Improving Size Selectivity of Shrimp Trawls in the Gulf of Maine with a Modified Dual-Grid Size-Sorting System

Abstract

Finfish bycatch has been drastically reduced since the Nordmøre grid became mandatory in 1992 in the Gulf of Maine trawl fishery for northern shrimp Pandalus borealis. However, the Nordmøre grid does not improve northern shrimp size selection, catching large numbers of small shrimp when they are mixed with large mature shrimp. To reduce the catch of small northern shrimp, a new dual-grid shrimp size-sorting system was developed and tested in a flume tank and at sea to evaluate its potential for reducing catch of juvenile and male shrimp in the Gulf of Maine northern shrimp fishery and to optimize rigging. These tests used a trouser trawl system consisting of one trawl leading to two separate cod ends, which compared a cod end with a dual-grid size-sorting system with another identical cod end without a size-sorting grid. The size-sorting grid had 11-mm bar spacing. The dual-grid size-sorting system significantly reduced the number of small northern shrimp with a carapace length of 22 mm or less and slightly increased catch of large shrimp with a carapace length of 23 mm or larger, when compared with the Nordmøre grid without a size-sorting grid. There was a slight reduction of northern shrimp catch rates due to the release of smaller shrimp but with no significant differences in the number or quantity of major bycatch species between the regular grid and the size-sorting grid. The new dual-grid system was practical to operate and a significant improvement over an earlier version of the size-sorting grid in design, rigging, and handling. The application may reduce small shrimp in other northern shrimp fisheries and other shrimp and prawn fisheries with suitable modification to the design.

Received July 15, 2011; accepted August 2, 2012

ACKNOWLEDGMENTS

This project was funded by the Saltonstall-Kennedy Grant of National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Award Numbers NA08NMF4270417 and NA10NMF4270394. Sea trials were conducted while the first author (P. He) was working at the University of New Hampshire, Durham, New Hampshire. Sea trials were assisted by Tom Cass and the crew of FV North Star. Lorraine Hu and Christian Panachu participated in laboratory sample measurement. Christopher Rillahan, Michael Pol, and David Chosid helped in statistical analysis. Thanks also go to Harold DeLouche for designing the trouser trawl section, Professor Daniel Georgianna for language edit of the manuscript, and Professor Saang-Yoon Hyun for help in statistics. We appreciate comments and suggestions of the editor, the associate editor, and three anonymous reviewers.