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.

A Review of Microplastics in China Marine Waters

Microplastics(<5 mm)are ubiquitous in the environment and can pose potential danger to the ecosystem and even human health.As the sink of microplastics,the ocean,especially the densely populated coastal area,has become a hotspot for research on microplastic pollution.In the last decade,the research of marine microplastics has been rapidly increasing in China.This review summarized the microplastic research conducted in China marine waters so far,and introduced the trends and progress of microplastic research in the four seas along the coast of China.We reviewed and compared the current sampling,extraction,and identification methodologies of China's microplastic research.According to the sampling method,the 30 reviewed studies were separated into two categories,trawl sampling and bulk sampling,to summarize relevant data,including abundance,sizes,shapes,colors and polymer types of microplastics.The main results showed that the distribution of microplastics in China's marine environment varied significantly,with offshore mariculture zones and the South China Sea being the most contaminated areas.Transparent,granules(or pellets)and fibers were the most dominant microplastic colors and shapes,and the size of microplastics was influenced significantly by the sampling method.Polyethylene(PE),polypropylene(PP)and polystyrene(PS)were the most common polymer types found in the China Sea,accounting for 49.96%,29.97%,and 12.38%of the total studies,respectively.Compared with other global data,China's coastal microplastic pollution is at an intermediate level and does not seem to be a major microplastic pollution source.

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.

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.

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.

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.

Smaller bar spacings in a Nordmøre grid reduces the bycatch of redfish(Sebastes spp.)in the offshore Northern shrimp(Pandalus borealis)fishery of eastern Canada

The offshore Northern shrimp(Pandalus borealis)bottom trawl fishery in eastern Canada currently uses 22 and 28 mm bar spacing Nordmøre grids to limit bycatch from using small mesh codends.However,a recent rebound of juvenile redfish(Sebastes spp.),that can pass through the grids,has greatly increased bycatch.To address this concern,this study investigated the effectiveness of 17 and 15 mm bar spacing Nordmøre grids in a twin-trawl(paired)configuration against the traditional 22 mm bar spacing grid.Size selectivity analyses showed that the 17 and 15 mm grids resulted in no significant reduction in shrimp catch across all length classes.The 17 mm grid significantly reduced redfish bycatch for all length classes and the 15 mm grid significantly reduced redfish bycatch for individuals larger than 95 mm total length.Less redfish entered the codend with the experimental grids,however,the overlap in width between redfish and Northern shrimp limits the overall sorting efficiency of the grids,leaving some redfish still vulnerable to capture.

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.

Refining tickler chains for penaeid trawls

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.

Improving penaeid-trawl efficiencies via ground gear with tickler chains

Penaeid trawls often have poor ecological efficiencies measured as unwanted bycatches(with high discard mortality)and<100%effectiveness for all penaeids interacting with the ground gear(with some escape mortality).This study investigated the utility of varying ground-chain size(8-vs 10-mm diameter(Ø)stainless-steel chain)and the present or absence of a tickler chain(4-mmØ)for improving efficiencies an Australian fishery diurnally targeting school,Metapenaeus macleayi and eastern king prawns,Penaeus plebejus.There were no treatment effects on wing-end spread and therefore areas trawled.Irrespective of the tickler chain,increasing the ground-chainØreduced catches of total penaeids(by 14%)and jellyfish,Catostylus mosaicus(by 39%)-results speculatively attributed to variation in ground contact and/or a slightly higher footrope in trawls with the 10-mmØchain.By comparison,installing the tickler chain had no effect on eastern king prawns,but increased catches of school prawns(by 1.13×)and bycatches of blue swimmer crabs,Portunus armatus(1.52×;a species with a low discard mortality)and jellyfish(1.23×),while reducing bycatches of two species with high discard mortalities:southern herring,Herklotsichthys castelnaui(by 38%)and squid,Uroteuthis sp.(37%).The results were attributed to the tickler chain producing(1)mechanical stimuli that augmented the capture of benthic-orientated school prawns(but not eastern king prawns owing to diel variations in emergence from the substrata),blue swimmer crabs and jellyfish;and(2)visual stimuli that possibly enabled some southern herring and squid to detect the trawl and escape.The data show tickler chains can improve trawl efficiency for penaeids with at least some positive effects on bycatch that should be rationalised in terms of species-specific vulnerabilities to unaccounted fishing mortality.

A review on fishing gear in China: Selectivity and application

In the 1980s,China’s offshore marine fishery resources declined,thus fishing gear selectivity gradually became a focus of scientific fishery research.Studies of fishing gear selectivity mainly focus on five kinds of fishing gear:trawls,shrimp beam trawls,stow nets,gillnets,and pots.Cover net method is the main method used in the comparative study of codend selectivity of both trawls and stow nets.These results showed that(1)Bottom trawls,targeting Larimichthys polyactis(the Little yellow croaker)and Trichiurus haumela(the Hairtail),should have a minimum mesh size of the codend no less than 60 mm in the Yellow and East China Seas.While the codend should use a square mesh with a minimum mesh size of 40 mm due to the variety of targeted species in the South China Sea.(2)Pair pelagic trawls,targeting pelagic species such as Engraulis japonicus(the Japanese anchovy),Pampus argenteus(the Silvery pomfret),Scomberomorus niphonius(the Spanish mackerel),as well as bottom species such as L.polyactis and T.haumela,should have a minimum mesh size of the codend no less than 56 mm in the Yellow Sea.(3)Shrimp beam trawls should use shrimp/fish separator devices(e.g.,separator panels,vertical separator panels,and sorting grids)to improve intra-species selectivity and reduce by-catch of non-target fish.(4)Canvas stow nets,targeting L.polyactis and T.haumela,should have a minimum mesh size of the codend no less than 55 mm.(5)Gillnets,the minimum mesh size should be determined for catching traditional economic species,but also the optimal mesh size for newly developed species.Such as,gillnets with mesh size of 35-38.71 mm targeting Harpodon nehereus(the Bombay duck),55 mm targeting L.polyactis and Nemipterus virgatus(the Golden threadfin bream),72 mm targeting Dentex tumifrons(the yellow sea breams)or Scomberomorus commersoni with body length of 280 mm,100 mm targeting P.argenteus,104 mm or 146 mm targeting 2-3 year or 6-7 year S.niphonius,120 mm targeting S.commersoni with body length of 380 mm,and 144 mm targeting Pampus chinensis(the Chinese pomfret).(6)Pots,the steel-framed round pots targeting Portunus trituberculatus(the swimming crab)should have a 33 mm height of escape vent or an 8 cm diameter escape hole,and the steel-framed round pots or accordion-shaped trap targeting Charybdis japonica(the Asian paddle crabs)should have a 30 mm height of escape vent installed on the side of pots.These results of fishing gear selectivity from the scientific basis for the implementation of fishing gear access systems,especially for identifying the minimum mesh size of fishing gear.In the future,the Fisheries Research Institute of China should conduct long-term investigations of selectivity in trans areas.Developing fishing gear access systems that are in line with a sustainable development is of great significance for the conservation and restoration of offshore fishery resources in China.

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.

Release of unwanted flat-bodied fish from a horizontal-bar grid system as revealed through comparative fishing trials and underwater video observations

Grid systems separate fish species primarily through physical means:fish size and body shape.On Georges Bank off the northeast USA,many species of flounders are overfished,and their catch needs to be reduced.Flat-bodied skates are also often discarded.We tested a European style horizontal-bar grid system to reduce these flat-bodied low quota species in a trawl targeting the haddock(Melanogrammus aeglefinus),whose population is robust.The grid system consisted of 4 grid sections,two on each side,with horizontal bars 70 mm apart.The alternating tow method was used to compare the catch characteristics between a trawl with a grid section and the same trawl without a grid section.A video camera was used to observe fish escape in the grid section.The results indicate that the grid system reduced the flounder catch rate(mainly winter flounder,Pseudopleuronectes americanus)by 51.3%,and skates(mainly winter skate,Leucoraja ocellata and little skate,Leucoraja erinacea)by 29.4%,while there were no differences in the catch of Atlantic cod(Gadus morhua).While haddock was reduced by 37%by weight,the reduction was primarily small undersized individuals.There was no reduction in large haddock greater than 50 cm.Video observations indicates that flounders and cod mainly escaped from the bottom half of the grid while haddock were from the top half.These finding suggest that this system reduced low quota flounders,as well as discarded skates and small haddock,while retaining Atlantic cod and large size haddock.Underwater observations indicate that differential spacing(narrower on the top and wider on the bottom)may improve the system performance by releasing more flat-bodied fish,that were observed to escape from the bottom part of the grid,while retaining more haddock,which typically escape from the top part of the grid.

Comparative studies on biochemical analysis of some economically important marine gastropods along Gulf of Mannar region,southeast coast of India

Objective:To signify the economic importance of molluscan-gastropod food by estimating its biochemical composition.Methods:Samples were collected from the trawl net bycatch at the fish landing center of Mandapam coast of the Gulf of Mannar region.The total protein,carbohydrate,lipid,ash and moisture contents were estimated from nine gastropods i.e.Phalium glaucum,Tonna dolium,Hemifusus pugilinus,Babylonia spirata,Xancus pyrum,Chicoreus ramosus,Harpa articularis,Ficus ficus and Babylonia zeylanica.Results:The percentages of protein(41.2%),carbohydrate(17.5%)and lipid(6.6%)contents were found highest in Babylonia spirata,followed by other gastropods.The maximum ash content was observed in Chicoreus ramosus(1.21%)and the maximum moisture content was observed in Phalium glaucum(83.71%).Conclusions:The results show that all the nine gastropods contain good sources of protein and other biochemical constituents and can be used for edible purposes to prevent starvation.