By-catch reduction in an ocean shrimp trawl from a simple modification to the trawl footrope

Abstract

Two footrope configurations commonly used in the ocean shrimp trawl fishery were fished side by side from a double-rigged vessel to compare catch rates of shrimp (Pandalus jordani) and by-catch. The control net footrope incorporated a traditional "tickler chain" groundline which runs below and in front of the fishing line of the trawl. The other net utilized a ladder chain groundline with a short roller section in the center, set to run under and slightly behind the fishing line. An acoustic mensuration system was used to monitor net efficiency in terms of net spread and rise. The trawl with the ladder/roller groundline caught 84% fewer slender sole (Eopsetta exilis), 49% fewer greenstriped rock fish (Sebastes elongatus) and 47% fewer juvenile rock fish (Sebastes spp.) (<8 cm total length) than with the tickler chain groundline. After allowing for a wider net spread with the ladder/roller gear, catches of shrimp and other fish species were comparable for both gears. These results indicate that ocean shrimp trawls rigged with a groundline that runs behind the fishing line of the trawl fish efficiently for shrimp and marketable fish while excluding unmarketable species. The exact mechanism behind the increased by-catch escapement is unclear. We recommend underwater camera work to determine how small fish escape capture, and also testing a trawl without a groundline, using only dropper chains to maintain a constant height of the fishing line above bottom.