Numerical optimization of trawl energy efficiency taking into account fish distribution

Abstract

This study reports on energy efficiency optimization regarding bottom trawls. Efficient fishing gear uses up only a small amount of energy per fish caught. Drag and mouth area during trawling operations affect energy efficiency. Drag causes the energy consumption and the trawl mouth area impacts the quantity of fish caught, hence an energy efficient gear has a low ratio drag on the mouth area. A novel numerical optimization technique using spatial fish distribution is presented in this work. The tool is based on a FEM mechanical model for trawls which consist mostly of netting panels sewn together. This tool is adapted to minimize an objective function namely the drag-to-mouth area ratio. This technique consists in modifying the design of all the panels of the trawl. In this paper the modifications are constant and quantified in terms of mesh number. Moreover the trawl mouth area takes into account the presence of fish within a given depth with respect to sea bottom and the value of the depth is adapted to the fish species of interest. Trawl design optimization with two uniform fish distributions at a given depth (6 m and 3 m above the sea bed) and one linear distribution at 6 m above the sea bed are compared. The application of this tool when designing a bottom trawl for research vessels leads to an energy economy ranging from 16% to 52% under certain assumptions.