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Tickler chains are simple additions to the ground gears of benthic trawls that can improve the catches of some target species, albeit with considerable variability. The purpose of this study was to decipher some of the technical variability associated with tickler chains used with generic, paired trawls targeting school prawns, Metapenaeus macleayi. In the first of two experiments, single tickler chains were either attached between the trawl otter boards or the wing ends and compared against each other and a control trawl (no tickler chain). Attaching the tickler chain to the otter boards significantly reduced wing-end spread and the trawled area without significantly affecting catches of school prawns. By comparison, attaching the tickler chain to the wing ends maintained spread and resulted in significantly more school prawns being caught (by 1.11 x) than in the control trawl, and also fewer numbers of one teleost species, pink-breasted siphonfish, Siphamia roseigaster. In experiment two, a single tickler chain was compared against one with multiple chains and one with a double chain; all attached at the wing ends. Trawls with the multiple- and double-tickler chains had the same wing-end spread as those with the single tickler chain, while retaining up to 1.17 x more school prawns, and for the double-chain tickler, 45% fewer southern herring, Herklotsichthys castelnaui. The results are attributed to me- chanical and visual stimuli affecting school prawns and some fish, respectively, and support maximising the surface area of tickler chains within the wing ends as a means for improving penaeid-trawl species selectivity without affecting swept area. [ABSTRACT FROM AUTHOR]

Highlights • Single- and double-concave 'simple anterior fish excluders' (SAFEs) were compared in an Australian penaeid-trawl fishery. • Both SAFE configurations maintained catches of the targeted penaeids. • The double-concave SAFE was slightly more efficient than the single-concave SAFE at reducing bycatch, but it complicated fishing operations. • The single-concave SAFE warrants ongoing refinement, testing and adoption among penaeid trawlers. Abstract Following recent international efforts to assess anterior modifications to trawls as a means for precluding unwanted catches, a promising device termed the 'simple anterior fish excluder' (SAFE) was modified for an Australian penaeid-trawl fishery. We tested a conventional penaeid trawl against those rigged with two SAFEs; each comprising a plastic banner located between the otter boards but orientated as either single- (original design) or double-concave configurations. Compared to the conventional trawl, both SAFEs caught significantly fewer absolute and standardised (ha–1 trawled) numbers of the most abundant non-target species, southern herring (Herklotsichthys castelnaui), although mean reductions were greater in the trawls with the double-concave (by 59 and 50%) than single-concave (44 and 36%) SAFE. Neither absolute nor standardised catches of the targeted school prawns (Metapenaeus macleayi) were affected. The results reiterate the generic utility of anterior modifications for reducing bycatch and maintaining function across different geometries. Configuring the SAFE as a double concave did partially improve its efficiency, but it also complicated fishing operations. Therefore, we conclude that owing to its simplicity, the single-concave design might be a preferable configuration for penaeid-trawl fisheries—pending assessments of other refinements. [ABSTRACT FROM AUTHOR]

The utility of fitting panels of mesh turned 45° (‘T45’ or ‘square-shaped’ mesh) in a generic penaeid trawl body was investigated for improving selectivity. Compared to a conventional trawl comprising 43-mm diamond-shaped mesh throughout the body and a codend made from 27-mm T45 mesh, replacing just the side panels (18% of the total trawl area) with those made from 32- and 35-mm T45 mesh generally improved size-selection for school prawns, Metapenaeus macleayi —which was the dominant species—and especially in the trawl with 35-mm T45 mesh. This trawl also had a trend of improved size selection for the much less abundant eastern king prawns, Penaeus plebejus . Commercial catches of both penaeids were not negatively affected by the modifications. No teleosts were significantly affected by T45 side panels—although the predicted mean numbers of species with individuals smaller than the 32- and 35-mm T45 meshes were slightly lower. We conclude that retroactively inserting T45 side panels with an appropriate mesh size into penaeid-trawl bodies could be a simple and effective strategy for improving size selection; either as an adjunct to T45 codends, or as an alternative modification. [ABSTRACT FROM AUTHOR]

The effects of reducing bar spaces in a generic Nordmore grid used in an Australian penaeid-trawl fishery were investigated within an objective of maximising bycatch reduction while maintaining target catches. Compared to a conventional Nordmore grid with 20 mm bar spaces, those with 16 and 18 mm bar spaces maintained the retained weights of school prawns, Metapenaeus macleayi, and with no effects on size selectivity, but caught incrementally lower weights of total fish, which for the 16 mm grid was significant (mean reduced by ~26%). The results imply that Nordmore grid bar spaces can be reduced to a size approaching the width of the largest penaeids with no commercial impacts. [ABSTRACT FROM AUTHOR]

In an attempt to improve selection and engineering efficiency in an Australian penaeid fishery, the mesh size and side taper of a generic trawl design were reduced (from 41 to 35 mm) and increased (from 1N3B to 1N5B), respectively and, within the revised design, the independent and combined utility of (i) increasing the frame-line hanging ratio, and (ii) replacing the diamond-orientated wing/side panel with square-orientated mesh were investigated. Compared to the 41-mm mesh conventional trawl, all of four smaller-meshed designs (comprising either loose or tight hanging ratios, and with diamond- or square-mesh wing/side panels) had lower drag (9–12%) and required less fuel (up to 4–12%). Some of the small-meshed trawls caught fewer unwanted fish—attributed to the steeper side panels allowing individuals to swim forward and escape. Within the small-mesh trawls, there were incremental improvements in size selectivity for the targeted school prawns, Metapenaeus macleayi associated with both modifications. But the square-mesh wing/side panels were by far the most effective, reducing the catches of sub-commercial school prawns by up to 72%. The results support simple, retrospectively fitted alterations to existing penaeid-trawl bodies to improve selection and efficiency. [ABSTRACT FROM AUTHOR]

Three experiments were conducted to compare the engineering and catching performances of a hydrodynamic otter board termed the ‘batwing’ (comprising a sled-and-sail assembly, configured to operate at 20° angle of attack – AOA and with minimal bottom contact) against three conventional designs (termed the ‘flat-rectangular’, ‘kilfoil’ and ‘cambered’ otter boards) with AOAs between ∼30 and 40°. Experiments involved paired penaeid trawls (7.35-m headlines). The first experiment compared the batwing otter boards against all other designs (using 41-mm mesh trawls). In experiment 2, the batwing was tested against the flat-rectangular design (with 32-mm mesh trawls). In experiment 3, the batwing and flat-rectangular otter boards were towed without trawls to facilitate estimates of their partitioned drag. Overall, compared to the conventional otter boards, the batwings had up to ∼86 and ∼18% less bottom contact and drag, respectively. Among the conventional otter boards, the trawls spread by the cambered design caught up to 13% more school prawns Metapenaeus macleayi attributed to their greater solid profile. No significant differences were detected among catches of fish in the trawls spread by the various otter boards. The results reaffirm that because otter boards contribute towards a large proportion of total system drag (estimated here at up to ∼56%), their appropriate configuration is essential to maximise the fuel efficiency of penaeid-trawl systems. [ABSTRACT FROM AUTHOR]

Two paired-trawl spreading mechanisms were compared, with a view towards progressing more environmentally benign penaeid-trawling systems. Identical twin trawls (7.35-m headline) were alternately towed in a “double rig” (four otter boards) with either 18- or 36-m bridles (“short-” and “long-” double rigs) and in a “dual rig” (36-m bridles with two otter boards and a sled). At similar speeds (∼1.31ms−1), wing-end spread per trawl was significantly greatest in the long-double (mean±SE: 4.50±0.05m), followed by the short-double (4.27±0.05m) and dual (3.86±0.05m) rigs. The dual rig required 24% and 20% less towing force and fuel and caught significantly fewer (by ∼60%) small mulloway, Argyrosomus japonicus per ha of foot-rope contact than both double rigs. Other standardised catches were not significantly different among configurations, although many, including school prawns, Metapenaeus macleayi were lower (by 25–39%) in the dual rig – which manifested as a relatively greater quantity of fuel required per kg caught (up to ∼1.5 times more). When school-prawn catches were combined across double rigs and compared against the dual rig, the latter caught significantly larger individuals, but fewer per ha of foot-rope contact (by 35%); an effect that was negated when catches were re-standardised to the contact of the foot-rope and otter boards. The results show that removing otter boards reduces benthic contact, towing force, fuel consumption and bycatch, but there are negative implications for penaeid catches that may require compensatory offsets. [ABSTRACT FROM AUTHOR]

Abstract: Implementing gear-based solutions to reduce bycatch in trawl fisheries can be difficult owing to fishers’ concerns over the fleet-wide applicability of results from discrete, highly controlled experiments associated with the development of such innovations. In New South Wales, Australia, a codend comprising a posterior section made from 35-mm square-shaped mesh (i.e. hung on the bar) was developed and subsequently recommended for partial implementation in a multi-species demersal trawl fishery (i.e. for use when eastern king prawn –Melicertus plebejus – is the primary target species). However, industry concerns over this design have to date prevented its implementation. Here, using direct twin-tow comparisons onboard a subset of the fleet, we quantitatively assessed the performance of the square-mesh codend, relative to the range of diamond-mesh codend designs currently being used, in terms of reductions in bycatch rates and shortfalls in catch rates of retained animals. Generalised linear mixed-effects modelling across all vessels showed that when eastern king prawn was the target species, the square-mesh codend significantly reduced bycatch rates (by ∼48%), but with an accompanying significant shortfall in catch rates of eastern king prawn (∼7%) and a substantial (though not statistically significant) shortfall in catch rates of an important component of the retained byproduct (whiting, Sillago spp., ∼36%). Partitioning of data by vessel revealed that such shortfalls in retained catch were evident for only some vessels, while the square-mesh codend performed as intended for the others. When school prawn (Metapenaeus macleayi) was the target species, substantial shortfalls in catches of school prawn (∼54%) in the 35-mm square-mesh codend confirmed that a codend of a smaller square mesh might be a more appropriate design under those circumstances. Potential strategies to address intra-vessel inconsistencies in performance of the square-mesh codend and thereby facilitate implementation of square-mesh codends in the fishery are discussed. [Copyright &y& Elsevier]

Abstract: 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 (7.26m 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 (143kg) and initial wing-end spreads (predicted means of 4.33 and 4.45m), 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. [Copyright &y& Elsevier]

Abstract: Since bycatch reduction devices (BRDs) were first tested in Australian penaeid-trawl fisheries almost 20 years ago, there have been ongoing refinements to a discrete range of legislated designs; mostly in response to dynamic fishery-specific operational requirements. This study assesses the relative utility of the latest versions of two generic BRDs that have evolved/devolved in an estuarine fishery. Compared with a codend containing no BRD, a simplified Nordmøre-grid and radial escape section significantly reduced the total weight of bycatch by 74 and 36%, respectively. Catches of the targeted school prawns (Metapenaeus macleayi) were also reduced by 4 and 11%, respectively, although, at least for the Nordmøre-grid, these losses were apparently offset by the practicality of the revised design in terms of a simpler and cheaper construction. The presence of large quantities of displaced submerged plants (e.g. Egeria densa and Hydrilla verticillata) on the trawl grounds at certain times (which preclude using the Nordmøre-grid) may similarly compensate for the reduced efficiency of the radial escape section. In addition to quantifying the performance of the revised BRDs, the results provide direction for their future refinement in this and other similar fisheries. [Copyright &y& Elsevier]

Broadhurst, M. K., Millar, R. B., and Uhlmann, S. S. 2009. Using a double codend to reduce discard mortality. – ICES Journal of Marine Science, 66: 2077–2081.Traditional technical strategies for mitigating collateral fishing mortality have involved improving gear selectivity (to reduce bycatch) and, more recently, concomitant changes to onboard handling procedures to reduce some of the negative impacts to the remaining discards. A less common approach is to modify gears physically to minimize deleterious catching mechanisms and subsequent mortalities during fishing. This study aimed to investigate the utility of the latter category of modifications for penaeid trawls by separating a codend into two compartments (termed a double codend) to alleviate interactions between catches. Compared with a conventional design, the double codend significantly reduced the immediate (from 17.1 to 13.8%) and short-term (22.5 to 17.1%) mortalities of discarded juvenile school prawns (Metapenaeus macleayi). The effectiveness of the double codend remained independent of other factors known to affect the fate of discarded juvenile M. macleayi, including cloud cover and, owing to minimal variability, the weight of the total catch. However, irrespective of the codend configuration, overall mortalities were also positively correlated with the quantity of jellyfish. We conclude that when combined with modifications to improve selectivity and appropriate onboard handling strategies, compartmentalizing codend catches could cumulatively reduce unaccounted fishing mortality. [ABSTRACT FROM PUBLISHER]

The potential for changes to onboard handling practices in order to improve the fate of juvenile school prawns (Metapenaeus macleayi) discarded during trawling were investigated in two Australian rivers (Clarence and Hunter) by comparing a purpose-built, water-filled sorting tray against a conventional dry tray across various conditions, including the range of typical delays before the start of sorting the catch (2 min vs. 15 min). Juvenile school prawns (n= 5760), caught during 32 and 16 deployments in each river, were caged and sacrificed at four times: immediately (T0), and at 24 (T24), 72 (T72), and 120 (T120) hours after having been discarded. In both rivers, most mortalities occurred between T0 and T24 and, after adjusting for control deaths (<12%), were greatest for the 15-min conventional treatment (up to 41% at T120). Mixed-effects logistic models revealed that in addition to the sampling time, method of sorting, and delay in sorting, the weight of the catch, salinity, and percentage cloud cover were significant predictors of mortality. Although trawling caused some mortalities and comparable stress (measured as L-lactate) in all school prawns, use of the water tray lessened the negative impacts of some of the above factors across both the 2-min and 15-min delays in sorting so that the overall discard mortality was reduced by more than a third. When used in conjunction with selective trawls, widespread application of the water tray should help to improve the sustainability of trawling for school prawns. [ABSTRACT FROM AUTHOR]

Two experiments were carried out to assess the short-term impacts to juvenile school prawns, Metapenaeus macleayi (Haswell) discarded from commercial seining and trawling in New South Wales, Australia, and the potential utility of modifications to operational and/or onboard handling practices designed to improve survival. For both gears, discards were handled according to two general categories of treatment (termed mild and extreme) chosen to represent the plausible limits of severity during existing commercial operations. A total of 600 trawled and 480 seined school prawns (from both treatments), along with appropriate numbers of controls (previously collected and housed in acclimation tanks), were placed into cages (in groups of 10). Subsets of each of these groups were then destructively sampled over periods of up to 5 days and examined for their mortality and physiological response (measured as levels ofl-lactate in the haemolymph). In both experiments, the temporal mortalities in the treatment groups ranged between 0% and 15% and, for the most part, were not significantly different to controls. Prawns in the treatment groups had similar, significantly elevated levels ofl-lactate approximately 40 min after being caught, but these returned to levels approaching baseline estimates within 48 h. Under the conditions examined, juvenile school prawns appear capable of withstanding a range of stressors associated with being discarded from active gears, and their fate is not greatly influenced by the duration of gear deployment and subsequent air exposure. However, further work is required to quantify other sources of unaccounted fishing mortality and, ultimately, the impacts that discarding has on stocks. [ABSTRACT FROM AUTHOR]