy core were similar to those
measured in the Polar Front Zone, where the eddy originated 27 days
before our sampling. These results show that, as for physical and
chemical tracers, the eddy maintained its biological characteristics
from its source waters creating a unique habitat compared to its
surrounding waters.