Lessening the Impact of the Northern Shrimp....
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GRANT NUMBER:  NA46FD0324           NMFS NUMBER:  93-NER-035

REPORT TITLE:  Lessening the Impact of the Northern Shrimp Fishery on Juvenile Groundfish in the Western Gulf of Maine

AUTHOR:  Daniel Schick and Michael Brown

PUBLISH DATE:  September 8, 1997

AVAILABLE FROM:  National Marine Fisheries Service, Northeast Regional Office, One Blackburn Drive, Gloucester, Massachusetts 01930-2298.   PHONE:  (978) 281-9256


The introduction of the Nordmore grate in the shrimp fishery may have affected the selectivity of commonly utilized mesh (1-3/4" stretched diamond mesh) with respect to northern shrimp.  The mesh selection study that forms the basis for selecting proper mesh size in the Gulf of Maine shrimp fishery was conducted in 1973 using nylon mesh in Yankee 36 two-seam otter trawls.  On the basis of that study, 1-3/4" stretched mesh is used in both the body and cod end of the net.  This mesh releases the smaller-sized male shrimp and retains the larger transitional and female shrimp.  The industry has been using polypropylene mesh for years, and no one can be sure what effect that change has had on the mesh selection characteristics of shrimp trawls.  Possible water flow changes through the cod end with the addition of the Nordmore grate system, coupled with the possible addition of square mesh in the cod end to enhance juvenile groundfish release, are two more factors to consider.  This study reexamines the mesh selection for shrimp.  Six configurations of cod end mesh with and without the Nordmore grate were tested for northern shrimp mesh selectivity: a small mesh cod end; a 1-3/4" diamond mesh cod end without the grate; and 1-3/4" diamond, 1-3/4" square, 1-1/2" square, 1-1/4" square, and 1-3/4" roped diamond mesh cod ends with the grate.  The small mesh cod end--approximately 1" stretched diamond mesh with a 1/4" mesh liner--served as a control net for establishing the selectivity curve for the 1-3/4" diamond mesh cod end without the grate.  The diamond mesh cod end then became the control for testing the other cod end/grate combinations.   The 1-1/2" square mesh cod end with the grate provided a mesh selection curve very similar to the 1-3/4" diamond mesh cod end, which is the industry standard.   The 50% selection was at 22 mm carapace length, right between the male and female shrimp in size.  The 1-1/4" square mesh showed a 50% selection at about 16 mm carapace length (a small male), and the 1-3/4" square mesh showed a 50% selection at about 27 mm carapace (a good-sized female).  The square mesh cod ends provided sharper selection curves for shrimp than the diamond mesh.

To determine the effectiveness of square mesh in the cod end behind the grate in increasing the release of small finfish, the grate and 1-1/4" square mesh cod end was compared to the grate and 1-3/4" diamond mesh cod end and the 1-3/4" diamond mesh without the grate at 12 inshore stations.  The square mesh cod end released more silver hake (32% decrease from control) than the diamond mesh with the grate (1% increase over control), but was either less effective or ineffective at reducing the catch of other finfish compared to the diamond mesh with the grate.  Although the Nordmore grate with a 1-1/2" square mesh cod end provided the best selection curve for shrimp, testing of this configuration was added after testing the other nets and has not been tested for finfish retention.  Habitat, benthic community, and predator-prey data from the project have been added to the Maine Department of Marine Resources database for further characterization of the inshore areas.

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