NMFS NUMBER: 93-NER-035
Lessening the Impact of the Northern Shrimp Fishery on
Juvenile Groundfish in the Western Gulf of Maine
Daniel Schick and Michael Brown
Marine Fisheries Service, Northeast Regional Office, One
Blackburn Drive, Gloucester, Massachusetts 01930-2298.
PHONE: (978) 281-9256
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.