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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 ×) 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 × more school prawns, and for the double-chain tickler, 45% fewer southern herring, Herklotsichthys castelnaui. The results are attributed to mechanical 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.

Penaeid trawls often have poor ecological efficiencies measured as unwanted bycatches (with high discard mortality) and

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.

Freshwater flows into estuaries influence fish populations through effects on recruitment, growth and mortality. We compared year class strength of a large sciaenid Argyrosomus japonicus with rainfall through a 16-year period in southeastern Australia, to understand the influence of freshwater input on this estuary-dependent and depleted population. Relative year class strength, estimated by back-calculating age composition data from commercial fishery landings, was positively related to rainfall within estuarine catchments (R2 = 52%). Commercial estuarine landings from a separate 29-year dataset were positively related to rainfall two and three years earlier (R2 = 21% and 44%, respectively). Year class strength of a key prey species, the penaeid prawn Metapenaeus macleayi, was also related to year class strength of A. japonicus (R2 = 39%), suggesting that rainfall influences recruitment of A. japonicus by affecting food availability. Several years of above average rainfall may be required to promote recruitment substantial enough to rebuild the population.

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.

Penaeid 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.

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.

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.

In an attempt to improve the selectivity and engineering performances of generic penaeid trawls, three established and one novel spreading-mechanism configurations were assessed: otter boards attached (1) with and (2) without 3.15-m sweeps to a 7.35-m headline trawl, and a beam rigged directly to a 9.19-m trawl (3) with and (4) without a horizontal wire and plastic streamers. Despite more surface area (7.5 vs 6.0 m), both beam-trawl configurations had significantly lower drag than the otter trawls (≤30%). When catches were standardised to per ha, the otter trawl with sweeps retained significantly more (1.3-2.4 times) school prawns, Metapenaeus macleayi (Haswell), than the other three configurations. Within systems, removing sweeps or adding a horizontal wire significantly reduced the unwanted catches of a key teleost (southern herring, Herklotsichthys castelnaui Ogilby) by 41 and 48%. The results illustrate the utility of simple anterior modifications for independently addressing penaeid-trawling environmental issues.

The importance of wing-end spread on the performance of a generic penaeid trawl was assessed to investigate the potential for confounding effects when comparing modified anterior sections designed to improve selectivity and fuel efficiencies. Two identical beam-and-sled assemblies were configured to allow two identical trawls (7.35 m headlines and footropes) to be adjusted to spread ratios (defined as wing-end spread ÷ headline length) of 0.5, 0.6, 0.7 and 0.8 and deployed ( n = 30 each) in a double rig across the same depth, current, towing speed and duration. Increasing spread ratio significantly increased drag (by up to 16%), without affecting absolute catch weights. However, when standardised to per ha trawled, significantly fewer targeted school prawns ( Metapenaeus macleayi ) and total bycatch by weight were retained in the wider-spread trawls. The significant reductions in standardised catch with increasing spread ratio were hypothesised to reflect either: (i) slightly reduced ground gear contact and headline heights offsetting the greater swept areas; or perhaps more likely (ii) steeper wing angles which increased the probability of mesh encounters and escape for school prawns and were less efficient for herding fish. Future research comparing modified trawl bodies should focus on maintaining similar spread ratios to minimise confounding effects. Similar logic applies to surveys using penaeid trawls to obtain relative abundance estimates.

The engineering and catching efficiencies of single-, double-, triple- and quad-rigged penaeid trawls (all with the same cumulative headline – 14.63 m and sweep lengths – 9.76 m) were compared across the same spatio-temporal scales in an attempt to identify the more environmentally benign configurations (in terms of species selectivity, fuel intensity and habitat contact). The relative system efficiencies reflected a complex array of interacting factors, but broadly environmental performance improved with an increasing number of nets in the configuration. The single rig had the least fishing capability due to having the lowest wing-end spread (a ‘spread ratio’ – SR to headline length of 63.50%), but the greatest drag, and required the most fuel (predicted mean of 2.88 L per ha trawled). Further, this system caught significantly more yellowfin bream, Acanthopagrus australis and fewer (but larger) of the targeted school prawns, Metapenaeus macleayi than the multi-trawl rigs. Compared to single rig, the double rig had an improved SR (68.76%) and was easier to tow (2.44 L per ha trawled). However, triple and quad rigs similarly achieved the greatest SRs (75.46 and 74.37%) and at lower drag and fuel consumptions (2.13 and 2.21 L per ha trawled, respectively). Providing effort is regulated, the latter two configurations, but especially triple rig (which had relatively less total benthic contact), could be the most suitable base from which to progress modifications to further reduce the environmental impacts of penaeid trawling.

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 (∼1.31 m s −1 ), wing-end spread per trawl was significantly greatest in the long-double (mean ± SE: 4.50 ± 0.05 m), followed by the short-double (4.27 ± 0.05 m) and dual (3.86 ± 0.05 m) 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.

As part of efforts to minimise unwanted environmental impacts of penaeid trawling, the importance of side taper (and therefore total length) on the (i) engineering performance and (ii) selectivity of a generic trawl was investigated in an Australian fishery. Four designs (‘1N2B two-seam’, ‘1N2B four-seam’, ‘1N5B two-seam’ and ‘1N5B four-seam trawls’) were tested; all identical (7.35-m headlines and 42-mm mesh) except for their tapers (1N2B—25° to the tow vs 1N5B—35°) and panel number (two vs four). Irrespective of panels, compared to the 1N2B trawls, the shorter 1N5B designs achieved significantly greater predicted mean wing-end spreads (4.7–5.6%), while reducing drag (up to 4.3%) and also the numbers (per ha trawled) of one teleost (southern herring, Herklotsichthys castelnaui ; by up to 66%). However, catches of the targeted school prawns ( Metapenaeus macleayi ), were also significantly lower in the 1N5B trawls (by up to 50%; and biased towards smaller individuals escaping). The catch reductions were attributed to the shorter 1N5B bodies increasing the probability of mesh encounters (both species) and/or allowing H. castelnaui to swim forward and escape.

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.

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.

The school prawn (Metapenaeus macleayi) is important among commercial prawn trawler operators, but its harvest is affected in a complex way by a number of interacting water quality, and other variables. In this study, using the Hawkesbury—Nepean River system as a case study, we use Pearson correlation and hierarchical agglomerative cluster analysis (HACA) to assess the influence of the selected water quality (n = 7), quantity (n = 1), and weather (n = 2) parameters on the prawn harvest. Using data records (n = 104) collected over a 9-y period, we found water temperature (r = 0.63, P < 0.01), dissolved oxygen (r = -0.59, P < 0.01), and rainfall (r = 0.26, P < 0.01) to be significantly correlated variables with prawn harvest. The HACA produced 3 distinct clusters of variables nutrient availability for prawns (the total nitrogen, the total phosphorus, reactive silicate, turbidity, and suspended solids), the physical river environment (temperature, rainfall, and river flow), and the biochemical river...

Estimating rates of mortality is important for assessing stocks and for effectively managing seafood resources. This study reports the first estimates of rates of instantaneous fishing mortality ( F ) and natural mortality ( M ) for the commercially important school prawn, Metapenaeus macleayi . Replicated tag-recapture experiments were conducted on two stocks (Clarence River and Wallis Lake) to estimate the catchability coefficient ( q ) and F , whilst M was estimated from meta-analyses. Experiments were conducted across spatial and temporal scales to consider variability between stocks and variability between fisheries targeting this resource. The catchability coefficient was fitted to tag-recapture data in a population model across a range of M values (0.001–0.025 per day) and average daily F values were calculated. Values of q ranged between 2.03E−04 and 5.43E−02 and, 1.29E−04 and 3.41E−03 for the Clarence River and Wallis Lake stocks, respectively. Average daily F ranged between 2.86E−03 and 1.69 for the Clarence stock and 1.71E−03 and 3.44E−02 for the Wallis stock. With one exception (the November 2004 experiment) there were no differences in q and F values between experiments conducted on the Wallis Lake stock. For the Clarence stock values of q and F varied between years and within years. Factors thought to contribute to this variability were changes in prawn behaviour, differences in sizes of prawns tagged between experiments and differences in the survival of tagged prawns. The catchability coefficient for the Clarence River stock was weakly correlated to greater rates of river discharge. Estimates of F for the Wallis stock, the Clarence stock in 2004–05 and estimates of M were comparable to those reported for other penaeid fisheries world wide. There was no consistent pattern in comparisons of q and F values between the predominantly trawling operation of the Clarence River fishery and the seine netting of the fishery in Wallis Lake.

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 Nordmore-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 Nordmore-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 Nordmore-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. (C) 2011 Elsevier B.V. All rights reserved.

Godwin, R. M., Frusher, S., Montgomery, S. S., and Ovenden, J. 2011. Telomere length analysis in crustacean species: Metapenaeus macleayi, Sagmariasus verreauxi, and Jasus edwardsii. – ICES Journal of Marine Science, 68: 2053–2058. Estimates of age and growth in crustaceans have been historically problematic and presented significant challenges to researchers. Current techniques of age determination provide valuable data, but also suffer from disadvantages. Telomeric DNA has been proposed as an age biomarker because it shortens with age in some species. In this study, the feasibility of using telomere length (TL) to estimate age was examined in the school prawn Metapenaeus macleayi and the spiny lobsters Sagmariasus verreauxi and Jasus edwardsii. Carapace length (CL) was used as a surrogate for age, and terminal restriction fragment assays were used to test the relationship between TL and size. Degradation of telomeric DNA with time during storage significantly influenced TL estimates, particularly for M. macleayi. TLs obtained from species in this study were 10–20 kb. No relationship between CL and TL was detected for any of the test species, and TL did not differ between male and female M. macleayi. TLs of J. edwardsii pueruli were unexpectedly shorter than those of J. edwardsii adults. The suitability of TL as an age biomarker in crustaceans may be limited, but further research is needed to elucidate telomere dynamics in these species with their different life histories and lifespans.

The relationship between the amount of square mesh in codends and selectivity was investigated for an Australian penaeid stow-net fishery. Three lengths (3, 2 and 1 m) of square-mesh codend made from 27-mm mesh hung on the bar were alternately tested with a conventional 34-mm diamond-mesh design during two covered-codend experiments. Compared with the conventional codend, the square-mesh configurations incrementally selected school prawns, Metapenaeus macleayi (Haswell) across narrower selection ranges (SR) and mostly at greater sizes at 50% retention (L), while retaining fewer fish. Irrespective of the mesh configuration or square-mesh codend length, there were significant differences between experiments (attributed to water flow) and impacts of catch weight on the selectivity of school prawns. Both variables had a negative relationship with L, while water flow similarly affected SR. This study reaffirms the utility of square-mesh codends as a key input control for managing the selectivity of penaeid-catching gears.

Broadhurst, M. K., and Millar, R. B. 2009. Square-mesh codend circumference and selectivity. – ICES Journal of Marine Science, 66: 566–572. Despite the wide-scale assessment and the use of square-mesh codends in demersal trawls, relatively few studies have tested the effects of configurations other than mesh size on their selectivity. We investigated the consequences of increasing the circumference of square-mesh codends used in an Australian penaeid fishery from the expected optimal configuration of ∼33% of maximum diamond-mesh extension to ∼56 and 75%. Three square-mesh designs comprised 27-mm polyamide mesh throughout and had the same length (100 bars, B), but different circumferences (90, 150, and 200 B, respectively). Paired simultaneous comparisons (using twin trawls) of each treatment codend against a small-meshed control revealed significant effects of circumference on the efficiency of the trawl for a small teleost (pink-breasted siphonfish, Siphamia roseigaster) and commercial size classes of school prawns (Metapenaeus macleayi). Compared with the 90 codend, pink-breasted siphonfish catches and prawn count (numbers per 500 g) were both significantly greater in the codends of larger circumference, and these effects are attributed to concomitant (i) convoluted or reduced lateral openings of meshes and (ii) lesser probabilities of organisms encountering meshes in the posterior section. These differences would not preclude the use of codends of larger circumference in the fishery, but they do highlight the need to select appropriate configurations in future studies to reduce the potential for including the confounding effects of different geometries.

Rotherham, D., Broadhurst, M. K., Gray, C. A., and Johnson, D. D. 2008. Developing a beam trawl for sampling estuarine fish and crustaceans: assessment of a codend cover and effects of different sizes of mesh in the body and codend. – ICES Journal of Marine Science, 65: 687–696. An experiment was carried out in the Clarence River (New South Wales, Australia) to test the hypotheses that fish and crustacean catches in an experimental beam trawl were affected by a codend cover and the sizes of mesh in the body and codend. The cover had no obvious effects on the catches retained in the codend. Similarly, in comparisons between trawl bodies made from 26- and 41-mm diamond-shaped mesh, there were no differences in the assemblages of fish caught, or in the mean numbers entering the codends. For one species of fish (Acanthopagrus australis), however, there were differences in the proportions caught between the trawl bodies across different size classes. There was also some evidence to suggest that mesh size in the body of the trawl influenced the size selection of school prawns (Metapenaeus macleayi). For most finfish, there were no differences in catches between codends made from 20-mm and from 29-mm mesh hung on the bar (i.e. square-shaped mesh). In contrast, mesh size in the codend was important for the size selectivity of school prawns, with smaller carapace lengths at 50% retention in the 20-mm codend. We conclude that use of a 41-mm mesh in the body and a 20-mm square mesh in the codend of the beam trawl would be appropriate for future sampling with this gear in estuaries of New South Wales. A similar experimental approach to ours is needed in adapting the beam trawl to estuaries in other parts of the world, or in developing other types of research trawl.

A covered-codend experiment was done to assess the size selectivity of a codend made from 27-mm mesh hung on the bar (i.e. square mesh) for school prawns (Metapenaeus macleayi) across 10 vessels within the New South Wales (NSW) estuarine penaeid-trawl fishery. Prior to starting the experiment, a codend cover (18-mm diamond-shaped mesh) was tested and demonstrated to have no effect on the catching efficiency of a typical estuarine trawl, thereby validating its use for assessing size selectivity. There was substantial variability in the estimated carapace lengths at 50% probability of retention (L50 ± S.E.: between 7.90 ± 0.97 and 13.76 ± 0.23 mm) and selection ranges (SR: between 2.70 ± 0.18 and 6.71 ± 1.72 mm) of the 27-mm square-mesh codend among trawlers and estuaries. Multi-level, generalised linear mixed-effects modelling demonstrated that this variation was explained by: (i) the catch weight of school prawns; (ii) the particular estuary being fished; and (iii) random between-tow and -trawler variability. The results are discussed in terms of the various operational, biological and environmental factors unique to each fishing operation. Despite the variability in the performance of the 27-mm square-mesh codend, an overall improvement in size selectivity compared to conventional diamond-mesh designs, combined with a high survival rate of escapees, justifies its adoption and legislation throughout the NSW estuarine penaeid-trawl fishery as a means for mitigating unwanted fishing mortalities. We also conclude that similar codends may have application in other penaeid-trawl fisheries around the world.

Various plastic strips and sheets (termed ‘simple anterior fish excluders’−SAFEs) were positioned across the openings of penaeid trawls in attempts at reducing the unwanted bycatches of small teleosts. Initially, three SAFEs (a single wire without, and with small and large plastic panels) were compared against a control (no SAFE) on paired beam trawls. All SAFEs maintained targeted Metapenaeus macleayi catches, while the largest plastic SAFE significantly reduced total bycatch by 51% and the numbers of Pomatomus saltatrix, Mugil cephalus and Herklotsichthys castelnaui by up to 58%. A redesigned SAFE (‘continuous plastic’) was subsequently tested (against a control) on paired otter trawls, significantly reducing total bycatch by 28% and P. saltatrix and H. castelnaui by up to 42%. The continuous-plastic SAFE also significantly reduced M. macleayi catches by ~7%, but this was explained by ~5% less wing-end spread, and could be simply negated through otter-board refinement. Further work is required to refine the tested SAFEs, and to quantify species-specific escape mechanisms. Nevertheless, the SAFE concept might represent an effective approach for improving penaeid-trawl selectivity.

A field experiment was carried out in southeastern Australia to assess the short-term mortality and stress incurred by juvenile school prawns (Metapenaeus macleayi) discarded from an estuarine trawler. Some 35% of the prawns died up to 72 h after being caught in a trawl, exposed to air during sorting and separation from the retained catch (as per normal commercial procedures), then discarded into replicate cages. Total mortality was partitioned into that caused by trawling (about 16% of mortalities), and by subsequent sorting and grading (about 19%). Assuming that the majority of the non-penaeid bycatch is excluded from trawls (by the use of bycatch reduction devices), the latter mortalities could be almost eliminated by sorting and separating unwanted school prawns in water-filled compartments. Emersion stress was measured as concentrations of l-lactate in the haemolymph, which were elevated for at least 40 min following capture, but similar among all trawled treatments. l-lactate levels decreased within the first 24 h post-capture, then remained constant over at least the next 48 h, and were greater than baseline levels. The potential benefits associated with subtle changes to handling practices onboard estuarine trawlers are discussed.

The selectivities and catch compositions of conventional 30-mm diamond-mesh codends and a new square-mesh design made from 20-mm mesh hung on the bar for river and lagoon penaeid seines in south-eastern Australia were compared. The square-mesh design significantly improved the selectivities of the river seine for school prawns, Metapenaeus macleayi (Haswell), and the lagoon seine for greasyback prawns, Metapenaeus bennettae (Racek & Dall), by increasing their carapace length at 50% probability of retention (L) and by decreasing between-haul variability in selectivity. The presence of weed reduced the L for greasyback prawns caught in the conventional diamond-mesh codend during the lagoon-seine experiment. The differences among codend performances are discussed in terms of the differing methods of operation and composition of catches between the seines. These differences highlight the need to develop and manage modifications to improve the selectivity of fishing gears on a fishery-specific basis.

Three manipulative experiments were done to estimate the selectivity of conventional and new sizes and configurations of mesh for school prawns, Metapenaeus macleayi, in three south‐eastern Australian recreational fishing gears (haul, push, and scoop nets). The treatment meshes examined were: (1) conventional‐sized, diamond‐shaped mesh used in all gears (20 mm in scoop nets and 30 mm in push and haul nets); (2) 30 mm in scoop nets; and (3) 40‐mm diamond‐ and (4) 23‐mm square‐shaped mesh in all gears. In all experiments, known quantities of school prawns (6–24 mm carapace length (CL)) were placed in purpose‐built enclosures, monitored to ensure no experimental‐induced stress (as measured by changes in L‐lactate in their haemolymph) and the replicate treatments of the various gear configurations deployed. Escapees from the various treatment nets were collected from the enclosures using fine‐meshed nets. Logistic selection curves were derived for all treatment nets and specific comparisons made with...