Engineering and catching performance of five netting materials in commercial prawn-trawl systems

Abstract

High-strength netting trawls have less drag due to the opportunity to use thinner twine. In prawn/shrimp trawl systems, where large otter boards spread the trawls to high spread ratios, further substantial drag reduction should occur due to the reduced size of otter boards required. Here we conducted two comparative experiments with commercial prawn trawls made from five respective netting materials including standard polyethylene (PE), high tenacity PE and three examples of Ultra High Molecular Weight (UHMW) PE. In the first experiment, a sequence of paired comparisons of engineering performance (drag and spread) was undertaken across all trawls individually connected to one of three pair of different size otter boards; in all cases codends were left open to avoid catch size confounding the measurements. In the second experiment, catch and selectivity performances were assessed by towing the five trawls simultaneously in a five-rig system, with location in the system randomly varied. The trawl-board combination with the best engineering performance for each high-strength material produced a range of drag reduction compared to the best combination for the standard PE trawl: ∼12% for high tenacity PE and ∼19–21% for the three UHMW PE trawls. The high-strength netting trawls caught larger prawns compared to the standard PE trawl, possibly due to (i) increased twine flexibility allowing smaller prawns to escape, and (ii) greater flow of water through the trawl causing increased catch efficiency for larger, more mobile prawns. A strong negative result was the large amount of bycatch for most high-strength netting trawls—it was concluded that this was driven in part by low fishing line tension, exacerbated by higher operating spread ratio. Some evidence was obtained that this could be mitigated by applying layback to the headline.