Modifying otter boards to reduce bottom contact: effects on catches and efficiencies of triple-rigged penaeid trawls

Published source details Broadhurst M.K., Sterling D.J. & Millar R.B. (2015) Modifying otter boards to reduce bottom contact: effects on catches and efficiencies of triple-rigged penaeid trawls. Fisheries Management and Ecology, 22, 407-418.
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This study is summarised as evidence for the following.

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	Modify the design or configuration of trawl doors Action Link

Modify the design or configuration of trawl doors

A replicated, controlled study in 2014 of an area of seabed in the Tasman Sea off New South Wales, Australia (Broadhurst et al. 2015) found that triple rigged otter trawls with different otter board designs (two types) caught higher amounts of one of six discarded fish species, and similar amounts of discarded catch overall (fish and invertebrates combined), compared to a conventional door type. For unwanted fish, catch rates of only one of six species, spiky flathead Ratabulus diversidens, differed between door designs and was lower with the conventional doors than either of the non-conventional designs (restrained: 0.03, batwing: 0.07, conventional: 0.01 kg/ha). Catches of long spine flathead Platycephalus longispinis, small-toothed flounder Pseudorhombus jenynsii, red bigeye Priacanthus macracanthus, long-finned gurnard Lepidotrigla argus and eastern bluespotted flathead Platycephalus caeruleopunctatus were similar between door designs (data reported as statistical model results). The average catch rates of all unwanted catch (fish and invertebrates combined) was not significantly different between “restrained” flat, rectangular doors (2.6–3.2 kg/ha), “batwing” doors (3.2–3.5 kg/ha) and the conventional flat, rectangular doors (4.1–4.6 kg/ha). In addition, catches of commercial target eastern king prawns Penaeus plebejus were similar across trawl designs (see paper for data). Fifteen trawl deployments/door type were made in July 2014 using a triple-rigged trawl fitted with either conventional flat-rectangular otter doors (2.0 × 0.8 m, angle of 42°), “restrained” flat-rectangular doors (same doors as conventional with a line attached to the central sleds to minimise door spread), or “batwing” doors, comprising a plastic “sail” mounted on a main stainless steel sled at an angle of 20°. Trawls were towed for an average of 8.6 km at an average depth of 55 m. Full details of trawl design are provided in the original study.

(Summarised by: Leo Clarke)