A Study on the Catch Losses and Mesh Selectivity Related to the Attachment of Marine Mammal Bycatch Reduction Devices on Midwater Trawl Gear

The National Institute of Fisheries Science in Korea is developing marine mammal bycatch reduction devices (BRDs) for high-risk fishing gear, such as trawls. We experimented with two BRD types (guiding nets) attached in front of codend at 30 degrees and 45 degrees angles, and catch losses and mesh size selectivity were investigated. Experimental fishing operations were conducted along the East Coast of South Korea where whales and dolphins are commonly sighted. The catch was classified according to fishing location, BRD type, codend, and covernet, with measurements recorded for body length, maximum girth, and weight. The average selectivity for each haul was analyzed using the 'selfisher' package. The catch loss rates with the BRD attached at tilt angles of 30 degrees and 45 degrees were 11% and 29% for common flying squid, 6% and 28% for sailfin sandfish, and 5% and 8% for pearlside. While the mesh selectivity rates for common flying squid and pearlside remained at 0.2-0.5 across all lengths and tilt angles, the mesh selectivity curve for sailfin sandfish was estimated. There were significant differences in catch loss between 30 degrees and 45 degrees angles, with the 30 degrees angle being more effective in catch loss. We observed a masking effect in the codend.