Effectiveness of northern shrimp trawls designed to reduce megafauna bycatch

Large Greenland sharks (Somniosus microcephalus) are captured as bycatch in northern shrimp (Pandalus borealis) bottom trawls by becoming stuck in the Nordmøre grid system. Grid systems do not easily exclude Greenland sharks due to their large size (up to 6 m in length). Thus, an additional or modified bycatch reduction device (BRD) should be considered to promote a quick escape. Two experimental BRD systems were designed and tested to facilitate the escape of large-sized Greenland sharks: 1) a large escape opening at the grid (increased from 113 to 250 cm) and 2) a sieve panel with a large escape opening before the traditional grid system. Catch rates (kg tow−1) and size selectivity were compared between the experimental and traditional treatments. Results for the large escape opening treatment versus the traditional gear showed no difference in northern shrimp catch rates (P-value = 0.237), however size selectivity was different between treatments (P-value = 0.033) with a slight reduction for large length sizes. Conversely, the catch rates of northern shrimp were significantly reduced for the sieve panel treatment (18% reduction; P-value < 0.001) but showed no difference in size selectivity (P-value = 0.388) across all length classes. No Greenland sharks were observed during sea trials, and total bycatch was minimal. In conclusion, the large escape opening showed promise as a technique to reduce megafauna bycatch, such as Greenland shark, by providing a large escape area while having minimal effects on northern shrimp catch. However, the sieve panel treatment, as tested, likely loses too much northern shrimp to be considered in the fishery.