Papers by David J Sterling
Fisheries Science, Dec 10, 2019
A benthic trawl's substrate contact (e.g. spreading mechanisms (i.e. otter boards) and ground gea... more A benthic trawl's substrate contact (e.g. spreading mechanisms (i.e. otter boards) and ground gears) determines both its catching efficiency and the extent of perceived habitat impacts. The potential for mitigating habitat impacts was investigated here via the novel 'batwing' otter board and 'soft-brush' ground gear. A purpose-built testing assembly towed treatments across three artificial habitat types with incrementally greater detachment thresholds (~ 8, 32 and 56 N), and the batwing and soft-brush were alternately compared against a conventional flat-rectangular otter board and three chain ground gears (6-, 8-and 10-mm diameter link), respectively. Overall, during 48 alternate deployments, the batwing removed up to 61% fewer of all habitats than the flat-rectangular otter board. By comparison, during 42 alternate replicates, the soft-brush ground gear failed to displace any habitat, while all three chain ground gears similarly removed between 3 and 5% of the two least resistant habitats (irrespective of position). The results imply the perceived impacts of penaeid trawls across sensitive areas can be reduced via simple modifications to spreading mechanisms and ground gears, along with appropriate spatio-temporal regulation. This study represents a unique approach to understanding the relative differences in impacts between ground gears and commercial-sized otter boards under controlled conditions.
Fisheries Research, Sep 1, 2016
Abstract The W trawl is a proposed innovation for more eco-efficient penaeid trawling—via reduced... more Abstract 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.
This paper reviews prawn trawl innovations investigated by a collaboration between the Australian... more This paper reviews prawn trawl innovations investigated by a collaboration between the Australian Maritime College – University of Tasmania and Sterling Trawl Gear Services over recent years. The review covers work on (i) implementing high strength netting in commercial trawls; (ii) quantifying drag due to T0 vs. T45 mesh orientation; (iii) the ‘W’ trawl system with reduced drag achieved by enhanced redirection of netting drag from the wings (and otter boards) onto centre-line ‘tongues’; (iv) batwing otter board with sail technology for superior lift-to-drag ratio; and (v) an alternative fuel-efficient steering mechanism.
The objectives of the review are: - examine the degree to which rising fuel costs have impacted o... more The objectives of the review are: - examine the degree to which rising fuel costs have impacted on different fisheries - examine new and existing technologies developed both within and outside of Australia in the field of increased fishing efficiency through reduced energy usage and innovation - examine opportunities for applying innovative solutions and developments which are most likely to produce the best return for the Australian fishing industry - develop a publication that scopes potential innovations, whether they be existing or have the potential for development, that reduce energy usage - provide advice on potential R&D that could assist industry in reducing energy usage.
AbstractThe Australian commercial fishing industry has been impacted by dramatic increases in th... more AbstractThe Australian commercial fishing industry has been impacted by dramatic increases in the price of diesel fuel. Combined with the global need to reduce greenhouse gas emissions, there is a clear need to introduce new efficiencies into fishing vessel ...
In prawn-trawling operations, otter boards provide the horizontal force required to maintain net ... more In prawn-trawling operations, otter boards provide the horizontal force required to maintain net openings, and are typically low aspect ratio (∼0.5) flat plates operating on the seabed at high angles of attack (AOA; 35–40°). Such characteristics cause otter boards to account for up to 30% of the total trawling resistance, including that from the vessel. A recent innovation is the batwing otter board, which is designed to spread trawls with substantially less towing resistance and benthic impacts. A key design feature is the use of a sail, instead of a flat plate, as the hydrodynamic foil. The superior drag and benthic performance of the batwing is achieved by (i) successful operation at an AOA of ∼20° and (ii) having the heavy sea floor contact shoe in line with the direction of tow. This study investigated the hydrodynamic characteristics of a generic sail by varying its twist and camber, to identify optimal settings for maximum spreading efficiency and stability. Loads in six degrees of freedom were measured at AOAs between 0 and 40° in a flume tank at a constant flow velocity, and with five combinations of twist and camber. The results showed that for the studied sail, the design AOA (20°) provides a suitable compromise between greater efficiency (occurring at lower AOAs) and greater effectiveness (occurring at higher AOAs). At optimum settings (20°, medium camber and twist), a lift-to-drag ratio >3 was achieved, which is ∼3 times more than that of contemporary prawn-trawling otter boards. Such a result implies relative drag reductions of 10–20% for trawling systems, depending on the rig configuration.
This paper reviews prawn trawl innovations investigated by a collaboration between the Australian... more This paper reviews prawn trawl innovations investigated by a collaboration between the Australian Maritime College – University of Tasmania and Sterling Trawl Gear Services over recent years. The review covers work on (i) implementing high strength netting in commercial trawls; (ii) quantifying drag due to T0 vs. T45 mesh orientation; (iii) the ‘W’ trawl system with reduced drag achieved by enhanced redirection of netting drag from the wings (and otter boards) onto centre-line ‘tongues’; (iv) batwing otter board with sail technology for superior lift-to-drag ratio; and (v) an alternative fuel-efficient steering mechanism.
Ocean Engineering, 2020
The greatly-stretched lateral opening of penaeid trawls is typically achieved by using relatively... more 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 largesized batwing boards might best be achieved using a two-mast configuration that incorporates three sails.
Fisheries Research, 2016
Abstract The W trawl is a proposed innovation for more eco-efficient penaeid trawling—via reduced... more Abstract 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.
The objectives of the review are: - examine the degree to which rising fuel costs have impacted o... more The objectives of the review are: - examine the degree to which rising fuel costs have impacted on different fisheries - examine new and existing technologies developed both within and outside of Australia in the field of increased fishing efficiency through reduced energy usage and innovation - examine opportunities for applying innovative solutions and developments which are most likely to produce the best return for the Australian fishing industry - develop a publication that scopes potential innovations, whether they be existing or have the potential for development, that reduce energy usage - provide advice on potential R&D that could assist industry in reducing energy usage.
Volume 6: Ocean Space Utilization, 2015
In prawn-trawling operations, otter boards provide the horizontal force required to maintain net ... more In prawn-trawling operations, otter boards provide the horizontal force required to maintain net openings, and are typically low aspect ratio (∼0.5) flat plates operating on the seabed at high angles of attack (AOA; 35–40°). Such characteristics cause otter boards to account for up to 30% of the total trawling resistance, including that from the vessel. A recent innovation is the batwing otter board, which is designed to spread trawls with substantially less towing resistance and benthic impacts. A key design feature is the use of a sail, instead of a flat plate, as the hydrodynamic foil. The superior drag and benthic performance of the batwing is achieved by (i) successful operation at an AOA of ∼20° and (ii) having the heavy sea floor contact shoe in line with the direction of tow. This study investigated the hydrodynamic characteristics of a generic sail by varying its twist and camber, to identify optimal settings for maximum spreading efficiency and stability. Loads in six degr...
Ecological Modelling, 2016
The delay difference model was implemented to fit 21 years of brown tiger prawn (Penaeus esculent... more The delay difference model was implemented to fit 21 years of brown tiger prawn (Penaeus esculentus) catch in Moreton Bay by maximum likelihood to assess the status of this stock. Monte Carlo simulations testing of the stock assessment software coded in C++ showed that the model could estimate simultaneously natural mortality in addition to catchability, recruitment and initial biomasses. Applied to logbooks data collected from 1990 to 2010, this implementation of the delay difference provided for the first time an estimate of natural mortality for brown tiger prawn in Moreton Bay, equal to 0.031 ± 0.002 week -1 . This estimate is approximately 30% lower than the value of natural mortality (0.045 week -1 ) used in previous stock assessments of this species.
A maximum-likelihood method for estimating natural mortality and catchability from catch and effo... more A maximum-likelihood method for estimating natural mortality and catchability from catch and effort data, with application to Moreton Bay Brown Tiger Prawn trawl fishery (Objective 5A
ICES Journal of Marine Science, 2008
Bishop, J., Venables, W. N., Dichmont, C. M., and Sterling, D. J. 2008. Standardizing catch rates... more Bishop, J., Venables, W. N., Dichmont, C. M., and Sterling, D. J. 2008. Standardizing catch rates: is logbook information by itself enough? – ICES Journal of Marine Science, 65: 255–266. The goal of the work was to maximize the accuracy of standardized catch per unit effort as an index of relative abundance. Linear regression models were fitted to daily logbook data from a multispecies penaeid trawl fishery in which within-vessel changes in efficiency are common. Two model-fitting strategies were compared. The predictive strategy focused on maximizing the explained variance, and the estimation strategy on finding realistic coefficients for important components of changing catchability. Realistic values could not always be obtained, because the regression factors were not orthogonal, and data on the presence of technology were sometimes unreliable or systematically incomplete. It was not possible to separate fishing power from abundance by analysing logbook data alone; it was necessa...
ICES Journal of Marine Science, 2016
Abiotic factors are fundamental drivers of the dynamics of wild marine fish populations. Identify... more Abiotic factors are fundamental drivers of the dynamics of wild marine fish populations. Identifying and quantifying their influence on species targeted by the fishing industry is difficult and very important for managing fisheries in a changing climate. Using multiple regression, we investigated the influence of both temperature and rainfall on the variability of recruitment of a tropical species, the brown tiger prawn (Penaeus esculentus), in Moreton Bay which is located near the southern limit of its distribution on the east coast of Australia. A step-wise selection between 60 environmental variables identified temperature as the most important environmental factor to explain the variations of recruitment between 1990 and 2014. Including temperature into the Beverton and Holt stock–recruitment relationship explained 69% of the recruitment variability compared with 9.5% when omitted. This analysis indicates that increasing temperatures have increased recruitment of brown tiger pra...
PLOS ONE, 2015
For prawn trawling systems, drag reduction is a high priority as the trawling process is energy i... more For prawn trawling systems, drag reduction is a high priority as the trawling process is energy intensive. Large benefits have occurred through the use of multiple-net rigs and thin twine in the netting. An additional positive effect of these successful twine-area reduction strategies is the reduced amount of otter board area required to spread the trawl systems, which leads to further drag reduction. The present work investigated the potential of redirecting the drag-strain within a prawn trawl away from the wings and the otter boards to the centre line of the trawl, where top and bottom tongues have been installed, with an aim to minimise the loading/size of the otter boards required to spread the trawl. In the system containing the new 'W' trawl, the drag redirected to the centre-line tongues is transferred forward through a connected sled and towing wires to the trawler. To establish the extent of drag redirection to the centre-line tongues and the relative drag benefits of the new trawl system, conventional and 'W' trawls of 3.65 m headline length were tested firstly over a range of spread ratios in the flume tank, and subsequently at optimum spread ratio in the field. The developed 'W' trawl effectively directed 64% of netting-drag off the wings and onto the centre tongues, which resulted in drag savings in the field of *20% for the associated 'W' trawl/otter-board/sled system compared to the traditional trawl/otter-board arrangement in a single trawl or twin rig configuration. Furthermore, based on previously published data, the new trawl when used in a twin rig system is expected to provide approximately 12% drag reduction compared to quad rig. The twin 'W' trawl system also has benefits over quad rig in that a reduced number of cod-end/By-catch Reduction Device units need to be installed and attended each tow.
The study investigated the effect of mesh orientation (square vs. diamond) on the hydrodynamic fo... more The study investigated the effect of mesh orientation (square vs. diamond) on the hydrodynamic forces in prawn trawls. Three ¼ scale models of an 8 fathom typical prawn trawl were built from Ultracross Dyneema©, an innovative high strength and soft material that is used for full-scale prawn trawl construction. Model 1 [Diamond Body with Square Sides] represents a conventional two seam trawl with diamond-mesh orientation and square mesh side-sections achieved by an all-bar side taper cut; Model 2 [Diamond Body with Diamond Sides] is a four seam trawl with conventional mesh orientation and diamond-mesh side panels; and Model 3 [Square Body] was a two seam trawl with square–mesh orientation (mesh rotated 45degrees). The models were streamed in the flume tank and their drag determined over a range of velocities. To obtain an accurate comparison of drag, and account for the effects of speed velocity gradients present in the flume tank, each model was tested twice: at the upper and lower ...
Fisheries Research, 2020
Various, low-cost modifications to the anterior sections of penaeid trawls have been shown to rea... more 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.
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Papers by David J Sterling
At the policy level an intention to manage trawl fisheries in a comprehensive way is backed by a mandated approach that is designed to capture all of the above issues and interests. That approach is termed Ecological Sustainable Development (ESD).
The work in this thesis is designed to produce a prediction tool for prawn trawling performance that is based on modelling the physical nature of prawn trawling activities. It is proposed that the resulting tool is essential for working to manage the multi-dimensional aspects of prawn trawling fisheries.
Three discrete objectives for the thesis are; to expand and improve an existing Prawn Trawling Performance Model (PTPM) so that it is more accurate and relevant to a broader range of questions, to evaluate the capacity of the PTPM to predict the performance characteristics of real prawn trawling operations in terms of both engineering and catching performance and to investigate the problem space surrounding prawn trawl fisheries to identify and develop applications for the model.
A rudimentary PTPM (Sterling 2000b) is expanded through the analysis of further empirical data collected for model and full-scale trawl gear. Eight area of improvement to the PTPM were considered and in all cases significant changes were made.
The accuracy of the new form of the model is here tested by comparing performance predictions with measurements of trawling performance for a variety of industrial trawl systems operated in the Queensland East Coast Trawl Fishery and also through comparing predicted trawling performance with prawn catches returned for trawlers operating in the Northern Prawn Fishery over the years 1970 to 2000.
In the first case, errors in predicting swept area rate, considered an important performance parameter, were less than 5%. Fine scale issues were explored using the available sea trial data and a number of areas of concern within the model are highlighted. These relate to accurately quantifying the forces involved in the interaction of the trawl gear with the seabed and accurately accounting for the interaction between components within trawl systems.
In the second case, the results suggest that between 50% and 60% of the variation in the seasonal catching performance of trawlers in the NPF is explained by predictions of swept area rate derived by the PTPM from the available data for that fishery.
A comprehensive survey of applications for the PTPM is conducted in context with approaching the management of prawn trawling fisheries using the principles of ESD as defined by the National Strategy for ESD (1992). The Northern Prawn Fishery is used as a case study to explore in finer detail applications for the PTPM. Issues arising from the implementation of some of the applications are discussed.