Abstract
The effects of reducing mesh size while concomitantly varying the side taper and wing depth of a generic penaeid-trawl body were investigated to improve engineering performance and minimize bycatch. Five trawl bodies (with the same codends) were tested across various environmental (e.g. depth and current) and biological (e.g. species and sizes) conditions. The first trawl body comprised 41-mm mesh and represented conventional designs (termed the ‘41 long deep-wing'), while the remaining trawl bodies were made from 32-mm mesh and differed only in their side tapers, and therefore length (i.e. 1N3B or ‘long’ and ∼28o to the tow direction vs 1N5B or ‘short’ and ∼35o) and wing depths (‘deep’–97 T vs ‘shallow’–60 T). There were incremental drag reductions (and therefore fuel savings – by up to 18 and 12% per h and ha trawled) associated with reducing twine area via either modification, and subsequently minimizing otter-board area in attempts to standardize spread. Side taper and wing depth had interactive and varied effects on species selectivity, but compared to the conventional 41 long deep-wing trawl, the 32 short shallow-wing trawl (i.e. the least twine area) reduced the total bycatch by 57% (attributed to more fish swimming forward and escaping). In most cases, all small-meshed trawls also caught more smaller school prawns Metapenaeus macleayi but to decrease this effect it should be possible to increase mesh size slightly, while still maintaining the above engineering benefits and species selectivity. The results support precisely optimizing mesh size as a precursor to any other anterior penaeid-trawl modifications designed to improve environmental performance.
Highlights
Benthic otter trawling relies on hydrodynamic forces created by hydrovanes dragged across the seabed to achieve correct net geometry, and is an energy intensive fishing method [1]
There was a significant difference in SMO between the 41conventional trawl and the four smaller-meshed designs (LM, p,0.001), but no significant differences among the latter (FDR, p.0.05; overall mean 6SE of 31.6160.08 mm)
The results from this study further highlight the potential for improving the engineering and biological performances of penaeid trawls by changing their anterior mesh (1) lateral openings
Summary
Benthic otter trawling relies on hydrodynamic forces created by hydrovanes (otter boards) dragged across the seabed to achieve correct net geometry (to capture the targeted animals), and is an energy intensive fishing method [1] This is especially the case for penaeid trawls which, owing to the small sizes of the targeted species and their distributions (often buried in the soft substratum) [2,3] require small mesh (typically 30–50 mm stretched mesh opening–SMO) and sufficient bottom contact pressure [4]. The mesh characteristics are important, since do the lateral openings influence what escapes or is retained (for small animals), the twine typically comprises .70% of the total system area for most penaeid trawling systems, and strongly affects drag
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