Penaeid Trawls Research Articles

Relative performance of two bycatch reduction devices for excluding juvenile mulloway, Argyrosomus japonicus and other fish from south-eastern Australian penaeid trawls

Mulloway, Argyrosomus japonicus is an economically valuable species across southern Australia, but the south-eastern population is considered depleted owing to excessive historical commercial and recreational fishing mortality. To address this concern, stock-recovery initiatives have been implemented in New South Wales, which include minimising unaccounted fishing mortality by regional penaeid trawlers targeting school prawns, Metapenaeus macleayi. This study presents four experiments to evaluate the relative effectiveness of two mandated bycatch reduction devices (BRDs) to reduce incidental catches of mulloway across volunteer estuarine penaeid trawlers. Both BRDs were ‘mechanical separators’ designed to partition catches based on size. The first design (termed the ‘blubber-chute’) is the most commonly used in the assessed fisheries and comprised a funnel of 96-mm mesh inside the extension that directed larger organisms through a posterior, top opening. The second design (‘Nordmøre-grid’), comprised a 400- × 872-mm aluminium grid with 20-mm bar spacings and inserted at 33° in the extension. Organisms larger than the bar spaces were directed upwards to an escape exit similar in size to that in the blubber-chute. The BRDs were assessed using covers on single-rigged trawlers and simultaneously without covers on double-rigged trawlers. Trawls with the BRDs had the same quantities of school prawns but those with the blubber-chute retained significantly more total incidental catches (by up to 5.2 ×) and mulloway > 20 cm TL (up to 9 ×) than those with the Nordmøre-grid. Relatively greater numbers of unwanted juveniles of another economically important species, tailor, Pomatous saltatrix, and others were also excluded by the Nordmøre-grid. The results support preferential use of the conventional Nordmøre-grid for eliminating the bycatch of mulloway >20 cm TL. Secondary BRDs that mitigate the capture of smaller fish warrant ongoing assessment.

Assessing T45 vs T0 and T90 meshes and covered codend effects in southeastern Australian penaeid trawls

Square-mesh codends (i.e. turned 45 degrees or ‘T45′) made from a minimum stretched mesh opening (SMO) of 27-mm mesh hung on the bar are mandated for estuarine-trawl fisheries targeting penaeids (mostly school prawns, Metapenaeus macleayi) in New South Wales, Australia. However, owing to variable mesh shrinkage over time, often slightly larger SMOs are supplied, and sometime too large to be oriented as T45 for the targeted school prawns (>13–15 cm carapace length; CL). During three experiments involving paired trawls with and without codend covers, we sought to investigate if, compared to a conventional 27-mm T45 codend, lateral openings in 30-mm SMO could be appropriately maintained by orientating diamond-shaped (‘T0′) meshes at narrow (experiment 1) or normal (experiment 2) circumferences, or turned 90 degrees (‘T90′; experiment 3). Analyses of size selection was done after first testing for species-specific cover effects, which required fish sizes to be assessed via a paired-hauls approach while sizes for school prawns were modelled using a covered-codend approach. Compared to the 27-mm T45 codend, only the T0 mesh hung at a normal circumference provided the same size selection for school prawns, with the two other alternative configurations producing wider selection. But, while incidental fish catches were low (14% of total catch), all new codends caught between ∼2 and 7 × more individuals of some small fish cross all sizes, including juveniles of an economically important species (tailor, Pomatomus saltarix). The data reiterate that among the codends tested, T45 mesh is the optimal configuration for regional penaeid trawls in terms of consistently maximining school prawn size selection and the escape of small fish, and so high-quality material in the required sizes needs to be sourced.

Improving the performance of a generic fisheye as a secondary bycatch reduction device in a south-eastern Australian penaeid-trawl fishery

The utility of a generic device (termed a ‘fisheye’) and its simple modification for cumulatively reducing bycatch in penaeid trawls was assessed during two experiments in a south-eastern Australian fishery. In the first experiment, conventional trawls (with no fisheye) were compared to simultaneously towed, identical trawls installed with a typical fisheye (termed ‘mesh fisheye’) comprising an open triangular frame terminating in a semi-elliptical vertical opening (350 × 150 mm) in the top of the trawl extension at 1.1 m anterior to the codend drawstring. Trawls with the mesh fisheye maintained catches of penaeids (mostly school prawns, Metapenaeus macleayi) but retained significantly less total bycatch (by 39 %) and fewer unwanted southern herring, Herklotsichthys castelnaui (by 60 %) and tailor, Pomatomus saltatrix (by 52 %). During the second experiment, trawls with the mesh fisheye were simultaneously compared to those with a modified design comprising a solid panel of aluminium in the lower half of the triangular frame (termed the ‘solid fisheye’). The solid fisheye excluded significantly more bycatch (by 52 %) and small southern herring (by 47 %), and also the numbers of several other species, including mulloway (by 38–41 %) without affecting penaeid catches. The improved performance of the solid fisheye was attributed to its greater anterior displacement of water and posterior area of lower perceived flow for fish. The results and simplicity of the fisheye modification support not only its regional use, but also testing in other national and international fisheries.

Adaptive Spatial Management to Deal with Postflood Inshore Bycatch in a Penaeid Trawl Fishery

AbstractAdaptive spatial management provides a way to balance the conservation of bycatch species and the potential economic effects of bycatch‐induced closures on a fishery, but implementation can be both data and resource intensive. We present a survey exploring a novel approach to adaptively manage an intermittent bycatch issue in an inshore trawl fishery where Sciaenidae bycatch species (including juvenile Mulloway Argyrosomus japonicus, a species of conservation concern) co‐occur with large aggregations of a valuable target species (eastern school prawn Metapenaeus macleayi), following flood‐induced egression from estuarine habitats. Diurnal sampling surveys followed three flood events and targeted permanent inshore closure zones (designed to protect the normally nocturnally harvested eastern king prawn Melicertus plebejus) and adjacent inshore nonclosure areas. Sciaenidae abundance was mostly <10 fish/ha trawled, but larger catches generally corresponded with larger catches of eastern school prawn, and these relationships appeared to be steeper within nonclosure zones. There were only minor differences in the size structure of eastern school prawn between zones, and patterns were event and location specific. Negligible eastern king prawn were encountered. The results show that temporarily allowing fishers access to permanent trawl closures following floods may support the exploitation of aggregated target species with reduced interaction with bycatch species of concern. The approach presented here is relevant to the consideration of adaptive spatial management in other trawl fisheries.

Modelling the potential consequences of adaptive closure management in a penaeid trawl fishery

Spatial management of fishing effort can be used to avoid catching undesirable size classes for target species, and improve yield-per-recruit for the exploited stock. Adaptive closure management has been proposed as a means to more effectively utilise spatial management, however these management provisions often lack quantitative evaluation which constrains the information available to inform decisions. We demonstrate the use of a spatially and size structured population dynamics model to evaluate the potential impact of spatial management on a multijurisdictional fishery for a highly migratory species (eastern king prawn, Penaeus [Melicertus] plebejus). Under current conditions in the fishery, the overall effect of closures on harvest was estimated to be comparatively minor, regardless of assumptions about how effort or fisher behavior are affected by spatial management. Alternative assumptions about the movement patterns of eastern king prawn had little influence on the impact of closures on overall harvest. However, when effort was increased to historic levels similar to those observed when the closures were implemented, a much greater impact on overall harvest was observed. The approach taken and simulation outcomes are discussed in the context of spatial management for both eastern king prawn, and penaeid fisheries more broadly.

Connectivity between a spatial management network and a multi-jurisdictional ocean trawl fishery

Spatial management is often applied in penaeid trawl fisheries to avoid non-optimal size classes of target species, by providing areas of refuge from trawl fishing effort. Efficient application of spatial management relies on having a detailed knowledge of animal movements, particularly for species with complex life histories. Using Eastern King Prawn (Penaeus (Melicertus) plebejus; EKP) as a case study, we examine how spatial and temporal factors can influence connectivity between a spatial management network designed to protect small prawns from capture by trawlers, and a multi-jurisdictional (Queensland [Qld] and New South Wales [NSW]) fishery. Over 7500 prawns were tagged across multiple years in three northern NSW spatial management areas, and 786 tagged prawns were recaptured across both jurisdictions. The majority of prawns tended to be recaptured from deeper waters adjacent to, or directly north of, the spatial management areas, but tagging location and release timing influenced recapture patterns. Cohorts tagged early in the growth season showed lower recapture rates, but were larger on average at recapture, and had migrated further. Prawns reached the fishery in the adjacent jurisdiction (Qld) within 54 days-post tagging, but for most cohorts the majority of recaptures were in NSW zones. Overall, there was good connectivity between all spatial management areas and the NSW fishery, and tagged prawns also reached the Qld spawning areas. This represents an optimal scenario for NSW fishers (who forgo capture of prawns within these spatial management areas): the prawns protected in the closure areas primarily contribute to harvest of larger sized prawns within NSW waters; and a portion of the protected prawns survive to reach the southern Queensland spawning grounds, which is the primary source of recruits for northern NSW nurseries. These findings are broadly relevant to defining the role and impact of established and proposed spatial management networks within penaeid fisheries.

Impact of Spatial Management on Nontarget Species in an Oceanic Penaeid Trawl Fishery

AbstractTrawling is generally an unselective fishing method that is employed to harvest a number of species. Capture of nontarget species that are subsequently discarded is an important issue for trawl fisheries, especially penaeid trawl fisheries. While gear modifications are a common approach to manage this issue, maintaining areas that are closed to trawling may also have a positive impact on these nontarget species. This study systematically examined the diversity and abundance of nontarget species across a network of areas closed to trawling (trawl closure areas) relative to adjacent areas that were open to trawling. Analysis of nontarget species from 209 trawls and 72,787 animals (across 216 taxa) revealed some impact on both abundance and diversity within trawl closure areas. Latitude was a key variable that was driving differences in the taxonomic assemblage among locations. Within locations, there were inconsistent impacts for different nontarget species in trawl closure areas. Length distributions showed size truncation of abundant nontarget species in fished areas relative to adjacent trawl closure areas. While the impact of trawl closure areas varied among species, it was clear that the spatial management in the fishery provided some refuge for a diversity of nontarget species, observed in patterns of species richness, abundance, and population size structure. Latitudinal breadth appears to be an essential component of the trawl closure network, but localized effects may be important for maintaining reproductive capacity for nontarget species. These data highlight the broader utility of trawl closure networks as a management tool for trawl fisheries.

Influence of top-panel orientation (lead-ahead) on targeted and incidental catches in a penaeid trawl

Various, low-cost modifications to the anterior sections of penaeid trawls have been shown to realise significant improvements in species selectivity. This study expands the existing options by quantifying the importance of top-panel orientation relative to the bottom panel (dictated by relative headline and footrope lengths; termed ‘lead-ahead' or ‘lead’) on targeted and incidental catches in an Australian penaeid-trawl fishery. Three trawls were compared in a double rig: (1) the conventional configuration with identical footrope and headline lengths (7.53 m) and a 200-mm diameter buoy attached to the latter (termed the ‘buoyed no-lead trawl’); (2) the same trawl, but with the buoy removed (‘no-lead trawl’); and (3) a trawl with the same headline length as above, but the footrope 1.14× longer, creating lead-ahead (‘lead trawl’). Between 11 and 26 deployments of each configuration were done over six fishing days. Compared to the buoyed no-lead trawl, both the no-lead and lead trawls had greater wing-end spreads and both caught less total bycatch (absolute and standardized weights ha–1 trawled reduced by up to 64 and 48 %, respectively) and fewer individual unwanted species, including jellyfish, Catostylus mosaicus (by up to 61 and 58 %) and southern herring, Herklotsichthys castelnaui (by up to 59 and 43 %). However, only the lead trawl maintained catches of the targeted school prawns, Metapenaeus macleayi. These target catches were reduced by 37 % in the no-lead trawl (without the buoy). The results imply a trawl without lead-ahead requires sufficient headline height to maximise catches of school prawns, but simply introducing lead-ahead and removing the buoy (lowering the headline) can maintain target catches while dramatically reducing bycatch. Similar subtle refinements may have application in other penaeid-trawl fisheries.

Seeking maximum effectiveness and efficiency for large multi-sail penaeid otter boards

The greatly-stretched lateral opening of penaeid trawls is typically achieved by using relatively large otter boards with an aspect ratio of ~0.5 at high angle of attack (AOA; ~40°) and low efficiency (lift-to-drag ratio; L/D ~1). Alternatively, the ‘batwing’ otter board utilises a unique rigging system to allow a flexible sail with an aspect ratio of ~1 to operate at a lower AOA (~20°). As a result, high effectiveness (lift) and efficiency (L/D ~3) are achieved for small- and medium-size gear. However, for large trawlers it is impractical to simply scale up the current batwing design as this will excessively increase the trawl's height and generate unnecessary drag. Hence, here we experimentally searched for greater hydrodynamic forces from a multi-sail configuration (comprising a jib and main sail) by varying the twist and camber of the two sails. The results suggest that the jib needs to be set at a low AOA compared to the main sail for balanced pressure loading of the two sails, and the main sail requires medium twist to control detrimental centre of pressure characteristics. Ultimately the desired objective for large-sized batwing boards might best be achieved using a two-mast configuration that incorporates three sails.

The effect of subsampling when monitoring bycatch in a penaeid trawl fishery

Fishery dependent and independent survey programs for multi-species trawl fisheries often employ subsampling to increase efficiency, but this is not usually accompanied by any quantitative analysis to optimise its application. Here, we measure the effect of subsampling on the information generated from trawl surveys, and provide relationships to assist the interpretation of data derived from such surveys, and confidence in associated estimates for future studies. A commercial fishing vessel was chartered to perform normal fishing operations, and various proportional subsamples (by weight) were sorted from each tow. Overall, 50 % of total species diversity in a sample would be detected if only 10 % of the total sample mass was sorted, however abundance estimates of individual species under different subsampling scenarios varied depending on their rarity. For common species, the abundance in a sample was estimated with reasonable accuracy from sorting only 10 % of the biomass, whereas sorting >60 % of the biomass was required to obtain an accurate estimate of abundance for rarer species. The careful consideration of program objectives is important in optimising the sampling approach employed, as subsampling may not be appropriate for detection of rare species. The patterns presented here provide a quantitative basis to support sampling and sorting methodology for multispecies trawl samples, as well quantifying the implications of subsampling on the confidence in estimates of diversity or abundance.

Adaptive bycatch reduction in penaeid trawls via rapid adjustments to headline height

AbstractPenaeid trawling is among the world's least selective fishing methods; a characteristic that has evoked spatial closures being implemented in some fisheries if certain bycatch limits are exceeded. For decades, considerable work has been done to develop modifications to penaeid trawls that reduce unwanted bycatches, with most focussed at the posterior section (i.e. codend). More recently, efforts have examined ways to prevent bycatch entry into trawls entirely—via modifications to anterior components. This study assessed the utility of proactively lowering the headlines of Australian penaeid trawls, using clips at the otter boards, to 68% and 54% of their conventional height, and demonstrated mean total bycatch reductions (by weight) of 69% and 79%, respectively, with no effects on the targeted Metapenaeus macleayi (Haswell). The results provide insights into the location and behaviour of various species in the water column preceding capture, and support a simple and easy method for regional fishers to use in situ to avoid excessive bycatch and associated fishing closures. More broadly, the data support ongoing efforts in other penaeid‐trawl fisheries to reduce bycatches via similar, rapid adjustments to anterior components, depending on species‐specific behaviours during capture.

Refining a simple anterior fish excluder (SAFE) for penaeid trawls

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.

Challenges in adopting turtle excluder devices (TEDs) in Brazilian penaeid-trawl fisheries

The incidental capture and mortality of endangered, threatened and protected (ETP) marine species remains a global problem. Among the most problematic fishing methods is penaeid trawling, which is responsible for one quarter of the world's discarded bycatch and remains a key threatening process to ETP species (turtles and some elasmobranchs). So-called ‘turtle excluder devices’ (TEDs) legislated among USA penaeid trawlers and subsequently fleets in other countries during the 1990s alleviated some concerns over impacts to ETP species, but adoption has not been global. One country characterised by resistance to TEDs is Brazil—despite federal legislation mandating usage for almost 25 years. The reasons for this deficit, reiterated here via interviews with 64 southern penaeid-trawler captains, are threefold. First, are perceived issues with mandated designs and minimal third-party expertise, leading to a sustained misconception that TEDs simply do not work in Brazilian penaeid trawls. Second, is the perpetuated belief of minimum negative impacts to ETP species; exacerbated by few quantitative data. Third, is jurisdictional failure to promote the concept that slight economic loss associated with not catching ETP species (elasmobranchs) is necessary for broader ecological sustainability. We propose TED use among Brazilian penaeid-trawl fisheries might be promoted via a co-management strategy facilitated by sustained education and technical expertise from research providers encouraging fishers to develop fishery-specific solutions. If industries can be encouraged to accept the concept of TEDs, they might then be expected to refine and develop ownership of the most appropriate configurations.

Quantifying the morphology of key species caught in the southern Brazilian penaeid trawl fishery as a precursor to improving selection

Penaeid trawls are poorly selective fishing gears; contributing towards approximately 27% of global marine fisheries discards. Various options are available for mitigating penaeid-trawl bycatch, including gear modifications such as ‘bycatch reduction devices’ (BRDs) or codend mesh-size regulations. A precursor to developing modifications is information about the key target and bycatch species in terms of their sizes and morphology. Here we describe the relationships between these characteristics for the southern Brazilian industrial penaeid-trawl fishery within a broader objective of proposing more selective trawl configurations. Catches were sampled during 37 tows. Fifty-two species were caught, including two loggerhead turtles, Caretta caretta , one green turtle, Chelonia mydas , as well as 61 individuals of seven ray species classified as Endangered or Critically Endangered. One penaeid ( Pleoticus muelleri ) and 11 teleosts were assessed for various morphological relationships. The data demonstrated that both the existing conventionally used 26 mm (stretched mesh opening; SMO) mesh and a legislated size of 30 mm SMO are too small. Using morphological relationships, we propose testing a minimum diamond-shaped mesh size of at least 35 mm and a square-mesh window in the top of the codend comprising at least 48 mm mesh. Such a configuration would probably retain penaeids and larger teleosts, but allow many small teleosts to escape. Anteriorly located grids are also required to reduce the bycatch of charismatic species like turtles and rays. Wide-scale use of such BRDs should considerably reduce bycatches and the ancillary impacts of regional penaeid-trawl fisheries.

T45 side panels improve penaeid-trawl selection

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.

Choosing anterior-gear modifications to reduce the global environmental impacts of penaeid trawls

Globally, penaeid-trawl fisheries are faced with three broad sustainability issues: (1) large bycatches; (2) acute benthic-habitat impacts; and (3) high energy consumption. Most resolution efforts have focused on i above, and via bycatch reduction devices (BRDs) installed in the posterior trawl (codend), which typically reduce total bycatches by 30–70%, but are poorly adopted owing to few perceived benefits by fishers. While mandated BRDs will remain a feature of selective penaeid trawling, solutions to habitat impacts and high energy consumption require changes to the anterior trawl, including the spreading mechanisms (e.g. otter boards, beams and sleds), ground gears, and net designs. Further, because such components ultimately determine which organisms enter the codend, it should be feasible to structure anterior-trawl modifications to address all three sustainability issues, including improving selection. We sought to review the feasibility of such an approach here, and located fifty-eight relevant articles: of which 45, 11 and 23 directly or indirectly focused on reducing bycatch, habitat impacts and energy consumption, respectively. Considering these articles, we propose a protocol for holistically improving the environmental efficiency of penaeid trawling involving: (1) selecting the most appropriate multi-net configuration; (2) reducing otter-board angle of attack to ~20°; (3) minimising twine area; and (4) optimising horizontal-trawl opening. Compared to conventional configurations, choosing alternatives within the above protocol could reduce total unwanted bycatches and habitat contact by >70%, while concomitantly lowering drag/fuel costs by >20%. The latter outcome might improve selective penaeid-trawl adoption among global fishing fleets.

Effects of knot orientation on the height and drag of a penaeid trawl

The knots used to make fishing nets are asymmetric which, in most cases for trawls, means they present either a positive or negative angle of attack (AOA) in horizontal panels, producing hydrodynamic side forces and changes in system geometry. The extent of associated displacement and drag were quantified in a flume tank using four, full-scale penaeid (shrimp/prawn) trawls with the same generic design, but with all possible combinations of knot orientation in the top and bottom panels. All four trawls had the same distance between the footrope and flume-tank floor at the wing end during a flow of 1.2ms−1, but the trawl with both panels at a positive AOA had a significantly greater footrope height at the centre (by up to 13mm). Further, compared to both trawls with their top panels orientated to have a positive AOA, the two with their top panels producing a negative AOA had up to 27% lower headlines and 10% less total drag. Differences were also observed among vertically partitioned drag at the wing ends—depending on knot orientation in the bottom panel, which led to greater force on the headline or footrope when at negative or positive AOAs, respectively. The magnitudes of observed variations in footrope and headline heights and drag are likely to affect the catching and engineering performances of penaeid trawls, and so knot orientation should be considered during attempts at improving selectivity and reducing drag. The results also reiterate a need for consistency among knot orientations to avoid confounding effects during fishery-dependant/independent surveys.

Progressively evaluating a penaeid W trawl to improve eco-efficiency

The W trawl is a proposed innovation for more eco-efficient penaeid trawling—via reduced energy intensity and benthic contact. A significant portion of the drag-generated, netting tension of the W trawl is directed away from the wings to centre-line bracing ropes that are connected to a central towing wire through an intermediary sled. This configuration facilitates substantially smaller otter boards than normal, and consequently less drag and seabed contact. Here we compared the engineering and catching implications of subtle, progressive rigging changes to a W trawl paired against a standard Florida flyer trawl; both rigged with another eco-efficient modification, termed the ‘batwing’ otter board (suitably sized for each trawl). The W-trawl refinements included: (i) securing netting at the wing ends to prevent sliding; (ii) moving the headline forward and footrope aft; and (iii) inserting dan lenos at the wing ends. The first two modifications resulted in a maximum drag reduction by the W trawl of up to 11% for a 1.14×increase in spread. Catches ha−1 of mostly retained (commercially important) large brachyurans and cephalopods were maintained, but penaeid catches were reduced in the W trawl (by at least 33%), which was mostly attributed to less than optimal ground-gear contact. This study provides a base for future research to concurrently reduce the energy intensity and habitat impacts of penaeid trawls, via holistic modifications.

Confounding effects of knot orientation in penaeid trawls

This study assessed the potential for confounding engineering and/or catching effects due to different knot (‘sheet bend’) orientations (and therefore positive or negative hydrodynamic lift) in the principal horizontal panels of a penaeid trawl. During 24 paired comparisons, orientating all meshes in the top and bottom belly panels of a trawl to produce a negative angle of attack (AOA) for all knots resulted in greater wing-end spread, and lower catches of school prawns, Metapenaeus macleayi per hectre trawled (means reduced by up to 26%) and three key species of schooling fish (by up to 67%), but more debris (4×) than when the meshes were orientated to produce a positive AOA. By comparison, catches of two other benthic fish remained unaffected by knot orientation. These catch differences were attributed to the negative-AOA trawl potentially assuming a considerably lower position in the water column (and species-specific orientations). The results demonstrate that it is essential to maintain consistency in knot orientation in penaeid trawls when testing gear modifications and/or during fishery-dependent or -independent surveys.

Twist-and-camber effects on the performance of simple hydro-sails for efficiently spreading penaeid trawls

Penaeid-trawling fuel efficiency can be enhanced by using otter boards with high lift-to-drag ratios (L/D), which generally occurs at low angles of attack (AOA). The batwing otter board is such a low AOA solution; incorporating a unique towing arrangement and a high aspect ratio, flexible, hydro-sail to spread trawls with minimal seabed contact. This study investigated the effects of twist and camber on the performance of two simple batwing sails in a flume tank. Six degree-of-freedom loads were measured for AOAs between 0° and 40° for five twist and camber combinations. The major findings were that: (1) changing the mast rake angle from 30° to 45° reduced L/D at the design AOA of 20° by 12%, considering each sail’s optimum setting; (2) stall angle increased for the higher rake angle, despite the associated sail having the greater aspect ratio; (3) irrespective of rake angle, camber was positively associated with lift whereas twist decreased drag and substantially increased L/D at the design AOA; and (4) a L/D ratio of ∼3 was achieved or exceeded at 20° AOA for both sails, which is ∼3 times greater than for contemporary otter boards, implying a 20% drag reduction for otherwise equivalent penaeid-trawling systems.