Herding of Antarctic krill (Euphausia superba) inside a trawl

The catch efficiency of a fishing gear depends on the design and performance of the gear and the species-specific behaviour of the targeted species. This includes the responses target species have to stimuli from gear components and their ability and inclination to escape. The Antarctic krill (Euphausia superba) fishery is the largest in the Southern Ocean. The fishing fleet uses trawls that vary in design and mesh configuration. We placed a downward-facing 333 kHz and an upward-facing 70 kHz split-beam echosounder just behind the fishing circle in a scientific macroplankton trawl during fishing off South Orkney Islands (Figure 3.3).

The two echosounders recorded acoustic observations of krill in the 6 m high trawl mouth alternately throughout six hauls. It was possible to track 264 and 237 individual krill from the 333 kHz and 70 kHz echosounder, respectively. We demonstrated that small-bodied krill of 3-5 cm body length were actively moving away from the netting panels of the trawl during towing while krill movements in the centre of the trawl body were random (Figure 3.4). The response patterns were consistent for both the upper and lower half of the trawl and are in line with a herding effect that leads to lower krill densities close to the netting panels. The swimming direction of the krill was the determining variable for this herding effect. Their swimming speed was similar irrespective of their swimming direction. Our findings indicate that there is potential of increasing mesh sizes of the gear designs using small-meshed trawl bodies to reduce the carbon footprint of the fishery for krill in Antarctica. The detailed description of the behaviour obtained for the small-sized krill highlights the potential of using split-beam acoustics to gain knowledge of fish responses to fishing gear and selective devices in dark and turbid waters where camera observations are limited.