Acoustic oceanography

Echosounders

Upward-looking echosounders with the electronic parts built into water- and pressure proof containers are deployed on the bottom or floating in anchored buoys (Fig. 1 a,b). The echosounders are cabled to shore for electricity and transfer of data, providing unlimited power and data storage capacity. This enables long registration periods with high temporal resolution. Such data have unveiled very dynamic behavior between day and night, with season and upon encounters with predators. Recent work has addressed the diel vertical migration and individual swimming behavior of krill, jellyfish and mesopelagic (deep-sea) fishes like the northern lanternfish Benthosema glaciale (Fig. 2). 

Jellyfish

Different fjords have different characteristics and pelagic fauna. The jellyfish Periphylla periphylla (Fig. 3 a,b) is exceedingly abundant in some fjords, facilitating studies of their biology. While jellyfish are very simple organisms, acoustic studies of individuals have revealed surprisingly dynamic behavior. 

Studies in the Oslofjord have particularly addressed krill and the small fish sprat (Sprattus sprattus). Since the upward-looking echosounders are places on the bottom, or floating in buoys in mid-waters, the studies have included how ice cover, as well as the darkening caused by snow on the ice, affect their distribution and behavior during winter (Fig. 4). 

Recording data

Novel acoustic studies of krill have established their individual swimming speed in deep water, as well as diel and seasonal feeding behavior by recording their release of fecal pellets (Fig. 5).

The research group is also involved in international studies. Currently, we cooperate with scientists at King Abdullah University of Science and Technology (KAUST), Saudi Arabia for studies in the Red Sea. One of the aim for the Red Sea studies is to compare the behavior of the lanternfish that occur in this warm, unproductive and clear-water ocean with that of lanternfish in murky and productive Norwegian waters. In the Red Sea we also study the behavior of squid in deep water. We have deployed the echosounders all the way down to 1500 m for detailed behavioral information at depth. This has provided unique information on the life of these organisms in the deep, dark ocean (Fig. 6).