The mesopelagic zone by definition is out of reach for most types of direct observation. Yet, Norwegian fjords offer unique opportunities facilitating mesopelagic studies. They are deep, house mesopelagic fauna, and are easily accessible. At the University of Oslo we take advantage of these near-shore “deep-sea” habitats using echosounders cabled to land stations, in this way obtaining data without being limited by ship time, power or data storage capacity. By deploying upward-facing echosounders at the bottom and in in buoys floating in the water column (see picture), we obtain high-resolution information on the mesopelagic fauna at different depths.

As part of our contributions to SUMMER, we exploit long-term and high-resolution data from such cabled fjord observatories. We address population behavior like seasonal patterns in diel vertical migration and distribution of mesopelagic acoustic scattering layers, as well as individual behavior. The latter is exemplified here by the swimming path and speed of the small mesopelagic fish Maurolicus muelleri as swimming towards the surface in the morning, its behavior revealed using so-called acoustic target tracking on data obtained from an echosounder floating in the water column.

Ecogram showing a population of Maurolicus muelleri ascending towards the surface in the morning  (upper). An echo trace of one individual is depicted lower left, with the 3-D swimming path and speed of same individual (selection marked by red) displayed lower right. Illustration by Svenja Christiansen.