Issues in Shrimp Trawl Bycatch and BRDs


The tropical penaeid shrimp species constitute a substantial portion of the total world shrimp and prawn catch. Trawling which is the most dominant benthic sea fishing, isprobably the least selective. This exercabate the bycatch(catch incidentalto the target species catch) problem. The bycatch is usually discarded. Thejuviniles of the target species is now deemed as bycatch as they are alsodiscarded-a practice which threatens the maintenance of biodervisity and long term sustainability (FAO,1995).The bycatch problem in the first place arises when different species with different sizes, body shapes and fleeing behaviour occur on the same fishing ground. I reckon it's as old as the then art of fishing itself. As most fisheries resources in the world are fully exploited or overexploited, the large discarded quantity (estimated at 25 million toones) represent the most significant source available to increase marine harvest levels. Introduction of better management regimes eg. shifting exploitation patterns towards larger individuals by using size selective fishing gears and techniques is another way to increase yields from the sea.In the shrimp trawl fishery, the bycatch problem is even more pronounced because of ususally the huge market price of shrimp as compared to other species in the shrimp trawl catch. The tendency is to reserve the preservation and storage space for shrimp therefore other species have to be sent overboard if utilization programs are not in place. Shrimp is a low volume high value species. For the internal rate of return (IRR)to remain positive, long periods are spent at sea to obtain a substantial volume of shrimp catch. The bycatch is therefore continiously discarded during the fishing period.

In temperate countries, the distinct difference in size between shrimp and fin fish species; the relatively few species and the passive behaviour of shrimp enhances the development of gear and tecniques for catching one species rather than another ie inter-specific selectivity. There is a large number of different species on the same ground with shrimp in tropical waters. Moreover, a size overlap between shrimp and fin fish have been observed. This make inter-specific selectivity unattainable in warm waters. In as much as a substantial proportion of global shrimp supplies originate from the tropics, the bycatch problem is as acute. But ironically, most studies on selectivity of shrimp trawls for bycatch reduction have been and is being done in higher lattitudes. Selective gear and techniques used in higher latitudes are not readily transferable to tropical waters as the success of a selective gear depends on the application of design appropriate to the fauna and fishing techniques used in a given fishery. Encouragement and support in this area is urgently needed. A logical approach to fishing gear selectivity in warm waters should focus on size selectivity. The fishing mortality of the juvinile component of the stock would be alleviated and growth over fishing brought undre control. Higher yields per recruit will be obtained while sorting time reduced during fishing operations. As size selectivity only ensures the exclusion of certain predetermined sizes form the catch, the bycatch and discard problem is not completely overcome by size selective gears. Bycatch utilisation progrms must be developed and instituted to address this problem of waste admist hunger in the tropics.

After years of experimenting with soft sorting devices(meshed net screen) tangling, gilling and clogging problems coupled with illegal modification of the flexible panels militated against their operational success as a separator device. Moreover, size may not be the only factor in size selectivity of shrimp. The many appendages of shrimp may make them prone to becoming entangled in the net. The operational inadequacies of the nets induced the testing of hard sorting devices; for example to exclude turtles(TEDS)in the gulf of mexico shrimp fishery in the late 70's (Ogren et al., 1977; Seidel, 1979; Siedel and Watson,1978 ; etc etc etc) which kicked off the development of bycatch reduction devices(BRDs) throughout those years and more recently, the development and introduction of a very successful BRD, NORDM=D8RE grid( a rigid separator device for inter-specific selectivity) used in separating shrimp from fish in the pandalus borealis shrimp fishery (see Isaksen et al., 1992). A tropicalised multi-species fishery versions of the nordm=F8re grid adapted for size selectivity (Top grid) have been recently tested in tropical waters (Mahika, 1992; Isaksen and Larsen, 1993 , Isaksen et al., 1995 ; Baio, 1996....analysis in progress). Reported studies indicate the potential of top grid to replace mesh size regulations in the tropics.

Although aquaculture accounted for 30% of the global marine supplies by 1992, an increase form less than 4% to 7% betwween 1984 and 1992,(FAO, 1995) the future of the sector's sustainable contribution to shrimp supplies is questionable. The concern of a host of authors is that rapid expansion of intensive commercial shrimp farming in it's present form in coastal areas of a given country can only be sustained for a couple of decades at most before diseases associated with self pollution causes drastic decreases in production. The capture of wild shrimp may remain to be the most economic and environmentally sustainable method of shrimp production. This would be true only if the resource is sustainingly utilized in harmony with the environment and capture practices that are not harmful to the ecosystem, resource or their quality are used. This embodies "Responsible Fishing". This is what BRDs stand for. Let it be resposible fishing now and forever.

Interested in multi-species BRDs for warm waters. Would appreciates contributions ,leads, references etc.


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