Confounding effects of knot orientation in penaeid trawls

Abstract

This study assessed the potential for confounding engineering and/or catching effects due to different knot (‘sheet bend’) orientations (and therefore positive or negative hydrodynamic lift) in the principal horizontal panels of a penaeid trawl. During 24 paired comparisons, orientating all meshes in the top and bottom belly panels of a trawl to produce a negative angle of attack (AOA) for all knots resulted in greater wing-end spread, and lower catches of school prawns, Metapenaeus macleayi per hectre trawled (means reduced by up to 26%) and three key species of schooling fish (by up to 67%), but more debris (4×) than when the meshes were orientated to produce a positive AOA. By comparison, catches of two other benthic fish remained unaffected by knot orientation. These catch differences were attributed to the negative-AOA trawl potentially assuming a considerably lower position in the water column (and species-specific orientations). The results demonstrate that it is essential to maintain consistency in knot orientation in penaeid trawls when testing gear modifications and/or during fishery-dependent or -independent surveys.