Comparison of Northeast Atlantic mackerel (Scomber scombrus) distribution patterns in the Norwegian Sea using lidar, sonar, and trawl

Abstract

In July 2002 the Institute of Marine Research (IMR), Bergen mapped the distribution and density of mackerel in the Norwegian Sea using lidar, sonar and trawl. A major objective was to test the efficiency of the lidar as a survey tool. Due to the lack of swim bladder and the fact that mackerel feeds close to the surface, traditional acoustic equipment is inefficient. The airborne NOAA fish lidar covered the same tracks as two commercial trawlers hired by IMR. The trawlers used a Simrad 24-36 kHz sonar to track the speed, volume, direction and depth of mackerel schools and they were trawling close to the surface. Most of the fish caught was mackerel (69% of catch weight) and the majority of the schools (64%) were recorded shallower than 40 meters. The mackerel was mainly distributed in the southern parts, while one of the trawlers also caught a significant amount in the northwest. These are the same areas where we got the strongest lidar return. The southern part of the surveyed area contained rich plankton layers showing up in the lidar return. These layers are easily distinguished from fish as they continue over long distances compared to the size of the schools. Fish data were therefore easily extracted during post-processing. The amount of plankton gradually decreased as we proceeded north giving clearer water and better lidar depth penetration. The lidar seems to be an interesting tool, giving plausible results, but still needs some development. Some aspects of future development are discussed.