Engineering and catch implications of variable wing-end spread on a penaeid trawl

The importance of wing-end spread on the performance of a generic penaeid trawl was assessed to investigate the potential for confounding effects when comparing modified anterior sections designed to improve selectivity and fuel efficiencies. Two identical beam-and-sled assemblies were configured to allow two identical trawls (7.35 m headlines and footropes) to be adjusted to spread ratios (defined as wing-end spread ÷ headline length) of 0.5, 0.6, 0.7 and 0.8 and deployed (n = 30 each) in a double rig across the same depth, current, towing speed and duration. Increasing spread ratio significantly increased drag (by up to 16%), without affecting absolute catch weights. However, when standardised to per ha trawled, significantly fewer targeted school prawns (Metapenaeus macleayi) and total bycatch by weight were retained in the wider-spread trawls. The significant reductions in standardised catch with increasing spread ratio were hypothesised to reflect either: (i) slightly reduced ground gear contact and headline heights offsetting the greater swept areas; or perhaps more likely (ii) steeper wing angles which increased the probability of mesh encounters and escape for school prawns and were less efficient for herding fish. Future research comparing modified trawl bodies should focus on maintaining similar spread ratios to minimise confounding effects. Similar logic applies to surveys using penaeid trawls to obtain relative abundance estimates.