发布求助
文献互助 智能选刊 最新文献

Bulletin of The Korean Society of Fisheries Technology最新文献

筛选
英文 中文
Variation of fisheries conditions of mackerel (Scomber japonicus) fishing ground for large purse seine fisheries 大型围网捕捞鲭鱼渔场渔业条件的变化
Bulletin of The Korean Society of Fisheries Technology Pub Date : 2011-05-31 DOI: 10.3796/KSFT.2011.47.2.108
H. Lee, Hyung-seok Kim
{"title":"Variation of fisheries conditions of mackerel (Scomber japonicus) fishing ground for large purse seine fisheries","authors":"H. Lee, Hyung-seok Kim","doi":"10.3796/KSFT.2011.47.2.108","DOIUrl":"https://doi.org/10.3796/KSFT.2011.47.2.108","url":null,"abstract":"In order to offer data about fisheries resources management and prediction of catch on large purse seine fisheries, the fluctuation of the fisheries condition and distributions of fishing ground for mackerel were analysed with monthly catch data for 1990.2009. The overall catch has decreased to about 70% since 1997, with approximately 70% of the mackerel (Scomber japonicus) catch and monthly fluctuations showing a similar pattern. Monthly distribution of fishing ground is like distribution of mackerel in large purse seine fishery. The main fishing grounds are near Jeju Island and the Yellow sea with the main fishing season existing between October to December. The catches fluctuations and distribution of fishing ground were related to the effect of regime shifts. Therefore, in order to prediction of catch on large purse seine fisheries should be studied these relationships.","PeriodicalId":211073,"journal":{"name":"Bulletin of The Korean Society of Fisheries Technology","volume":"41 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2011-05-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"116455166","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 8
Design and performance estimation of fish-luring system using the water cooling typed LED lamp 水冷式LED灯诱鱼系统的设计与性能评估
Bulletin of The Korean Society of Fisheries Technology Pub Date : 2011-05-31 DOI: 10.3796/KSFT.2011.47.2.079
Bong-Seong B Ae, N. Heui-ChunA, Ki-Jin K Won, Seong-Wook P Ark, Chang-Doo P Ark, Kyoung-Hoon L Ee
{"title":"Design and performance estimation of fish-luring system using the water cooling typed LED lamp","authors":"Bong-Seong B Ae, N. Heui-ChunA, Ki-Jin K Won, Seong-Wook P Ark, Chang-Doo P Ark, Kyoung-Hoon L Ee","doi":"10.3796/KSFT.2011.47.2.079","DOIUrl":"https://doi.org/10.3796/KSFT.2011.47.2.079","url":null,"abstract":"A fishing lamp is fishing gear to gather fish in the night. But the cost of oil which is used to a light fishing lamp,goes significantly up to almost one hundred million won for 50 tonnage vessels and forty million won in case ofvessels less than 10 tonnages. This cost has almost taken 30–40% of total fishing costs. As oil price increases, thebusiness condition of the fishery gets worse and worse. Therefore, it is very urgent to develop an economicalfishing lamp, to solve the problem of fishery′s business difficulty. This research aims at developing a fishing lampfor squid jigging fishery and hairtail angling fishery using the LED, which has excellent efficiency and durability.One fishing lamp has about 160Watt capacity and five fishing lamps are installed one aluminium panel in whichsea water flows to emit generated heat from LED to outside. Developed fishing lamp lights to an effectivedirection of jigging and angling operation. This fishing lamp can be controlled to light the direction of fish shoalbecause the aluminium panel can be controlled to up and down direction. The wave length of fishing lamp haswhite and blue color. White color light is to gather fish shoal of horizontal direction and blue color light is togather fish shoal of vertical direction. After development of this fishing lamp, 60–110 units are established on theboat, and operated fishing. Consequently, in the case of squid jigging, spent energy was reduced to 39%, in thecase of hairtail angling, 68% of spent energy was reduced. And the catch was more than another boat.Keywords: LED fishing lamp, Squid jigging, Hairtail angling","PeriodicalId":211073,"journal":{"name":"Bulletin of The Korean Society of Fisheries Technology","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2011-05-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128613433","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 12
Performance characteristics of a multi-directional underwater CCTV camera system to use in the artificial reef survey 一种用于人工礁调查的多向水下闭路电视摄像系统的性能特点
Bulletin of The Korean Society of Fisheries Technology Pub Date : 2011-05-31 DOI: 10.3796/KSFT.2011.47.2.146
Dae-jae Lee
{"title":"Performance characteristics of a multi-directional underwater CCTV camera system to use in the artificial reef survey","authors":"Dae-jae Lee","doi":"10.3796/KSFT.2011.47.2.146","DOIUrl":"https://doi.org/10.3796/KSFT.2011.47.2.146","url":null,"abstract":"Underwater CCTV camera systems are increasingly replaced the traditional net approach of assessing the species, numbers and aggregation patterns of marine animals distributing around the artificial reefs installed in the inshore fishing grounds, in particular, in relation to the biological investigation of behavior and distribution patterns of target fishes. In relation to these needs, we developed a multi-directional underwater CCTV camera system to use in detecting and tracking marine animals in the artificial reef ground. The marine targets to be investigated were independently tracked by using a camera module toward the bottom and four camera modules installed in the interval of in horizontal plane and inclination of in vertical plane of the CCTV system without the overlap of video frames by each camera module. From the results of several field tests at sea, we believe that the developed multi-directional underwater CCTV camera system will contribute to a better understanding in evaluating the effect of artificial reefs installed in the inshore fishing grounds.","PeriodicalId":211073,"journal":{"name":"Bulletin of The Korean Society of Fisheries Technology","volume":"27 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2011-05-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"133029045","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 6
Seasonal variation and species composition of fish species in artificial reefs in the Shinyang-Ri coastal waters off Jeju island, Korea 韩国济州岛新阳里近海人工鱼礁鱼类的季节变化和种类组成
Bulletin of The Korean Society of Fisheries Technology Pub Date : 2011-05-31 DOI: 10.3796/KSFT.2011.47.2.118
J. Kim, Ha-Won Kim, S. Huh, S. Kwak
{"title":"Seasonal variation and species composition of fish species in artificial reefs in the Shinyang-Ri coastal waters off Jeju island, Korea","authors":"J. Kim, Ha-Won Kim, S. Huh, S. Kwak","doi":"10.3796/KSFT.2011.47.2.118","DOIUrl":"https://doi.org/10.3796/KSFT.2011.47.2.118","url":null,"abstract":"This study was investigated seasonal variation and species composition of fish species at artificial reefs (octangle three-stage compartment type and dice type)and natural rock in the Shinyang-Ri coastal waters off Jeju Island with visual census method in January 2009, April 2009, July 2009, September 2009, December 2009, March 2010, June 2010, and September 2010. A total of 44 fish species was occurred, and the dominant fish species were Apogon semilineatus, Chromis notata, Trachurus japonicus, Sebastes thompsoni and Apogon doederleini. The number of individuals of fishes was higher at April 2009 and March 2010, however, lower at June 2010 and September 2010. The number of fish speices was higher at artificial reefs than those of at natural rock. Apogon semilineatus was dominant at octangle three-stage compartment type and the dice type artificial reefs, whereas Chromis notata was for natural rock. These results indicated that artificial reefs were good habitats for a variety of fish species in the Shinyang-Ri coastal waters off Jeju Island.","PeriodicalId":211073,"journal":{"name":"Bulletin of The Korean Society of Fisheries Technology","volume":"71 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2011-05-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132692059","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 5
Relative efficiency of monofilament and multifilament nylon gill net forMarbled sole (Pleuronectes yokohamae) in the western sea of Korea 单丝和多丝尼龙刺网在韩国西部海域形成大理石纹鳎(Pleuronectes yokohamae)的相对效率
Bulletin of The Korean Society of Fisheries Technology Pub Date : 2010-11-30 DOI: 10.3796/KSFT.2010.46.4.281
In-Ok Kim, Chang–Doo Park, Samkwang Cho, Hyun-Young Kim, Bongjin Cha
{"title":"Relative efficiency of monofilament and multifilament nylon gill net forMarbled sole (Pleuronectes yokohamae) in the western sea of Korea","authors":"In-Ok Kim, Chang–Doo Park, Samkwang Cho, Hyun-Young Kim, Bongjin Cha","doi":"10.3796/KSFT.2010.46.4.281","DOIUrl":"https://doi.org/10.3796/KSFT.2010.46.4.281","url":null,"abstract":"To estimate the mesh selectivity of monofilament and multifilament gill net for Marbled sole, Pleuronectes yokohamae, the field testes were carried out 12 times with five different mesh sizes (86.6, 101.0, 121.2, 137.7, 151.5mm) in the western sea of Korea, 2007-2009. The master curve of selectivity was estimated by the extended Kitahara`s method. In the field testes, the total number of species and catch were 26 and 987, respectively. The catch number of marbled sole was 728 and occupied 73.8% in total catch. The optimum values of l/m for 1.0 of retention probability in monofilament and multifilament gill net were estimated 0.288 and 0.307, respectively and l/m was estimated to be 0.189, 0.203, 0.213, 0.222 and 0.230 in case of monofilament gill net and 0.171, 0.191, 0.205, 0.216 and 0.227 in case of multifilament gill net when the retention probability were 0.1, 0.2, 0.3, 0.4 and 0.5, respectively. The 0.5 selection range of multifilament gill net was wider about 1.5 times than that of monofilament gill net according as multifilament gill net was 0.216 and monofilament gill net was 0.148. So the multifilament gill net has a low selectivity than that of the monofilament gill net. To estimate the optimum mesh size on first maturity length 19.5cm of marbled sole, the retention probability value of 0.1 was adopted in consideration of using the gill net for marbled sole at present. The optimum mesh size were estimated to be 103.2mm and 114.0mm in monofilament gill net and multifilament gill net, respectively, on first maturity length 19.5cm of marbled sole.","PeriodicalId":211073,"journal":{"name":"Bulletin of The Korean Society of Fisheries Technology","volume":"17 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2010-11-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126070746","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 10
Estimation of economic benefits of biodegradable fishing net by using contingent valuation method (CVM) 基于条件价值法的生物可降解渔网经济效益评估
Bulletin of The Korean Society of Fisheries Technology Pub Date : 2010-08-31 DOI: 10.3796/KSFT.2010.46.3.265
Seong-Wook Park, Hyeok-Jun Kwon, Seong-Kwae Park
{"title":"Estimation of economic benefits of biodegradable fishing net by using contingent valuation method (CVM)","authors":"Seong-Wook Park, Hyeok-Jun Kwon, Seong-Kwae Park","doi":"10.3796/KSFT.2010.46.3.265","DOIUrl":"https://doi.org/10.3796/KSFT.2010.46.3.265","url":null,"abstract":"The main purpose of this study is to estimate willingness to pay (WTP) by the general publics, assuming thatthey pay tax or charge for protecting marine living resources and environment through developing andsupplying biodegradable fishing nets. This study employed a contingent valuation method (CVM) which isan econometric method. The survey was conducted by using both double-bounded dichotomous choice andopen-ended survey. Tobit model was used for the analysis. The variables included concerns about marineenvironment and fishing net discarded, sex, age profile, number of family members, educational level andpersonal disposable income. Annual average WTP per family for the biodegradable fishing net developmentand supply was estimated at 5,294 won and national WTP amounted to some 84.2 billion won. This includesboth of use and non-use value of biodegradable fishing nets.Keywords: Contingent valuation method (CVM), Biodegradable fishing net, Conservation, Ecosystem,Tobit model","PeriodicalId":211073,"journal":{"name":"Bulletin of The Korean Society of Fisheries Technology","volume":"5 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2010-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"128618957","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 8
Fish length dependence of target strength for striped beakperch, bluefin searobin and konoshiro gizzard shad caught in the artificial reef ground of Yongho Man, Busan 在釜山龙湖湾人工礁场捕获的条纹喙鲈、蓝鳍海鲈和河野沙鱼的目标强度对鱼的长度依赖性
Bulletin of The Korean Society of Fisheries Technology Pub Date : 2010-08-31 DOI: 10.3796/KSFT.2010.46.3.239
Dae-jae Lee
{"title":"Fish length dependence of target strength for striped beakperch, bluefin searobin and konoshiro gizzard shad caught in the artificial reef ground of Yongho Man, Busan","authors":"Dae-jae Lee","doi":"10.3796/KSFT.2010.46.3.239","DOIUrl":"https://doi.org/10.3796/KSFT.2010.46.3.239","url":null,"abstract":"Species of fish such as striped beakperch, bluefin searobin and konoshiro gizzard shad are commercially very important due to their high demand in the Korean market. When estimating acoustically the abundance of stocks for these species, it is of crucial importance to know the target strength (TS) to the length dependence. In relation to these needs, the TS experiments were conducted on three different species in an acrylic salt water tank using two split-beam echo sounders of 70 and 120 kHz. The TS for these three species under the controlled condition was simultaneously measured with the swimming movement by a DVR system and analyzed as a function of fish length (L) and frequency (or wavelength λ). The equation of the form TS a log (L) b log (λ) c was derived for their TS-length dependence. The best fit regression of TS on fork length for striped beakperch was estimated as TS 35.67 log (L, m) 15.67 log (λ, m) 46.69 (r","PeriodicalId":211073,"journal":{"name":"Bulletin of The Korean Society of Fisheries Technology","volume":"85 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2010-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"130597333","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 7
Movement range and behavior of acoustic tagged abalone (Haliotis discus hannai) in Jeonnam marine ranch 全南海洋牧场声标记鲍鱼的运动范围和行为
Bulletin of The Korean Society of Fisheries Technology Pub Date : 2010-08-31 DOI: 10.3796/KSFT.2010.46.3.232
Bokyu Hwang, H. Shin
{"title":"Movement range and behavior of acoustic tagged abalone (Haliotis discus hannai) in Jeonnam marine ranch","authors":"Bokyu Hwang, H. Shin","doi":"10.3796/KSFT.2010.46.3.232","DOIUrl":"https://doi.org/10.3796/KSFT.2010.46.3.232","url":null,"abstract":", were measured by anacoustic telemetry technique. The shape of the sea bottom of the experimental area was surveyed by abathymetry system and three self-recording type acoustic receivers were used for monitoring the behaviorand measuring the movement range. The abalones (WA1-WA4) attached acoustic tags were released andmeasured the movement during ten months. Three abalones (WA1, WA3 and WA4) were successivelydetected around the released point during the experiment and were moved to the V2 area where water depthis deeper than the V1 area. The change of inhabitation depth was also detected from the depth sensor ofWA4. As the result, abalones were moved to deeper water area accordance with the decrease of the watertemperature. The moved ranges of abalones were approximately 200 400m from the release point.Keywords: Acoustic telemetry, Movement behavior of abalone, Acoustic tag, Detection rate","PeriodicalId":211073,"journal":{"name":"Bulletin of The Korean Society of Fisheries Technology","volume":"108 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2010-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115077778","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 1
In situ side-aspect target strength of Japanese anchovy (Engraulis japonicus) in northwestern Pacific Ocean 西北太平洋日本凤尾鱼的原位侧向靶强度
Bulletin of The Korean Society of Fisheries Technology Pub Date : 2010-08-31 DOI: 10.3796/KSFT.2010.46.3.248
Hyungbeen Lee, D. Kang
{"title":"In situ side-aspect target strength of Japanese anchovy (Engraulis japonicus) in northwestern Pacific Ocean","authors":"Hyungbeen Lee, D. Kang","doi":"10.3796/KSFT.2010.46.3.248","DOIUrl":"https://doi.org/10.3796/KSFT.2010.46.3.248","url":null,"abstract":"Acoustic side-aspect target strength (TS) of living Japanese anchovy (Engraulis japonicus) was measured at 120kHz during in situ experiments. The data were collected by lowering and horizontally projecting the splitbeam transducer into the anchovy school. For analysis and interpretation of the side-aspect TS data, acoustic theoretical model, based on the fish morphology, and dorsal-aspect TS data were used. Total length of the anchovy ranged from 6.6 to 12.8cm (mean length 9.3cm). The side-aspect TS distributed between 40 and 55dB, has an obvious length dependency. The mean side-aspect TS of the anchovy was 47.8dB, and the TS was about 2dB higher than mean TS generated from dorsal-aspect measurements. With reference to maximum TS, the results of the side-aspect TS were distributed within the range of the theoretical and dorsal-aspect TS. Apparently these tendency indicates that side-aspect TS measured from the study is useful data. These in situ measurements of side-aspect TS can be applied to improve acoustic detection and estimates of the anchovy, and is necessary to measure with a various frequency and length for making enhance data.","PeriodicalId":211073,"journal":{"name":"Bulletin of The Korean Society of Fisheries Technology","volume":"58 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2010-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126548289","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 5
A study on the bycatches by mesh size of spring-net-pot in Geo-je & Tong-young waters of Korea 韩国巨济、东永水域弹簧网箱副渔获物的网目大小研究
Bulletin of The Korean Society of Fisheries Technology Pub Date : 2010-08-31 DOI: 10.3796/KSFT.2010.46.3.204
Bongjin Cha, Samkwang Cho, Gun-ho Lee
{"title":"A study on the bycatches by mesh size of spring-net-pot in Geo-je & Tong-young waters of Korea","authors":"Bongjin Cha, Samkwang Cho, Gun-ho Lee","doi":"10.3796/KSFT.2010.46.3.204","DOIUrl":"https://doi.org/10.3796/KSFT.2010.46.3.204","url":null,"abstract":"Test fishing was carried out using six kinds of different mesh sizes (20, 22, 24, 28, 35, 40mm) for spring-net-pot to study bycatches according to the mesh size and catches survey was done for another one (meshsize : 22mm, entrance round : 350mm) in Geo-je Tong-young waters of Korea. On the first seaexperiment, it was thought that suitable mesh size of spring-net-pot catching conger-eel over 35cm withdecreasing the catches of conger-eel (","PeriodicalId":211073,"journal":{"name":"Bulletin of The Korean Society of Fisheries Technology","volume":"23 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2010-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"123592791","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 5
首页 上一页 下一页 尾页
0
×
引用
GB/T 7714-2015
复制
MLA
复制
APA
复制
导出至
BibTeX EndNote RefMan NoteFirst NoteExpress
×
提示
您的信息不完整，为了账户安全，请先补充。
现在去补充
×
提示
您因"违规操作"
具体请查看互助需知
我知道了
×
提示
现在去查看 取消
×
提示
确定
请完成安全验证×
相关产品
×
本文献相关产品
联系我们：info@booksci.cn Book学术提供免费学术资源搜索服务，方便国内外学者检索中英文文献。致力于提供最便捷和优质的服务体验。 Copyright © 2023 布克学术 All rights reserved.
京ICP备2023020795号-1
ghs 京公网安备 11010802042870号
Book学术文献互助
Book学术文献互助群
群 号：481959085
Book学术官方微信