The Use of a High Powered Strobe Light to Increase the Catch of Antarctic Krill by a 1-m2 MOCNESS

Wiebe, P.H., C. Ashjian, S. Gallager, C. Davis, and N. Copley

Adult Euphausia superba are difficult to capture with the size of nets typically used by oceanographers due in part to their net avoidance capabilities. A study was done during the first broad-scale survey cruise of the Southern Ocean GLOBEC Program (NBP01-03) in April-May 2001 to evaluate the use of a high powered strobe light (1500 W with a beam fan angle of 30°) with a 3 second flashing rate to reduce the avoidance effect. Three horizontal tows were done in Marguerite Bay (Western Antarctic Peninsula), a region with high numbers and biomass of adult krill. Each tow consisted of a series of paired down and up casts through a set depth interval (e.g., 50-90 m), with each successive net sampling both a down and up cast. The strobe light was set to either "on" or "off" while each net was open. During a tow, four of the eight nets (335 µm mesh) sampled with the strobe flashing and four sampled with the strobe off, in a random sequence. Total displacement volume was significantly increased (p<0.05) on average by a factor of ~ 1.5 when the strobe light was on. The increased biovolume was due to the enhanced catch (factor of ~2) of adult krill in the 15 to 60 mm size range. They accounted for most of the biovolume. There was not an enhanced catch of krill in the 5 to 15 mm size range. In addition, the average size of the krill was not changed substantially by having the strobe light on. These preliminary results suggest that krill avoidance of nets can be overcome by smart counter-measures.