Using a horizontal-separator panel in an Australian fish trawl to quantify species-specific changes in vertical orientations during capture

Abstract

Owing to few data describing species-specific vertical orientations during capture in Australian fish trawls, but the need for information to progress modifications to improve selectivity and reduce operational costs, a generic trawl was fished with upper and lower (‘dual’) extensions/codends, and with and without a horizontal-separator panel in the posterior 50% of the body. The two configurations facilitated quantifying the vertical orientations of species from the mid to posterior trawl body based on catches. During 32 deployments, 13 and eight commercial and non-commercial species/groups were caught in sufficient quantities for analyses. The commercial species entered the posterior trawl at varying heights, but many individuals (including the most abundant, tiger flathead, Platycephalus richardsoni and especially larger fish) initially orientated low in the mid trawl, and then moved upwards at the posterior trawl to present similar catches in each of the dual codends. Eastern school whiting, Sillago flindersi was the only commercial species showing the opposite behaviour, entering higher in the mid trawl before orientating downwards. Non-commercial catches were biased towards rays, skates and stingarees (46%) and all except smooth stingrays, Bathytoshia brevicaudata mostly entered the lower posterior trawl, with proportionally fewer than commercial species eventually rising upwards. The data benefit prioritizing future modifications for regional testing, including lowering the headline and increasing mesh size in the anterior, upper trawl body to reduce drag (which might not greatly affect catches of commercial species) and/or changing the lower anterior trawl body to stimulate key fish to rise up earlier, and potentially facilitate separating unwanted elasmobranchs, possibly via bottom-opening grids or larger meshes/drop-out panels in the posterior trawl. More broadly, the data reiterate the benefits of horizontal-separator panels for providing insight into species-specific behaviour in trawls, supporting their application in other fisheries.