REPORT: The CRIMAC SFI starts with a bang on herring in Kvænangen and in the Norwegian Sea

By Egil Ona, IMR

The new SFI at IMR, which will use broadband echo sounders to identify marine animals as acoustic targets, got off to a great start on the herring cruise with G.O. Sars. The trip went, after extensive calibration of the equipment in Tromsø, first to the Kvænangen fjord to target juvenile herring of the 2013 and 2016 year class and then to the Norwegian Sea on the edge of the international zone west of Røst to find the large,  old herring.

A whole group of 16 Covid-free researchers and engineers from IMR, Kongsberg Maritime and Scantrol Deep Vision had their measuring new equipment tested on a cooperative layer of young herring in Kvænangen and shoals of “old” 370-450 gram herring in the Norwegian Sea. The main objectives that have been achieved thus far include:

• Measurements of spectral frequency response (echo dialect) for schools of juvenile herring and large herring from 18-420 kHz, as well as equivalent measurements of individual fishes with the Target Strength probe inside the herring layers.
• Measuring acoustic backscatter of herring near the sea surface at night, using both the KayakDrone and G.O. Sars so that density and avoidance of the research vessel and the silent electro-drone could be quantified. Here, humpbacks and killer whales have a fiesta, so the herring is sensitive to large moving objects. To put it mildly, the herring was more afraid of the large G.O.Sars than the silent drone.
• Testing a new version of the Kongsberg Maritime fishing sonar, SU90, which can be used to follow schools from more than two kilometers in front of the vessel until they enter the trawl, behind the vessel.
• Testing a mechanical selection system, controlled by the Deep Vision system, to open and close the codend based upon trawling time and depth. Now one can in principle shoot the trawl without retaining catch, collect a sample at the desired depth, and then close off the codend before hauling. This is the start of a modern selection system for trawls to retain only desired species, sizes, and amounts.
• Processing broadband acoustic data with the LSSS software, which has now been rebuilt to receive the huge amounts of data from the broadband echo sounder, and display continuous catch information from the Deep Vision system along the path of the trawl haul from the surface to 300 meters depth and back

We have also installed the latest equipment for measuring current speed and direction under the vessel, a combined broadband echo sounder and acoustic Doppler system, ADCP, from Kongsberg Maritime, which was first tested under the CRISP SFI. We have used it to measure the fish’s migration speed and avoidance, in addition to continuous current measurements from the surface to a depth of 350 m. As an echo sounder, it has a very narrow beam that can resolve individual fishes in schools, shoals and layers. This lays the groundwork for measuring the sizes of fish inside schools, a “holy grail” for both researchers and fishermen.

The survey dessert, species outside the main plan, included two age classes of blue whiting in the Norwegian Sea, saithe that hunted herring in Kvænangen, and finally a large concentration of “kruttåte”,  translated to “gun powder feed”, (winged snails, probably Limacina retroversa) that we encountered just before the arriving at large herring in the Norwegian Sea. It was all-hands-on-deck in the middle of the night Wednesday when the plankton layer was detected. The target strength probe was lowered into the layer, which can only be seen on high-frequency echo sounders of 150 – 450 kHz, and made acoustic measurements of individual organisms with acoustic signatures one-millionth that of a herring. Identity of the organisms was verified using plankton nets and microscopy. The discovery was especially interesting when we saw that the planktonic layer of snails used the ocean current to jump on the “train to Voss” without a ticket. Snails are usually slow movers, but by travelling like stowaways they can move faster over large distances.

Target Strength probe takes a late-night swim in pursuit of plankton

A great voyage, with fantastic people in various measurement teams, says acoustician and chief cat-herder Egil Ona.