Measuring distribution and density of sprat in Norwegian fjords using a kayak drone
Abstract
Sprat is a key species in several Norwegian fjords, as an important prey for fish, sea mammals and sea birds. The sprat populations are monitored with routine acoustic-trawl surveys using a large research vessel, which provides the main scientific basis for the sprat quota advice. Previous survey observations indicate that a significant fraction of the sprat population can be found close to the sea surface and thereby in the acoustic blind zone of the echo sounder mounted below the vessel. In addition, fish may avoid the approaching vessel, and thereby measure less acoustic density. Furthermore, a large research vessel cannot operate in shallow waters and is difficult to maneuver close to the shore. Experiments were carried out in several side arms of the Sognefjord and Hardangerfjord to identify any potential differences in observed distribution and density from the research vessel and the Kayak Drone. Furthermore, the kayak drone was drifting above dense concentrations of sprat close to the surface as the research vessel passed by at 10 knot survey speed. This was done to study potential vessel avoidance reactions. During the day, acoustic densities were similar across platforms. At night, however, the Kayak Drone (USV) recorded higher acoustic densities than during the day. This difference may be due to depth-dependent acoustic target strengths, where the target strength decreases with depth. The RV Kristine Bonnevie recorded similar densities both day and night but had a larger acoustic blind zone due to its deeper transducer (6 m versus 1 m for the Kayak Drone). This affected the observed vertical distribution, with sprat observed deeper at night with the RV Kristine Bonnevie. A diving response from sprat when the RV passed over could reduce target strength, explaining the lower NASC. Future sprat surveys with smaller USVs will need to account for platform-dependent differences in blind zones, avoidance, and target strengths. Measuring distribution and density of sprat in Norwegian fjords using a kayak drone Måling av fordeling og tetthet av brisling i norske fjorder med en kajakkdrone