Matt K. Broadhurst

Optimal positioning and design of behavioural-type by-catch reduction devices involving square-mesh panels in penaeid prawn-trawl codends

Two experiments were done in the oceanic penaeid prawn-trawl fishery in New South Wales to investigate (i) the optimal positioning of behavioural-type by-catch reduction devices (BRDs) involving square-mesh panels; and (ii) the relative performances of two industry-developed square-mesh panels against a composite square-mesh panel developed by scientists. In the first experiment, three codends, each containing one square-mesh panel (located on the tops of the codend at distances of 0.7, 1.2 and 1.6 m anterior to the last row of meshes respectively) significantly reduced the catches of some small fish and total discards compared with a control codend that held no BRD. Rates of reduction significantly increased with proximity of the square-mesh panel to the end of the codend. There was, however, a significant reduction in catches of prawns from the codend containing the square-mesh panel at 0.7 m. Based on these results, the optimal position for these sorts of BRDs was determined to be at 1.2 m anterior to the last row of meshes. In the second experiment, at this position, two codends containing industry-developed plastic and metal square-mesh panels were less effective in excluding by-catch than a codend containing the composite square-mesh panel. The results are discussed in terms of species-specific differences in behavioural responses and swimming ability and the future development and testing of behavioural-type BRDs in penaeid prawn trawls.

Utility of morphological data for key fish species in southeastern Australian beach‐seine and otter‐trawl fisheries: Predicting mesh size and configuration

Abstract Samples of 16 key fish species targeted by oceanic prawn trawlers and oceanic and estuarine beach seiners in southeastern Australia were measured for up to 10 morphological dimensions that included: total (Wt) and eviscerated weights (EWt); total (TL), fork (FL), natural (NL), standard (SL), dorsal standard (DSL), and anal standard lengths (ASL); and maximum body height (MH), width (MW), and girth (MG). All partial lengths were indexed against TL to provide linear relationships and, where there were sufficient data, tested for intra‐specific sexual dimorphism. Nearly all species that were sexed demonstrated at least some morphological divergence, and especially for transverse measures; probably owing to different reproductive conditions. Sex‐specific or combined regressions of DSL and ASL and transverse measures versus TL were used to identify the most appropriate: (1) partial length for compliance purposes; and (2) mesh sizes and configurations for the different fishing gears used. Using relevant data describing the smallest commercial‐sized fish species (stout, Sillago robusta and red spot whiting, S. flindersii— 150mm TL) retained in prawn trawls, a new square‐mesh codend (made from 35‐mm mesh hung on the bar) was designed and constructed. The selectivities of this square‐mesh codend and a conventional diamond‐mesh design (40‐mm mesh) were then modelled and compared in a case study. The conventional codend was demonstrated to be non‐selective for the sizes of eastern king prawns and red spot whiting targeted. In contrast, the square‐mesh codend selected red spot whiting across narrow selection ranges (18.88–15.67 mm) and at lengths of 50% retention (143.39–146.32mm TL) that closely corresponded to the estimated maximum mesh opening. The square‐mesh codend also improved selection for eastern king prawns, without impacting on commercial catches. Based on these results, similar predictions were made for appropriate mesh sizes and configurations in oceanic and estuarine beach seines; both of which were demonstrated to comprise minimum mesh sizes that would be non‐selective for the sizes of all targeted fish.

Strategies for Improving the Selectivity of Fishing Gears

Few fishing methods and gears are entirely selective for the targeted species and their sizes. The majority of gears have incidental catches (collectively termed ‘bycatch’) that vary from isolated occurrences in some hook-and-line fisheries to large numbers of juveniles of key species in trawl fisheries. Of primary concern is the contribution that the mortalities of such bycatches may have on subsequent stocks. Over the past 20 years, extensive efforts have been directed towards addressing this issue by modifying problematic fishing gears (especially trawls) and practices. Whilst this work has facilitated considerable reductions in bycatches (up to 80% in some cases), very few (if any) of the changes made to existing gears are 100% effective. There remains, therefore, a substantial mortality of unwanted individuals in most fisheries. To work more comprehensively towards the ultimate goal of achieving perfect selectivity, we propose that, in addition to conventional methods used in recent decades to modify fishing gears, a more lateral approach should also be adopted involving completely alternative gears. Specifically, we propose a strategy that: (1) examines the boundaries of what is realistically achievable in modifying poorly selective gears using established bycatch reduction protocols; and (2) determines the utility of alternative gears that, because of their design and/or operation, have selective mechanisms which could be applied to problematic gears. In this paper, the logic involved in the first approach is discussed and data supporting the benefits of the second approach are presented. Reducing the discarding of small prawns Project No. 2001/031 88 NSW Dept of Primary Industries The issue of bycatch From the earliest evidence of fishing more than 90 000 years ago (Yellen et al. 1995) to the present day, humans have exponentially advanced their harvesting methods. The clear focus of these developments has been to maximize the catches of an ever-increasing diversity of targeted species, with little or no regard for the incidental catches (termed ‘bycatch’, sensu Saila 1983). A progression from simple harpoons, hooks and traps deployed from the shore, through nets set from boats, to the industrial factory trawlers of developed countries has culminated in technology which, in many cases, far exceeds the sustainability of local resources. This excess was evident at the end of the 20 century by the collapse of many commercially-important stocks, a plateau in the world’s total landed wild catch (at less than 100 million tonnes) and the volumes of bycatch discarded in pursuit of targeted catches (Alverson et al. 1994). While recognition of the potentially negative impacts of unchecked fishing technology date back to the 14 century (Dyson 1977), it is only during the last few decades that coordinated attempts have been directed towards improving the selectivity of fishing gears (Kennelly and Broadhurst 2002). Relevant reviews of the published literature suggest that nearly all fishing gears and methods have received at least some attention (e.g. gillnets – Hamley 1975; longlines – Løkkeborg and Bjordal 1992; traps – Mahon and Hunte 2001), although the majority of effort has been directed towards benthic trawl fisheries (e.g. Kennelly 1995) and especially those targeting shrimp (Andrew and Pepperell 1992; Broadhurst 2000). This has occurred in response to the disproportional ratio of retained-to-discarded catches and the amount of unwanted catch discarded each year by shrimp trawlers; estimated to represent between 30 and 60% of the total world harvest of wild fisheries resources (Alverson et al. 1994). While the absolute volume of bycatch associated with shrimp trawling clearly makes it one of the most the most problematic fishing methods, many other gears including fish trawls, seines, gillnets, traps and longlines have, in recent times, been identified as having significant selectivity issues and have consequently been associated with prolonged calls for improvements coming from a variety of environmental groups, recreational fishers, interacting commercial fisheries and the general public. Solving bycatch problems During the past 2 decades, problems surrounding the issue of bycatch has shifted the focus of fishing gear technology from catching as much of the target species as possible (with little regard for collateral impacts) to improving selectivity, both in terms of the species targeted and their desired sizes (Kennelly and Broadhurst 2002). In many cases, the successful development and adoption of solutions to improve selection in problematic gears can be summarized in a simple framework (see also Kennelly and Broadhurst 1996; Kennelly 1997; Broadhurst 2000) which involves industry and researchers each applying their respective areas of expertise to the particular problem. This framework comprises five key steps: (1) quantifying bycatches (mostly via observer programs), (2) identifying the main bycatch species and their sizes of concern, (3) developing alterations to existing fishing gears and practices that minimize the mortality of these species, (4) testing these alternatives in appropriately-designed field experiments and (5) gaining acceptance of the new technology throughout the particular fishery and interested stakeholders. The protocol for completing the framework is quite straightforward and has been described with numerous examples by Kennelly and Broadhurst (1996), Kennelly (1997) and Broadhurst (2000). The crucial and most difficult step (3 above) is the actual development of appropriate solutions that improve the selectivity of existing fishing gears for the targeted catch and so reduce unwanted bycatch. Depending on the type of gear and its particular problems, solutions may involve simple adjustments to operational procedures and/or existing components of the gear, like changing the size and/or shape of meshes or hooks. Alternatively, for many towed gears, more complicated modifications that include physical bycatch reduction devices (BRDs) may need to be invented or modified from other fisheries (Broadhurst 2000). Owing to their relative complexity, these types of modifications frequently require detailed adjustment and reassessment to exclude specific sizes of individuals or species, yet maintain targeted catches (Kennelly and Broadhurst 2002). Project No. 2001/031 Reducing the discarding of small prawns NSW Dept of Primary Industries 89 While the above framework summarizes several successful attempts at addressing the problems of bycatch in different fisheries throughout the world (Kennelly 1997), in many cases the established protocols for improving inherently problematic gears has restricted fishing technologists in terms of working towards the ultimate goal of perfect selectivity. A reason for this is that to ensure the industry adoption and acceptance of modified designs that reduce bycatch (i.e. step 5 above), nearly all researchers have aimed to achieve 100% retention of the targeted species (during step 3). Theoretically, it should be possible to dramatically improve the selectivity of most fishing gears, provided some concomitant sacrifice in their overall efficiency is permitted. The issue would then become what is an acceptable loss of the targeted catch in order to improve selectivity and reduce bycatch. An extreme solution for achieving ‘perfect selectivity’ may be to reorder the above logic and, using traditional gears and established bycatch reduction methods, approach a 100% exclusion rate of unwanted catch at any cost to the desired catch. This approach could be appropriate in tightly-regulated fisheries where there is imminent threat of closure due to discarding. Reductions in gear efficiency could also be offset via some compensatory increases in the value of the targeted catch through ‘eco-labeling’. This sort of strategy would not be feasible, however, in the vast majority of countries and especially those where artisanal fisheries represent the main source of income for communities. For these fisheries, bycatch reduction clearly needs to be maximized with minimal impact on the efficiency of the gear for the targeted catch. Maximizing gear development within existing bycatch reduction frameworks To approach maximum bycatch reduction with no loss of the targeted catch (during step 3 of the framework described above), there needs to be a general estimate of what is achievable for particular gears. As a starting point, this requires an assessment of the limits of established modifications for improving selectivity. For many conventional towed gears, different sizes and/or shapes of mesh are among the simplest alterations and their utility is often (or at least should be) defined first. Under the framework proposed by Broadhurst (2000), this involves testing beyond what might intuitively be appropriate, so that the limits of a particular range of mesh sizes or shapes can be quantified and defined. If the solution to reducing particular bycatch species of concern is not apparent within the boundaries of the simple alterations tested, then more complex modifications (including physical BRDs) warrant examination. Specific designs of BRDs should also be tested to define their limits. For example, if mechanical-sorting grids are required to exclude organisms larger than the targeted species, then a range of configurations that include very narrow and wide bar spacings and small and larger profiles or angles of orientation should be examined (e.g. Broadhurst et al. 2004b). Similarly, because factors like relative water flow strongly influence the performance of BRDs that operate by exploiting differences in the behavior of species (Broadhurst et al. 1999a), these sorts of modifications need to be tested at different positions throughout the gear (e.g. Broadhurst et al. 2002). Coherent hypotheses encompassing the full range of key factors influencing the performance of mod

Effects of angling on post-release mortality, gonadal development and somatic condition of Australian bass Macquaria novemaculeata

This study assessed the protracted effects of two angling treatments (mild and harsh) on the post-release mortality, gonadal development and somatic condition of Australian bass Macquaria novemaculeata in a pond-based experiment. Angled fish were destructively sampled (along with controls) three times: immediately before, and 1 or 4 weeks after release into cages. Compared with the other groups, harshly angled fish had similar, low immediate and short-term mortalities (< or =5%), but significant delayed mortality (25%) and fed only minimally. None of these fish ejected ingested hooks. Forty-six per cent of captive fish (across all groups) and 20% of wild fish had non-ripening gonads (stage I) prior to, or during, the experiment. In females with ripening ovaries (stages II or III), neither angling treatment significantly reduced standardized gonadal mass. The mean per cent of atretic oocytes increased among females in all groups, but was significantly greater in those that were harshly angled. The results demonstrated that the gonadal development of M. novemaculeata could be suppressed or impaired (by angling, handling and confinement), and that further research is warranted. In particular, the timing and severity of angling in relation to the stage of gonadal development could have important implications for the introduction of temporal restrictions to angling.

Influences of hanging ratio, fishing height, twine diameter and material of bottom-set gillnets on catches of dusky flathead Platycephalus fuscus and non-target species in New South Wales, Australia

Three experiments were done to test for the influences of different (i) hanging ratios (E=0.5, 0.65 and 0.8); (ii) fishing heights (25 and 12 meshes); and (iii) twine diameters (0.41, 0.56 and 0.62 mm) and materials (multifilament nylon and multimonofilament nylon polyamide) on catches and by-catches in the estuarine gillnet fishery for Platycephalus fuscus in New South Wales, Australia. In each experiment, the various 100-m treatment panels comprising 80-mm (nominal) mesh rigged according to the different configurations being examined were configured in a single gang between 1090 and 1310 m in length, and fished according to commercial practices. The results showed no significant differences between different hanging ratios or twine diameters for the numbers, weights and size compositions of catches and by-catches. Twine material had an effect on only one key by-catch species (Acanthopagrus australis), with fewer caught in panels made from multifilament nylon compared with multimonofilament nylon. In contrast to the above modifications, lowering the fishing height of the floatline significantly reduced total by-catch by up to 46% and the individuals of key by-catch species (Mugil cephalus, A. australis and Girella tricuspidata) by between 60 and 85% with no effect on catches of targeted P. fuscus, or legally retained byproduct, Portunus pelagicus. The results are used to provide directions for the future management of this fishery and have relevance to other similar bottom-set gillnet fisheries.

Utility and efficiency of multi-mesh gill nets and trammel nets for sampling assemblages and populations of estuarine fish

Two replicate multi-mesh gill and trammel nets, each comprising five 30 m long panels made from different-sized mesh (38, 54, 70, 90 and 100 mm stretched mesh openings) were fished in a south-east Australian barrier estuary over seven nights to evaluate their potential as sampling gears for fishery-independent surveys of estuarine fish assemblages. There were no differences in composition and structure of assemblages, mean abundance, or diversity of catches between the two types of net. The composition and structure of catches differed between mesh sizes, with the panels made from 38 and 54 mm mesh retaining significantly more fish and species than the larger-sized meshes. The two smallest mesh sizes were important for capturing sub-adults and juveniles of some species. Based on a greater precision of catch per unit effort (CPUE) estimates, less sampling effort and greater ease of use, the multi-mesh gill net was a better sampling unit than the trammel net for assessments of estuarine fish populations.

Simulated escape of juvenile sand whiting (Sillago ciliata, Cuvier) through square-meshes: Effects on scale-loss and survival

Abstract Two laboratory experiments were done to assess effects of simulated escape through square-meshes on the scale-loss and survival of (i) non-fatigued and (ii) fatigued small sand whiting (Sillago ciliata). In experiment 1, non-fatigued fish that were forced through square-meshes (treatment fish) showed no significant difference in scale-loss compared to fish that did not pass through square-meshes (control fish), although there was a 50% difference in mean scale-loss immediately posterior to their maximum height. In experiment 2, fish were fatigued to exhaustion by swimming against a current of 0.7 to 0.8 knots for 15 min. Fatigued fish that were then forced through square-meshes showed significantly more scale-loss across their entire body than did the fatigued control fish (difference in means of between 67% to 84%). In both experiments the total scale-loss on treatment fish was quite low (1.4–4%) and there were negligible mortalities (only 2 treatment fish died in experiment 1) over the duration of each experiment (30 days). We conclude that the composite square-mesh panel currently used to reduce by-catch in the NSW oceanic prawn trawl fishery is likely to cause negligible damage and mortality of small sand whiting.

Mortality of discards from southeastern Australian beach seines and gillnets

Two experiments were done in an Australian estuary to quantify the mortalities and contributing factors for key species discarded during 8 and 9 deployments of commercial beach (or shore) seines and gillnets, respectively. In both experiments, bycatches (2347 individuals comprising 16 species) were handled according to conventional practices and assessed for immediate mortalities before live samples of selected species were discarded into replicate cages along with appropriate controls, and monitored for short-term mortalities (< or =10 d). All of the seined or gilled fish were alive prior to discarding. During the beach seine experiment, 20% of caged seined-and-discarded surf bream Acanthopagrus australis (n = 290) were dead after 5 d, with most mortalities occurring between the second and fifth day. In the gillnet experiment, 42 and 11% of gilled-and-discarded A. australis (n = 161) and lesser salmon catfish Neoarius graeffei (n = 67), respectively, died during a 10 d monitoring period, mostly within the first 5 d. There were no deaths in any controls for these fish. Mixed-effects logistic models revealed that the mortality of A. australis discarded from both gears was significantly (p < 0.01) and negatively correlated with their total length, while N. graeffei had a significantly (p < 0.05) greater (5-fold) probability of dying when jellyfish Catostylus sp. were present in the gillnet. Simple modifications to the operations of beach seines and gillnets and/or post-capture handling procedures, such as close regulation of size selectivity for the target species, careful removal of fish from meshes, and abstention from setting during high abundances of jellyfish will maximise the survival of discarded bycatch.

Increasing codend mesh openings: an appropriate strategy for improving the selectivity of penaeid fishing gears in an Australian estuary?

This study investigates the effects of increasing the lateral mesh openings in codends on the size and species selectivity of lagoon and river seines and a stow net used to target penaeid prawns (eastern king (Penaeus plebejus), school (Metapenaeus macleayi) and greasyback prawns (M. bennettae)) in Wallis Lake, New South Wales, Australia. Compared with conventional codends made from 36-mm diamond-shaped mesh, new designs made from 25- and 29-mm mesh hung on the bar (i.e. square-shaped mesh) significantly reduced the catches of non-target fish (by between 58 and 95%) and improved size selection for the targeted penaeids in the stow net and lagoon seine. In contrast, owing to gear-specific operational characteristics such as a slower hauling speed, there were few detectable effects of altering mesh openings in the codend of the river seine. The results are discussed in terms of the differences in the gears used and their particular selection mechanisms. We conclude that, pending further trials, codends made from a mesh size approaching 29 mm, hung on the bar, would provide appropriate size- and species-selection for Wallis Lake stow nets and lagoon seines. Further research is required, however, to examine the utility of operational changes to river seines and/or alterations to mesh size and configuration in the wings and body to improve selectivity.

Mitigating discard mortality from dusky flathead Platycephalus fuscus gillnets.

The mortalities and contributing parameters were estimated for key species discarded during commercial gillnetting (80 mm mesh) targeting dusky flathead Platycephalus fuscus in a southeastern Australian estuary. Bycatches (1470 individuals from 16 species over 11 deployments) were assessed for their immediate mortalities onboard the gillnetter, before subsamples (570 individuals from 11 species) were discarded into cages and monitored for their short-term fate over 4 d. Appropriate controls were concurrently caged and monitored. Blood samples were taken from some live meshed-and-discarded yellowfin bream Acanthopagrus australis and luderick Girella tricuspidata and analysed for plasma cortisol and glucose. Concomitantly angled fish were similarly sampled (to provide baseline estimates of blood physiology). The immediate mortalities of the abundant species ranged between 0 (undersize blue swimmer crab Portunus pelagicus <6 cm carapace length) and 70% (undersize P. fuscus <36 cm total length [TL]). Water temperature had a statistically significant positive relationship with the immediate mortality of G. tricuspidata and large-tooth flounder Pseudorhombus arsius, and TL had a significant negative relationship with the immediate mortality of black sole Synaptura nigra. Compared to baseline estimates, mean plasma cortsiol concentrations in meshed-and-discarded G. tricuspidata and A. australis were significantly greater, and approached levels comparable to most teleosts after peak stress. Mean glucose concentrations were not concomitantly elevated, possibly reflecting limited time between stress and sampling for some individuals. Short-term mortalities occurred throughout the entire 4 d monitoring period for most species and ranged from 0 (yellowfin leatherjacket Meuschenia trachylepis) to 29% (A. australis). Water temperature and TL were identified as having significant impacts similar to those described above on the delayed fate of A. australis and G. tricuspidata. The partitioned mortalities were combined to provide estimates of overall mortality (+/- SE) for the main species that ranged between 5.9 +/- 3.3% (P. pelagicus) and 76.9 +/- 7.8% (undersize P. fuscus). Discard mortality in this fishery could be mitigated by allowing fishers to retain a small percentage of undersize P. fuscus, restricting the deployment of nets in water temperatures >16 to 17 degrees C, and encouraging the careful removal of catches from meshes.