Reducing small shrimps in the Gulf of Maine pink shrimp fishery with a new size-sorting grid system

Reducing small shrimps in the Gulf of Maine pink shrimp fishery with a new size-sorting grid system Pingguo He, Institute for the Study of Earth, Oceans and Space and NH Sea Grant University of New Hampshire, Durham, NH. (603) 862-3154, [email protected] Vincent Balzano, F/V “North Star”, Saco, Maine, USA INTRODUCTION Since the implementation of the Nordmore Grid in the Gulf of Maine pink shrimp trawl fishery in the early 1990s, fish bycatch has been drastically reduced. However, the Nordmore Grid does not improve shrimp size selection and a large amount of small shrimps is still landed. This paper reports two designs of size-sorting grid system. In the new designs, the size-sorting grid was installed in front of the main Nordmore grid. The design was tested at the flume tank and at sea in the pink shrimp (Pandalus borealis) fishery in the Gulf of Maine. Both parallel tows involving two vessels and alternating tows using one vessel were made to compare the size-sorting grid system with a conventional Nordmore grid. Both size sorting designs significantly reduced small shrimps in the catch. This was indicated by a reduction of count (number per kg) by 38 and 45 respectively. There was a reduction of shrimp catch rates, presumably due largely to the release of small shrimps. There were no significant differences in the number or amount of major bycatch species between the commercial grid and the two experimental grid designs. The designs were practical to operate and easy to install. Their application may have wider implications in reducing small shrimps in the pink shrimp fishery in the Gulf of Maine and in other areas. • Size-species grid No funnel (SSNF) • Size-species grid With funnel (SSWF) Funding for this cooperative research project was provided by: RESULTS DISCUSSIONS MATERIALS AND METHODS Fishing Vessels: F/V “North Star”, F/V “Persistence” Fishing Grounds: Gulf of Maine, off Portland, Maine Fishing Depth: 37-49 m Fishing Season: March – June 2006 Tow Duration: one hour The new size sorting grid was installed in front of the main Nordmore grid, and was found effective in improving size selectivity of the shrimp. The increase in size is indicated by large reductions in counts. The SGNF grid (Size-Species Grid No Funnel) worked very well with a count reduction of 38 shrimp/kg with an acceptable reduction in shrimp catch (16%). Small shrimps of < 20 mm CL may have counted for the majority of the reduction in the catch. The size grid seemed not to influence the function of the main Nordmore grid as indicated by the low amount of finfish bycatch in the codend. The new size-sorting grid which was installed in front of the Nordmore grid showed great potential of reducing small shrimps in the pink shrimp fishery in Gulf of Maine. The technology may also be used in other pink shrimp fisheries with suitable adjustment to the grid spacing to match available shrimp size in the area and desirable sizes in the catch. Application to other shrimp and prawn fisheries may also be possible with additional experimentation. CONCLUSIONS Gear Design Pink shrimps (Pandalus borealis) are widely distributed in the northern Pacific, Atlantic and Arctic oceans. Because small mesh codends are used for harvesting the species, bycatch was considerable before the Nordmore grid became mandatory in 1992 in Gulf of Maine (Howell and Langan, 1992). However, a Nordmore grid cannot reduce small shrimps which pass through the 25 mm (1") grid spacing. Landing of small undersize shrimps represent resource wastes as well as a reduction in landed value. Various projects have been carried out to reduce small shrimps. Square mesh codends were tested in a number of fisheries. Though there was some reduction in small shrimps, mesh brokerage and loss of market size shrimps were reported (Hickey et al., 1993). Valdemarsen (1989) proposed a funnel-shape design, called Radial Escape Section (RES), installed in the rear section of the belly to force small shrimps out of the trawl. The device was shown to reduce small shrimps but blocking of the funnel by debris and complicated rigging prevented wide use of the design. A dualgrid system was tested in Newfoundland and in the Gulf of Maine (DFO, 1998; Schick et al., 1999). In both tests, the size sorting grid was installed behind the Nordmore grid. They had limited success due probably to lowered flow rates at the second grid (FTU, 1996). We here report two size sorting grid designs with the size grid in front of the main Nordmore grid. Nordmore grid Size sorting grid Small shrimp exit Finfish exit Nordmore grid Size sorting grid Small shrimp exit Finfish exit Size sorting No Funnel 0.00 0.05 0.10 0.15 0.20 5 10 15 20 25 30 35 Carapace length (mm) R el at iv e Fr eq ue nc y SSNF Control Size sorting No Funnel 0.00 0.05 0.10 0.15 0.20 5 10 15 20 25 30 35 Carapace length (mm) R el at iv e Fr eq ue nc y SSNF Control Size Grid with Funnel 0.00 0.05 0.10 0.15 0.20 5 10 15 20 25 30 35 Carapace length (mm) R el at iv e Fr eq ue nc y SSWF Control Size Grid with Funnel 0.00 0.05 0.10 0.15 0.20 5 10 15 20 25 30 35 Carapace length (mm) R el at iv e Fr eq ue nc y SSWF Control Size Grid 136 shrimp/kg Control 174 shrimp/kg Reduce by 38 shrimp/kg Shrimp catch reduction 16% Size-species grid No Funnel (SSNF) Size Grid 147 shrimp/kg Control 192 shrimp/kg Reduce by 45 shrimp/kg Shrimp catch reduction 43% Size-species grid With Funnel (SSWF) The SSNF Grid The SSNF grid caught much larger shrimp than the control. Mean counts (number per kg) were 135.5 (SE 6.75) for the experimental tows and 173.6 (SE 7.11) for control tows, which were significantly different (p-value <0.01). This represents a reduction in count of 38.1 shrimp/kg (17.3/lb). Carapace length frequency distribution indicates that the lengths where shrimps appear to be lost from the size-sorting grid was from 20 mm CL or below with the largest amount of shrimp belonging to the 19 mm size class. The K-S test indicated that the size distribution was significantly different (p-value <0.01, z statistic 5.68). The SSWF Grid The SSWF grid caught much larger shrimp than the control using the regular Nordmore grid. Mean counts (number per kg) were 147.2 (SE 7.52) for the experimental tows and 192.3 (SE 7.23) for control tows, which were significantly different (p-value <0.01). The difference was 45.2 count/kg (20.5 count/lb). Carapace length frequency distribution indicates that the lengths where shrimp appear to be lost from the sizesorting grid is from 22 mm CL or below with the largest amount of shrimp belonging to the 20 mm size class. The K-S test indicates that the size distribution were significantly different (pvalue <0.01, z statistic 5.02).