Development of low drag trawls for energy efficient fishing

Abstract

Trawling is the most energy intensive fishing method and fuel cost alone constitute upto 75% of operational expenditure.It was reported that to catch one kilogram of fish, trawling requires 0.8 kg of fuel. Drag is the single most important factor contributing to the fuel consumption and thereby energy efficiency and profitability of trawl operations. Drag of trawl depends on factors like the design and rigging of the net and the operating conditions. According to Wileman (1984), the warp contributes 5%, sweeps-4%, otterboards-20%, floats-3%, foot rope-10% and netting- 58% to the total drag. Use of smaller otterboards, adoption of optimized towing speed, thinner twines and large mesh size to reduce twine surface area and opting selective shape of the mesh can bring down the drag and fuel consumption. Similarly, drag reduction is achieved by adopting optimized trawl design with special features. Cutaway top belly shrimp trawl is an example of design improvement for drag and bycatch reduction. Estimation of drag of commercial trawls in Kerala reveal that itranges from 1.37 to 48.94 kN and it is more for fish trawl, followed by cephalopod andleast for shrimp trawl. Stronger materials will permit the use of thinner twines, leading to less twine area for trawl fabrication. Comparative trials carried out with 24.47 m fish trawls made of high density polyethylene (HDPE) and ultra-high molecular weight polyethylene (UHMWPE) revealed that the average reduction in drag of UHMWPE trawl was 17% with an average reduction in fuel consumption by 10%. The fuel consumption per kilogram of fish captured was estimated as 2.9 liters for HDPE trawls and 1.9 liters for UHMWPE trawls. It is concluded that material substitution, coupled with improvement in trawl design can help significantly in reducing the drag and fuel consumption to improve the economic feasibility of trawl operations. Besides, by reducing the fuel consumption, a drastic cut in carbon emission can be made.