Hydrodynamic Performance of a Newly-Designed Pelagic and Demersal Trawls Using Physical Modeling and Analytical Methods for Cameroonian Industrial Fisheries

This study proposed the newly-designed Pelagic and demersal trawls for the fishing vessels operating in Cameroonian waters in pelagic and demersal fishing grounds. The engineering performances of both trawls were investigated using physical modelling method and analytical method based on the predicted equations. In a flume tank, a series of physical model tests based on Tauti’s law were performed to investigate the hydrodynamic and geometrical performances of both trawls and to assess the applicability of the analytical methods based on predicted equations. The results showed that in model scale, the working towing speed and door spread for the pelagic trawl were 3.5 knots and 1.85 m, respectively, and for the bottom trawl net they were 4.0 knots and 1.8 m. At that speed and door spread, the drag force, net opening height, and wing-end spread of the pelagic model trawl were 36.73 N, 0.89 m, and 0.86 m, respectively, and the swept area was 0.76 m2. Bottom trawl speed and door spread were 30.43 N, 0.38 m, and 0.45 m, respectively, and the swept area was 0.25 m2. The maximum difference between the experimental and analytical results of hydrodynamic performances was less than 56.22% and 41.45%, respectively, for pelagic and bottom trawls, the results of the geometrical performances obtained using predicted equations were close to the experimental results in the flume tank with a maximum relative error less than 12.85%. The newly developed pelagic and bottom trawls had advanced engineering performance for high catch efficiency and selectivity and could be used in commercial fishing operations in Cameroonian waters.

Fishing Production and Fishing Changes in Hong Kong after the Ground Trawl Ban of 31st December 2012:A Geospatial Evaluation

From data published by the Hong Kong SAR(HKSAR)government in their two sole fisheries surveys of 2006 and 2016/2017,the current authors produced regional maps using spatial interpolation to more accurately describe and estimate the geographic coverage of changes in fishing production in Hong Kong waters since the ground trawl ban of 2012 December 31st.These suggest the fishing industry has adapted to smaller craft,and that fishing production increased in several areas in the period after the ground trawl ban came into effect.In addition,the maps enable a smoother assessment to be made of the geospatial changes in fishing production which have occurred since the ground trawl ban and suggest a‘workaround’by fishermen.In particular,small fishing craft known as sampans are able to take advantage by being more suitable vessels for areas such as narrower or shallower bays.Marine plastics pollution is also a proxy indicator of these activities,as evidenced by discarded fishing gear that includes plastic nets,floats,and other fishing boat equipment.

Improving Energy Efficiency in New Trawlers Fishing Gear in Mediterranean Sea

The objective of this work is to show the benefits of a new eco-systemic fishing gear installed in three bottom trawlers after one year using it. The study has been based on fuel consumption reduction for the three vessels and the catch in two of them. The new system minimizes the impact of the fishing gear on the seabed, with a reduction on the tow resistance. This generates significant fuel savings that improves the economical result of the fishery, helping the ship owners in the return of investment first, then in the future viability of the operation. Apart of the evident savings due to the fuel consumption reduction, in the long term, the ship owner will notice also savings on maintenance, both because the winches are towing with less tension （longer life for warps, brakes and hydraulic system） and the engine is running at low rpm＇s （longer life of the engine and between breakdowns）. The new fishing gear does not require any modification on the way that the fisherman is working, only replacing and/or modifying some parts or components of the fishing gear. The implementation is easy and the adjustments required could be done in a couple of days.

Analyses of Trawling Track and Fishing Activity Based on the Data of Vessel Monitoring System(VMS): A Case Study of the Single Otter Trawl Vessels in the Zhoushan Fishing Ground

The original purpose of Vessel Monitoring System(VMS) is for enforcement and control of vessel sailing. With the application of VMS in fishing vessels, more and more population dynamic studies have used VMS data to improve the accuracy of fisheries stock assessment. In this paper, we simulated the trawl trajectory under different time intervals using the cubic Hermite spline(c Hs) interpolation method based on the VMS data of 8 single otter trawl vessels(totally 36000 data items) fishing in Zhoushan fishing ground from September 2012 to December 2012, and selected the appropriate time interval. We then determined vessels' activities(fishing or non-fishing) by comparing VMS speed data with the corresponding speeds from logbooks. The results showed that the error of simulated trajectory greatly increased with the increase of time intervals of VMS data when they were longer than 30 minutes. Comparing the speeds from VMS with those from the corresponding logbooks, we found that the vessels' speeds were between 2.5 kn and 5.0 kn in fishing. The c Hs interpolation method is a new choice for improving the accuracy of estimation of sailing trajectory, and the VMS can be used to determine the vessels' activities with the analysis of their trajectories and speeds. Therefore, when the fishery information is limited, VMS can be one of the important data sources for fisheries stock assessment, and more attention should be paid to its construction and application to fisheries stock assessment and management.

Impacts of the sampling design on the abundance index estimation of Portunus trituberculatus using bottom trawl

In the survey of fishery resources,the sampling design will directly impact the accuracy of the estimation of the abundance.Therefore,it is necessary to optimize the sampling design to increase the quality of fishery surveys.The distribution and abundance of fisheries resource estimated based on the bottom trawl survey data in the Changjiang River(Yangtze River)Estuary-Hangzhou Bay and its adjacent waters in 2007 were used to simulate the"true"situation.Then the abundance index of Portunus trituberculatus were calculated and compared with its true index to evaluate the impacts of different sampling designs on the abundance estimation.Four sampling methods(including fixed-station sampling,simple random sampling,stratified fixed-station sampling,and stratified random sampling)were simulated.Three numbers of stations(9,16 and 24)were assumed for the scenarios of fixed-station sampling and simple random sampling without stratification.While 16 stations were assumed for the scenarios with stratification.Three reaction distances(1.5 m,3 m and 5 m)of P.trituberculatus to the bottom line of trawl were also assumed to adapt to the movement ability of the P.trituberculatus for different ages,seasons and substrate conditions.Generally speaking,compared with unstratified sampling design,the stratified sampling design resulted in more accurate abundance estimation of P.trituberculatus,and simple random sampling design is better than fixed-station sampling design.The accuracy of the simulated results was improved with the increase of the station number.The maximum relative estimation error(REE)was 163.43%and the minimum was 49.40%for the fixed-station sampling scenario with 9 stations,while 38.62%and 4.15%for 24 stations.With the increase of reaction distance,the relative absolute bias(RAB)and REE gradually decreased.Resource-intensive area and the seasons with high density variances have significant impacts on simulation results.Thus,it will be helpful if there are prior information or pre-survey results about density distribution.The current study can provide reference for the future sampling design of bottom trawl of P.trituberculatus and other species.

Spatiotemporal factors affecting fish harvest and their use in estimating the monthly yield of single otter trawls in Putuo district of Zhoushan, China

We used generalized additive models (GAM) to analyze the relationship between spatiotemporal factors and catch, and to estimate the monthly marine fishery yield of single otter trawls in Putuo district of Zhoushan, China. We used logbooks from five commercial fishing boats and data in government's monthly statistical reports. We developed two GAM models: one included temporal variables (month and hauling time) and spatial variables (longitude and latitude), and another included just two variables, month and the number of fishing boats. Our results suggest that temporal factors explained more of the variability in catch than spatial factors. Furthermore, month explained the majority of variation in catch. Change in spatial distribution of fleet had a temporal component as the boats fished within a relatively small area within the same month, but the area varied among months. The number of boats fishing in each month also explained a large proportion of the variation in catch. Engine power had no effect on catch. The pseudo-coefficients (PCf) of the two GAMs were 0.13 and 0.29 respectively, indicating the both had good fits. The model yielded estimates that were very similar to those in the governmental reports between January to September, with relative estimate errors (REE) of <18%. However, the yields in October and November were significantly underestimated, with REEs of 36% and 27%, respectively.

Size selectivity of combined square and diamond mesh codends of shrimp beam trawl for banded scad Caranx(Atule)kalla in the northern South China Sea

The shrimp beam trawl fishery in the northern South China Sea is characterized by high by-catch and poor selectivity problems.Among by-catch species,banded scad(Caranx(Atule)kalla)is economically the most important one.The aim of this study is to improve the selective properties of codends for banded scad.Size selectivity of 2 traditional diamond mesh codends,with mesh size 25 and 30 mm(defined as D25 and D30,respectively),and 4 new combined square and diamond mesh codends,with25 mm square-mesh and 25 mm diamond-mesh(S25+D25),30 mm square-mesh and 25 mm diamondmesh(S30+D25),35 mm square-mesh and 25 mm diamond-mesh(S35+D25),and 35 mm square-mesh and18 mm diamond-mesh(S35+D18),were tested for banded scad in shrimp beam trawl fishery of the South China Sea.A total of 54 valid hauls were finished using the covered codend method,and 5750 banded scad were caught.Selective parameters were obtained using the logistic equation with the maximum likelihood method,by incorporating the between-haul variation.The results show that the present minimum mesh size for shrimp beam trawl in the South China Sea,25-mm diamond mesh size,is insufficient to release immature banded scad.The S35+D25 codend is proved to be the most eff ective codend to release immature banded scad,and its 50%retention length(L _(50))is larger than the minimum landing size(MLS=63.5 mm)of banded scad.However,the 95%confidence intervals of L _(50) overlap among the three combined mesh codends,the S25+D25,S30+D25 and S35+D25 codend.For a sustainable exploitation of banded scad,larger mesh sizes are suggested to be investigated for the combined mesh codends.

Fuzzy robust path tracking strategy of an active pelagic trawl system with coordinated ship and winch regulation

A fuzzy robust path tracking strategy of an active pelagic trawl system with ship and winch regulation is proposed.First,nonlinear mathematic model of the pelagic trawl system was derived using Lagrange equation and further simplified as a low order model for the convenience of controller design.Then,an active path tracking strategy of pelagic trawl system was investigated to improve the catching efficiency of the target fish near the sea bottom.By means of the active tracking control,the pelagic trawl net can be positioned dynamically to follow a specified trajectory via the coordinated winch and ship regulation.In addition,considering the system nonlinearities,modeling uncertainties and the unknown exogenous disturbance of the trawl system model,a nonlinear robust H2 /H∞ controller based on Takagi-Sugeno(T-S) fuzzy model was presented,and the simulation comparison with linear robust H2 /H∞ controller and PID method was conducted for the validation of the nonlinear fuzzy robust controller.The nonlinear simulation results show that the average tracking error is 0.4 m for the fuzzy robust H2 /H∞ control and 125.8 m for the vertical and horizontal displacement,respectively,which is much smaller than linear H2 /H∞ controller and the PID controller.The investigation results illustrate that the fuzzy robust controller is effective for the active path tracking control of the pelagic trawl system.

Relationship between trawl selectivity and fish body size in a simulated population

Trawl is a main fishing gear in Chinese fishery,capturing large fish and letting small ones at large.However,long-term use of trawl would result in changes of phenotypic traits of the fish stocks,such as smaller size-at-age and earlier age-at-maturation.In this study,we simulated a fish population with size characteristics of trawl fishing and the population produces one generation of offspring and lives for one year,used trawl to exploit the simulated fish population,and captured individuals by body size.We evaluated the impact of the changes on selectivity parameters,such as selective range and the length at 50% retention.Under fishing pressure,we specified the selectivity parameters,and determined that smaller selection rates and greater length at 50% retention were associated with an increased tendency towards miniaturization.

Robust Optimal Output Tracking Control of A Midwater Trawl System Based on T-S Fuzzy Nonlinear Model

A robust optimal output tracking control method for a midwater trawl system is investigated based on T-S fuzzy nonlinear model. A simplified nonlinear mathematical model is first employed to represent a midwater trawl system, and then a T-S fuzzy model is adopted to approximate the nonlinear system. Since the strong nonlinearities and the external disturbance of the trawling system, a mixed H2/H∞ fuzzy output tracking control strategy via T-S fuzzy system is proposed to regulate the trawl depth to follow a desired trajectory. The trawl depth can be regulated by adjusting the winch velocity automatically and the tracking error can be minimized according to the robust optimal criterion. In order to validate the proposed control method, a computer simulation is conducted. The simulation results indicate that the proposed fuzzy robust optimal controller make the trawl net rapidly follow the desired trajectory under the model uncertainties and the extemal disturbance caused by wave and current.

Trawling算法在Web结构挖掘中的应用

在Web信息检索中,如何能够提取出与某个主题信息相关的网页变得异常重要,web结构挖掘作为web数据挖掘的一个重要方面,主要挖掘web潜在的链接结构模式,通过分析一个网页链接和被链接数量以及对象来建立web自身的链接结构模式,可以用于网页归类,本文探讨了Trawling算法在Web结构挖掘中的应用。

Potential Ecosystem Level Effects of a Shrimp Trawling Fishery in La Paz Bay, Mexico

The shrimp trawling fishery is the most important one in Mexico in value terms and given its putative environmental, societal and economical implications, it is also the most difficult to manage. Although this fishery was restricted from national bays and estuaries since the 1970’s, local fisheries cooperatives recently claimed access to shrimp stocks within La Paz Bay by using an artisanal fleet and a low impact trawling net. This study is aiming at simulating some ecosystem level effects of such a potential fishing effort release. We explored the response of three ecosystem indicators under two different exploitation scenarios: 30% and 80% of shrimp biomass removal. The indicators were: relative ecosystem biomass distribution as function of trophic level, trophic replacement and interaction strength, all computed from the outputs of a mass balance dynamic model (Ecopath with Ecosim) of this ecosystem. Our results suggest that moderate fishing scenario (30%) would not cause major changes in either indicator whilst the scenario of strong fishing pressure (80%) seems to increase not only the fish resources variability at the population level but also the variability of the overall biomass, hence potentially reducing ecosystem stability.

Effects of Trawl Selectivity and Genetic Parameters on Fish Body Length Under Long-Term Trawling

Long-term fishing pressure affects the biological characteristics of exploited fish stocks. The biological characteristics of hairtail(Trichiurus lepturus) in the East China Sea are unable to recover because of long-term trawling. Fishing induces evolutionary effects on the fish's biological characteristics. Evidence of these changes includes small size at age, a shift to earlier age structure, and early maturation. Natural and artificial selection usually affect the fish's life history. Selection can induce different chances of reproduction, and individual fish can give a different genetic contribution to the next generation. In this study, analysis of time-dependent probability of significance and test of sensitivity were used to explore the effects of fish exploitation rate, mesh size, and heritability with long-term trawling. Results showed that fishing parameters were important drivers to exploited fish population. However, genetic traits altered by fishing were slow, and the changes in biological characteristics were weaker than those caused by fishing selection. Exploitation rate and mesh size exhibited similar evolutionary trend tendency under long-term fishing. The time-dependent probability of significance trend showed a gradual growth and tended to be stable. Therefore, the direction of fishing-induced evolution and successful management of fish species require considerable attention to contribute to sustainable fisheries in China.

Simplified Approximate Expressions for the Boundary Layer Flow in Cylindrical Sections in Plankton Nets and Trawls

Trawls and plankton nets are basically made up of conical and cylindrical net sections. In conical sections the flow will pass through the inclined net wall with a noticeable angle of attack, and then the flow, filtration and drag can be suitably modelled e.g. by a pressure drop approach [1]. In cylindrical and other non-tapered net sections, such as foreparts and extension pieces in trawls and plankton nets, the flow is directed along the net wall and is best considered in terms of a boundary layer. Boundary layer theory and turbulence models can be used to describe such flow, but this requires extensive numerical modelling and computational effort. Simplified approximate formulas providing a qualitative description of the flow with some quantitative accuracy are therefore also useful. This work presents simplified parametric expressions for boundary layer flow in cylindrical net sections, including the boundary layer thickness and growth rate along the net, the filtration velocity out of the net wall, the decrease in mass flux through the net due to the growing boundary layer, and the effect of twine thickness, flow (towing) velocity and the dimensions of the net. These expressions may be useful for assessing the existence and extension of a boundary layer, for appropriate scaling of boundary layer effects in model tests, for proper placement of velocity measurement probes, for assessing the influence on filtration and clogging of plankton net sections, and more.

Using tickler chains to promote more environmentally benign trawls in an Australian penaeid fishery

There has been an increasing emphasis on reducing the environmental impacts of penaeid trawls by modifying their anterior sections.In one Australian estuary,this approach has culminated in a new generic trawl comprising a slightly smaller mesh size,and steeper and shorter side panels than traditional trawls,combined with a top panel extending forwards of the bottom panel(termed‘lead-ahead’)and no headline floats.This study sought to investigate if an additional simple,cumulative modification(tickler chain)might help promote adoption of the new trawl,via improved penaeid catches.Six volunteer fishers towing pairs of either the traditional or new trawls in double rigs were asked if an observer could accompany fishing trips,and alternately attach a light tickler chain(~3 kg)anterior to the ground gear of one trawl.Regardless of the trawl design,the tickler chains significantly increased the catch weights of penaeids,but relatively more so in the new trawls(by 1.11×vs 1.08×).The tickler chains also significantly increased the number and weight of total bycatch in the traditional trawls(by up to 1.22×),but not in the new trawls-although there were variable species-specific effects.In terms of total effects,the data support using tickler chains to increase the efficiency of the new trawls and their cumulative benefits,and could be promoted to facilitate broader inter-and intra-fleet adoption.

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.

南海区单船有袖单囊拖网网囊网目对金线鱼的选择性研究

金线鱼(Nemipterus virgatus)是南海区拖网渔业的重要经济种类。为提高拖网对金线鱼的选择性,在南海区使用套网法对网目尺寸分别为30、35、40和45 mm的4组网囊开展了试验。使用双重拔靴法拟合网囊对金线鱼的选择性参数和选择性曲线,比较网囊间的选择性差异,并估算网囊的捕捞方式指数。结果显示,随着网目尺寸的增大,50%选择体长(L50)增大,选择性曲线的置信区间变窄,网囊对金线鱼幼鱼的渔获比例减小。试验网囊的L50均小于金线鱼的开捕规格(15.0 cm),网囊对金线鱼的抛弃率大于67%。研究表明,增大网目尺寸能够优化网囊的选择性,但选择性仍需进一步提升,才能更好地促进金线鱼拖网渔业的可持续发展。

长江口桁拖网最小网目尺寸的选择性研究

为完善东海区桁拖网最小网目尺寸标准,2017年6月和9月分别于长江口(30°50′~31°24′N,122°12′~122°26′E)近海使用主机功率88.2 kw和总吨位30 t的渔船进行套网法25 mm和35 mm网囊网目的渔获选择性试验,供渔政主管部门制定网目尺寸标准参考。本研究利用Logistic模型分别拟合虾蟹类优势种安氏白虾(Palaemon annandalei)、三疣梭子蟹(Portunus trituberculatus)和葛氏长臂虾(Palaemon gravieri)的选择性曲线,利用极大似然法估算模型参数。结果表明,桁拖网共捕获渔获物47种,包括鱼类23种,虾类13种,蟹类6种,其他类5种;虾蟹类优势种为安氏白虾、三疣梭子蟹和葛氏长臂虾,体长范围分别为17~45 mm、13~130 mm和18~52 mm。在25 mm标准网目尺寸下,安氏白虾、三疣梭子蟹和葛氏长臂虾的选择率为50%所对应的体长或甲宽(L_(50))分别为33.784 mm、30.850 mm和32.930 mm,选择系数(SR)分别为10.100 mm、7.978 mm和9.094 mm。各优势种的平均逃逸率均低于50%,三疣梭子蟹的逃逸率显著低于其他优势种(P<0.05)。研究表明,当网囊网目尺寸由25 mm增大至35 mm时,能够有效释放本研究海域的虾蟹类,建议适当增大现行网目尺寸标准。

基于增强回归树的西非北部沿海底拖网主要商业头足类CPUE与主要环境因子的关系

【目的】研究西非北部沿海不同环境因子对章鱼、墨鱼和鱿鱼等主要商业头足类单位捕捞努力量(CPUE)影响的规律,为合理规划商业性捕捞生产和科学渔业管理等提供参考。【方法】基于2019-2021年西非北部沿海底拖网渔业数据,使用增强回归树模型(BRT)分析主要商业头足类对于海表面温度(SST)、叶绿素质量浓度、海表面盐度、海表面高度异常,溶解氧浓度、降水量、经度、纬度、海底深度、离岸距离和时间等环境因子的响应规律。【结果与结论】BRT模型在渔业研究中尤其在数据处理方面较传统模型优势明显。本研究以相对重要性值划分11个环境因子对于商业头足类(章鱼、墨鱼和鱿鱼)CPUE影响的强弱:强重要性(10%以上)、一般重要性(5%~10%)和弱重要性(5%以下)。BRT拟合结果表明,SST、纬度、降水量和离岸距离对章鱼CPUE影响有强重要性,纬度、SST、叶绿素质量浓度对墨鱼CPUE影响有强重要性,而降水量、溶解氧浓度、SST对于鱿鱼CPUE影响有强重要性。纬度对章鱼和墨鱼的CPUE影响有强重要性,较高的CPUE分别出现在24°N以南(章鱼)和23°N―27°N之间(墨鱼);离岸距离对章鱼和鱿鱼CPUE影响均在一般重要性以上,离岸30~70 km范围内CPUE较高;3种头足类中,鱿鱼CPUE受时间影响最大,较高CPUE出现在1月和7月。环境因子中,SST对3种头足类影响均有强重要性,相对重要性均在20%以上,其中对于章鱼和墨鱼影响效应相反,章鱼趋暖而墨鱼趋寒;溶解氧浓度与鱿鱼CPUE呈正相关;叶绿素质量浓度对墨鱼CPUE影响有强重要性,叶绿素质量浓度较低时CPUE较平稳,但当质量浓度超过4 mg·m^(-3)时CPUE却逐渐下降。降水量对于3种头足类CPUE影响均在一般重要性以上,随降水量增加,鱿鱼CPUE有所上升,而章鱼和墨鱼CPUE先升高后降低。

Identifying the turbulent flow developing inside and around the bottom trawl by Electromagnetic Current Velocity Meter approach in the flume tank

The Knowledge of turbulent flow developing inside and around the bottom trawl net is of great importance not only for improving the hydrodynamic performance of the gear but also for the selectivity via the fish response,such as the herding response or escape behavior.The 3-D Electromagnetic Current Velocity Meter(ECVM)measurements were performed to investigate the effect of turbulent flow on the bottom trawl net performance and to analyze the turbulence intensity and velocity ratio inside and around different parts of the trawl net.Proper orthogonal decomposition(POD)method was applied in order to extract the phase averaged mean velocity field of turbulent flow from each available ECVM instantaneous velocity.The results demonstrated the existence of turbulence flow,consisting of turbulent boundary layer flow and the turbulence due to the trawl wake developing all inside and around the bottom trawl net.Increasing input streamwise velocity results in faster trawl movement and a significant turbulent flow.The maximum turbulence intensity inside and around trawl wing,square part,first belly,second belly,third belly,cod-end is 0.95%,1.34%,3.40%,4.10%,4.25%and 3.80%,respectively.It was found that the mean velocity field in a turbulent flow inside and around trawl net cod-end recovered on the average was~77.58%of the input streamwise velocity.It is~12.92%,~13.07%,~11.40%,~13.00%and~0.45%less than that inside and around trawl wing,square part,first belly,second belly,and third belly of the bottom trawl net,respectively.The turbulent flow behavior depends strongly on the structure oscillation,input streamwise velocity and,porosity of the net structure.It is necessary to take into account the velocity reduction inside and around a different part of the trawl net to improve the entire drag force determination,cod-end design,and further selectivity control of the fishing gear.