There are behavioral differences between these species which can be exploited to provide a method for separating individuals and allowing an escape route so that trawling for flounder can continue without unnecessary negative effects on the recovery of cod populations. Flounder scuttle along the sea floor when disturbed and are seldom found at any height off the bottom. Cod, on the otherhand, swim just above the sea floor and will quickly rise above disturbances. They are also more likely to react to visual clues in searching for escape routes. A modified Nordmore grate with horizontal slits of decreasing size provides a method which will allow flounder to pass through the grate into the cod end while directing cod up the grate towards an escape chute. If the net functions as expected it will provide a means for fishermen to continue to harvest flatfish without interfering with recovery of cod stocks. The proposed design involves fairly simple modifications to the standard bottom trawl.
Two Nordmore style grates will be built. One will be of plastic and a second, heavier version, will be of steel. The grates will be approximately 40 inches wide and 48 inches in the vertical plane. The slits or openings of the grate will be in the horizontal plane and will decrease in aperture from 10 inches at the bottom of the grate getting progressively smaller in 2 inch increments (8 inches, 6 inches, 4 inches) until the 2 inch size is reached which will then remain constant for the upper portion of the grate. The grate(s) will be fitted into a standard inshore bottom trawl.
Preliminary field trials held in shallow water in Ipswich Bay. The net will be towed with buoys attached to the top of the grate (and elsewhere) to keep the entire net just under the surface. This will facilitate visual observation of the performance and stability of the rig as it is towed underwater. An observer will ride in a 19 ft powered skiff adjacent to the net. A portable self-contained video system will be used to tape the performance. Any necessary adjustments will be made to establish smooth water flow through the system. We will experiment with both both the plastic and the steel grate to to determine whether the differing weights affect the structure of the trawl under tow. A choice will be made between the two grates and a second grate will be built of the material deemed most suitable.
A key factor will be determining the angle of incline of the grate which produces the best bycatch separation. This will be done by completing a series of 15-minute tows with the grate at different angles. The initial tow will be at a near vertical plane. Ten short tows will be made with a cod end attached to the escape chute to catch those fish which would have escaped. The catch in both portions of the net (i.e. those that have gone through the grate into the regular cod end and those which gone up the grate, through the escape chute into a separate cod end) will be recorded. After ten tows the top of the grate will be released and reattached at the next mesh some 3 inches further back thus increasing the angle of incline of the grate. The appropriate meshes will have pre-attached color-coded wire ties to expedite change. The towing sequence will then be repeated. Ten tows will be made with the grate angled at eight different positions with a final angle created by attachment some 24 inches back. The escape chute will be adjusted each time so that rear end of the opening is flush with the top of the grate. The angle which produces the greatest separation of codfish from flounders will be established.
The final phase of the project will be a field test to produce statistically valid data which demonstrates the effectiveness of the design in reducing bycatch of cod. Two vessels will be used and they will tow in parallel. Four one-hour tows will be made on each of four sequential days. On the first day, both vessels will tow with the standard unaltered groundfish gear. On the second day one vessel will tow with the experimental design while the other tows with standard set (no grate). The experimental/control positions will be reversed on the third day and in the final four tows both vessels will have the experimental gear. The experiment will be repeated to produce a total of 32 tows with each of the experimental and control nets.
| Parallel one-hour tows (four per day) | ||
|---|---|---|
| Day 1 | Control | Control |
| Day 2 | Control | Experimental |
| Day 3 | Experimental | Control |
| Day 4 | Experimental | Experimental |
| Days 5 - 8 repeat the above sequence | ||
The composition of the catch will be recorded by a biologist and statistical tests used to demonstrate significant differences. A report and a short video which shows the grate under construction and functioning in the water will be supplied to the Northeast Consortium.
We will video the net in the process of catching fish to demonstrate how fish react to the grate. There are potential problems with lighting and sediment disturbance while videotaping in many of the traditional fishing areas. We will try to avoid this by starting a tow in a shallow sandy part of Ipswich Bay where visibility is good and trawl eastward towards deeper water to areas where there should be a mixture of groundfish. A camera will be mounted on the gear to focus on the grate and escape chute. A line will be run on to the vessel where we will view the activity on the pilothouse monitor and record continuously. It is expected that we will encounter mixed species on the bottom and be able to illustrate behavior of different species before light and sediment disturbance problems become an issue at increased depth.
Allan Michael of Magnolia, MA will provide biologists on specified trips and will be responsible for summarizing and analyzing the data and producing a written report.
Ronald Smolowitz, a fisheries engineer, will provide technical advice on the construction and performance of the gear design.
The 40ft vessel Maria Rose, owned by Robert Fisher of Rockport, MA will be used in the in the paired trawling.
Bill Lee
4 Sunset Lane
Rockport MA 01966
978 546 2748
978 502 5993