Research Document - 2008/050
A Review of Current Studies on Scallop Rake Modifications to Reduce Groundfish Bycatch in the Canadian Offshore Scallop Fishery on Georges Bank
By S.J. Walsh
Abstract
The management of bycatch is fundamental to Canada’s ecosystem based fishery conservation and management approach. In the Canadian sea scallop fishery on Georges Bank, the capture and mortality of yellowtail flounder, Atlantic cod and haddock, are measured and managed to bycatch limits for these stocks. The Canadian scallop fishery on Georges Bank is managed under Department of Fisheries and Oceans (DFO) Scotia Fundy Offshore Scallop Integrated Fisheries Management Plan (IFMP) that uses Total Allowable Catch (TAC) and meat counts as its two key management measures. This management plan includes a host of other control measures including Enterprise Allocations (EAs), bycatch quotas, dockside monitoring, regular observer coverage, satellite vessel tracking, etc. The industry have made efforts to manage their bycatch quotas by changing their fishing strategy, using acoustic seabed mapping to locate scallop beds and effectively fishing with reduced effort. However, because of seasonal movements and fluctuating changes in abundance of some groundfish species, it may be necessary to look at more than one conservation measure, or a combination of measures, to manage finfish bycatch in the Canadian scallop fishery on Georges Bank. Area/time closures are one conservation measure used to manage bycatch, and Canada has used this measure to manage the cod and yellowtail bycatch in the scallop fishery during cod spawning on Georges Bank. Gear based technical conservation measures can also be used to manage size and species bycatch, and, worldwide, have a long history in conservation research and management. Along with incentives and other management strategies, a key approach to managing bycatch of finfish and small scallops is to design and operate selective scallop rakes using knowledge of individual fish behaviour, including the target species, rake hydrodynamics and fishing practices.
Since the mid-1990s, both the Canadian and American scallop fishing industries have studied the engineering performance of scallop rakes (dredges) and potential modifications to reduce bycatch of finfish, while minimizing the loss of scallops. As a result, the 2004 USA management plan has regulated the mesh size in their twine top (equivalent of a Canadian rope back) panels to be set at 10 inch diamond or square mesh, and the ring size in the rake bags to be set at 4 inches. In Canada, there are no regulations pertaining to the construction or rigging of a scallop rake. Similar to American scallop fishers on Georges Bank, the Canadian scallopers generally fish twin New Bedford style rakes ranging in size from 14 feet on the wetfish vessels to 17 feet and, occasionally, a 20 feet rake on the factory freezer vessels. The ring size in the bag of rakes is mainly 3 inches, although some factory freezer vessels are using 4 inches. The wetfish trawlers and some freezer trawlers generally use a 5 or 6 inch diamond mesh rope back which may be hung square, while some factory freezer trawlers use 16 inch square mesh rope backs.
This report reviews and summarizes published and grey literature from Canada, the USA and the international community on gear modifications that have been proposed or used to reduce the bycatch of groundfish in scallop dredge fisheries. In the 11 major reports documenting trials to reduce finfish bycatch in the Canadian and American scallop rake fishery, there were some promising modifications to the frame (bale and bridle bars) of the rakes. The most encouraging modification to the rake frame was the use of a cookie sweep mounted just in front of the cutting bar. This appears effective at reducing catches of yellowtail, skates and other flatfish while increasing the catches of scallops, and works by scaring or herding fish out of the way before they enter the bag path. However, it may only be suitable on smoother substrates, and further testing is warranted. Overall, the most promising modifications to reduce finfish bycatch was an increase in mesh size of the rope back/twine top panel and an increase in ring size of the rake bags. Although most of the rope back/twine top mesh studies focused on diamond mesh panels, there were some investigations into using panels where the diamond mesh was hung square, i.e., the diamond mesh netting was turned 45 degrees. This way of rigging the mesh panel is common among Canadian scallop fishers, while using regular diamond mesh in the top panel is common among American scallop fishers on Georges Bank. Round fish like cod and haddock should escape easier through either a square or a diamond mesh opening of sufficiently large size. The laterally compressed shape of flounders should favor escapement through elongated diamond meshes more so than through square meshes.
An attempt was made to combine the results from several disparate studies to look at trends in catches and bycatch reduction. Regression analysis was chosen to examine the form and significance of an overall relationship between bycatch reduction and mesh size increase in the rope back/twine top panel. Although it provided a quantitative method to combine the data sources and determine the direction of the effect, the results of the combined data from square and diamond mesh rope backs/twine top panel experiments should be considered equivocal; i.e., uncertain with respect to significance because of the varied nature (and quality) of the data extracted from the four reports used in the analysis. From these four reports, only one report gave any measure of uncertainty for estimates of percent reduction in mean catches. However, the individual experiments and the combined analysis do demonstrate a central tendency for a reduction in finfish bycatch with increasing mesh size in the rope back/twine top panels. Loss of scallops may become significant at larger mesh sizes. The question whether large square mesh rope backs retained or released more flatfish also could not be conclusively answered from the limited studies examined.
The overall conclusion from the synthesis of the experimental results in the individual reports on gear modifications and the regression analysis carried out on a limited data set is that increasing the mesh size in the top panel shows the greatest promise for large reduction in finfish bycatch. Other gains are expected in reducing catches of juvenile flatfish and scallops by increasing the ring size in the rake bag. A larger ring size should reduce fish mortality and improve yield from the scallop resource by promoting harvest of larger scallops with higher meat weights. The majority of the current Canadian scallop fleet fishing Georges Bank are using 5 and 6 inch diamond mesh size rope backs hung regular or on the square, and are using 3 inch ring size in the rake bags. The analysis suggests that these small mesh sizes in the diamond and square rope back and ring size in the rake bags may not be all that effective at releasing large amounts of finfish bycatch and small scallops. This may already be recognized by fishers because some Canadian factory freezer vessels have been and are currently fishing knotless 16 inch square mesh with a 4 inch ring size in their rake bags.
With the use of any conservation measures, whether it be area/time closures, bycatch quotas or gear based modifications, one should remember that temporal and spatial changes in the dynamics and abundance of groundfish species can shift the complexity of the problem thereby necessitating continuous adjustment in solving the problem. The various gear modifications examined vary in effectiveness depending on the dynamics and abundance of groundfish species and flexibility is required to deal with these fluctuations over time.
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