Effects of otter boards on catches of an Australian penaeid trawl

The effects of otter boards on the catches of Metapenaeus macleayi were quantified within the broad aim of promoting more hydrodynamic and environmentally benign trawl-spreading mechanisms. Two identical trawls (7.26 m footropes) were attached to 2.89-m sweeps and simultaneously fished in a double-rig; each alternately attached to either two otter boards or a rigid 6.0-m beam with parallel sleds (configured not to impact on the behaviour of M. macleayi). Both trawl configurations had the same total weight (143 kg) and initial wing-end spreads (predicted means of 4.33 and 4.45 m), although the otter boards caused significantly more variable wing-end spread during towing. Replacing the otter boards with the beam significantly reduced system drag (by 10%) and the weights and numbers of school prawns per hectare swept by the footropes (by 33%); with the latter attributed to the concomitant reduction in lateral bottom contact of the system. Standardised catches of the most abundant teleost Herklotsichthys castelnaui were also significantly lower (by 79%) in the beam trawl. The results support the potential for compensatory adjustments to penaeid-trawl parameters (e.g. footrope length) to offset reductions in the bottom contact of spreading mechanisms that are modified to minimise drag and/or habitat impacts.

► Paired trawls were attached to either a beam or otter boards to quantify the effects of the latter on catches.
► Catches of targeted Metapenaeus macleayi were lower in the beam trawl; attributed to the absence of otter boards and their efficiency for disturbing and directing buried individuals towards the trawl mouth.
► The beam trawl also caught ∼80% fewer individuals of the most abundant fish (Herklotsichthys castelnaui).
► The results highlight the importance of spreading mechanisms for affecting penaeid-trawl species selectivity.