摘要?底拖網(wǎng)捕撈是最有效的捕撈作業(yè)方式之一,但一些學(xué)者和漁業(yè)管理人員已經(jīng)觀察到其對(duì)海底環(huán)境影響的嚴(yán)重性和持久性。聲納、視頻、遙感、漁船監(jiān)控系統(tǒng)等技術(shù)可以應(yīng)用于拖網(wǎng)對(duì)海底環(huán)境壓力評(píng)價(jià),聲納與視頻輔助可以從微觀角度評(píng)價(jià)拖網(wǎng)對(duì)海底沉積物影響,遙感影像可以用拖網(wǎng)軌跡寬度、長(zhǎng)度量化評(píng)價(jià),漁船監(jiān)控系統(tǒng)可以用累計(jì)距離、累計(jì)功率距離等參數(shù)量化評(píng)價(jià)。聲納與視頻適合小范圍,遙感適合大范圍評(píng)價(jià),但兩者在水體渾濁的情況下都不適用,漁船監(jiān)控系統(tǒng)時(shí)空分辨率都較高,可以很好地應(yīng)用于拖網(wǎng)對(duì)海底環(huán)境壓力評(píng)價(jià)。
關(guān)鍵詞?捕撈學(xué);底拖網(wǎng);環(huán)境壓力;漁船監(jiān)控系統(tǒng)
中圖分類號(hào)?X834;S975文獻(xiàn)標(biāo)識(shí)碼?A
文章編號(hào)?0517-6611(2019)20-0057-04
doi:10.3969/j.issn.0517-6611.2019.20.016
開(kāi)放科學(xué)(資源服務(wù))標(biāo)識(shí)碼(OSID):
Analysis of Environment Pressure Assessment Method for Bottom Trawling
ZHANG Sheng?mao
(Key Laboratory of East China Sea Fishery Resources Exploitation, Ministry of Agriculture, East China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Shanghai200090)
Abstract?Bottom trawling is one of the most effective fishing methods. But some scholars and fishery managers have observed the seriousness and persistence of its impact on the seabed environment. Sonar, video, remote sensing, fishing vessel monitoring system and other technologies can be used to evaluate the bottom environmental pressure of trawl. Sonar and video can be used to evaluate the impact of trawl on the bottom sediment from a microscopic perspective. Width and length of trawl trajectory can be quantitatively used to evaluate the affection by remote sensing images. Accumulated distance and accumulated power distance can be used to evaluate the affection by fishing vessel monitoring system. Sonar and video are suitable for small area. Remote sensing is suitable for large area evaluation. But both of them are not applicable in the case of turbid water. The space?time resolution of fishing vessel monitoring system is high, which can be well applied to the evaluation of bottom environmental pressure by trawl.
Key words?Fishing science;Bottom trawling;Environment pressure;Vessel monitoring system
20世紀(jì)90年代后,隨著國(guó)際漁業(yè)管理的加強(qiáng),漁業(yè)活動(dòng)監(jiān)控和管制已成為保護(hù)海洋漁業(yè)資源的必要組成部分[1]。國(guó)際上,漁船監(jiān)控管理技術(shù)以GPS(global positioning system)實(shí)時(shí)監(jiān)控為主,輔以SAR(synthetic aperture radar)遙感和光學(xué)遙感影像的漁船分布監(jiān)控,并結(jié)合具體應(yīng)用,構(gòu)建了許多實(shí)用的漁船監(jiān)控系統(tǒng)(vessel monitoring system,VMS)[2]。船位監(jiān)控系統(tǒng)最初是為了漁船管理和漁船安全[3-5],同時(shí)也為漁業(yè)科學(xué)研究提供了一種新的數(shù)據(jù)來(lái)源。1996年歐盟要求歐洲所有長(zhǎng)度大于24 m的漁船強(qiáng)制安裝VMS,到2012年,VMS的強(qiáng)制安裝范圍擴(kuò)大到了所有長(zhǎng)度大于12 m的漁船,美國(guó)、澳大利亞、新西蘭等漁業(yè)國(guó)家都相繼應(yīng)用VMS[6],許多發(fā)展中國(guó)家與地區(qū)也建立了類似的監(jiān)控系統(tǒng)。中國(guó)2006年開(kāi)始基于北斗衛(wèi)星導(dǎo)航系統(tǒng),在海南構(gòu)建北斗漁船船位監(jiān)控系統(tǒng)[7],我國(guó)近海漁船已安裝北斗終端約6萬(wàn)艘[8]。VMS數(shù)據(jù)記錄了漁船實(shí)時(shí)的船位、航速、航向、轉(zhuǎn)向率等動(dòng)態(tài)信息,已被廣泛應(yīng)用于漁船狀態(tài)識(shí)別、捕撈努力量估算、漁民行為分析、漁場(chǎng)判別、捕撈追溯等[9]。
VMS漁船監(jiān)控具有覆蓋廣、可靠性高、實(shí)時(shí)性強(qiáng)、可全天候工作等特點(diǎn),在強(qiáng)化漁業(yè)管理、保障漁船航行作業(yè)安全、履行相關(guān)國(guó)際義務(wù)等方面發(fā)揮了重要作用。基于衛(wèi)星遙感技術(shù)的漁船監(jiān)控能夠較準(zhǔn)確獲取漁船時(shí)空分布信息,雖不具有全球衛(wèi)星導(dǎo)航系統(tǒng)對(duì)漁船實(shí)時(shí)監(jiān)控的功能,但對(duì)于進(jìn)行事后的漁業(yè)捕撈生產(chǎn)評(píng)估、漁業(yè)資源管理、漁船生產(chǎn)效果評(píng)價(jià)等仍具有一定的科學(xué)價(jià)值和應(yīng)用意義。
1?拖網(wǎng)作業(yè)對(duì)海底環(huán)境的影響
拖網(wǎng)是目前海洋漁業(yè)生產(chǎn)中效益最高的漁具,也是對(duì)環(huán)境破壞較大的一種漁具,在全球各地均有使用。拖網(wǎng)的網(wǎng)板犁剎、沉綱刮割、網(wǎng)囊拖抹,引起環(huán)境破壞[10]。拖網(wǎng)會(huì)擾動(dòng)海底的底泥等物質(zhì),使沉積物懸浮造成水質(zhì)混濁、引起水體污染[11-12];拖網(wǎng)拖曳過(guò)程出現(xiàn)“犁地填溝”現(xiàn)象,海洋大陸斜坡上的峽谷皺褶被掃平,平滑的泥沙被犁出溝壑[13];有學(xué)者在西班牙沿海進(jìn)行調(diào)查時(shí)發(fā)現(xiàn),一些地方海底環(huán)境與自然狀態(tài)相比有很大差異,海底表面平滑的泥沙經(jīng)拖網(wǎng)拖曳產(chǎn)生許多溝壑。拖網(wǎng)長(zhǎng)期拖曳漁場(chǎng),不僅危及生物的生存和繁殖,而且嚴(yán)重破壞了海洋生境和生態(tài)平衡[14-16],不加強(qiáng)管理與保護(hù),海底將形成“海底沙漠”;拖網(wǎng)漁船所過(guò)之處的幾百米范圍內(nèi),也會(huì)對(duì)海底管線、電纜及其他水下結(jié)構(gòu)物[17]造成嚴(yán)重的威脅或損害。在撈走大量魚蝦的同時(shí),還把海洋生物賴以生存的海底家園“犁”了一遍,剩下的海洋生物會(huì)更難生存。美國(guó)地球資源衛(wèi)星對(duì)1999年墨西哥灣拍攝的遙感影像顯示,每個(gè)長(zhǎng)條沉積物痕跡末端的白點(diǎn)都是獨(dú)立的漁船,拖網(wǎng)捕撈使海底沉積物產(chǎn)生密集的劃痕[18],對(duì)底層魚類棲息地造成巨大破壞[19]。
鑒于海底拖網(wǎng)捕魚作業(yè)對(duì)生態(tài)系統(tǒng)的影響,許多國(guó)家和地區(qū)都對(duì)這一作業(yè)方式進(jìn)行了限制。如為了恢復(fù)海洋漁業(yè)資源量,維持海岸漁民的生活,美國(guó)海洋大氣局宣布減少拖網(wǎng)過(guò)度捕魚和保護(hù)必要的魚類生境。為了維護(hù)和合理利用沿海漁業(yè)資源,加強(qiáng)漁場(chǎng)管理,中共中央國(guó)務(wù)院早在1955年就劃定了渤海、黃海及東海機(jī)輪拖網(wǎng)漁業(yè)禁漁區(qū)。
2?拖網(wǎng)海底環(huán)境影響評(píng)價(jià)
2.1?聲納與視頻輔助拖網(wǎng)海底影響評(píng)價(jià)
局部、小范圍的拖網(wǎng)海底量化影響可以通過(guò)側(cè)掃聲納和水下視頻評(píng)估[20],側(cè)掃聲納能在2~3 knot速度下,獲取幅寬200 m、分辨率20 cm的數(shù)據(jù),水下視頻能在1 knot速度下,獲取 1~2 m幅寬、分辨率 1~2 cm數(shù)據(jù),使用該方法在伊拉克利翁灣區(qū)200 m水深分析拖網(wǎng)對(duì)粉質(zhì)黏土沉積物的影響。在英國(guó)東部海域約28 km×12 km面積,使用側(cè)掃聲納與傳統(tǒng)生物抽樣方法相結(jié)合,映射拖網(wǎng)作業(yè)范圍內(nèi)海底棲息地和相關(guān)的底棲生物群落的分布,每個(gè)聲區(qū)內(nèi)基底的分布一般是均勻的,沉積物類型從卵石、粗礫石到泥沙變化[21]。
2.2?遙感數(shù)據(jù)輔助拖網(wǎng)海底影響評(píng)價(jià)
資源衛(wèi)星可以觀測(cè)到拖網(wǎng)作業(yè)產(chǎn)生的軌跡[22],從而推算出軌跡寬度、長(zhǎng)度等信息,多個(gè)國(guó)家都在使用拖網(wǎng)作業(yè)(圖1),如瓜亞斯厄瓜多爾(圖1A)、美國(guó)路易斯安那州(圖1B、C)、菲律賓呂宋島(圖1D)、墨西哥索諾拉、馬來(lái)西亞等,借助遙感圖像可以揭示拖網(wǎng)對(duì)海底沉積物的影響規(guī)模。
2.3?VMS輔助拖網(wǎng)海底影響評(píng)價(jià)
利用VMS數(shù)據(jù)評(píng)估拖網(wǎng)漁業(yè)活動(dòng)對(duì)海洋環(huán)境影響的研究較多,如通過(guò)對(duì)VMS數(shù)據(jù)進(jìn)行挖掘,估算英國(guó)馬恩島扇貝底拖網(wǎng)船捕撈強(qiáng)度的分布狀況,量化分析了拖網(wǎng)捕撈對(duì)海洋底棲環(huán)境的影響[23]。網(wǎng)格化計(jì)算愛(ài)爾蘭凱爾特海底拖網(wǎng)漁船的捕撈強(qiáng)度,探討高時(shí)空分辨下底拖網(wǎng)捕撈對(duì)海洋環(huán)境的影響[24]。計(jì)算愛(ài)爾蘭坎布里亞海岸挪威拖網(wǎng)漁船的拖網(wǎng)次數(shù)和拖拽范圍,并與實(shí)地海底采樣相結(jié)合,分析了底拖網(wǎng)對(duì)海洋底棲環(huán)境、資源豐度以及生物多樣性的深刻影響[25]。在英格蘭和威爾士海借助VMS數(shù)據(jù),分析出2007年拖網(wǎng)海底影響強(qiáng)度為0.000 2~30(即1 km2的格網(wǎng)內(nèi)每年可能已被捕撈到30次)[26]。國(guó)外大部分VMS數(shù)據(jù)時(shí)間間隔0.5~2 h,通過(guò)線性插值方法重構(gòu)漁船軌跡[27-28],提高分析精度。
3?基于北斗船載終端的漁船作業(yè)監(jiān)測(cè)
中國(guó)東黃海拖網(wǎng)捕撈漁船最多,主要分布在專屬經(jīng)濟(jì)區(qū)內(nèi)(圖2),黃海是一個(gè)近似南北向的半封閉淺海,西北以遼東半島南端老鐵山角與山東半島北岸蓬萊角連線為界,南以中國(guó)長(zhǎng)江口北岸啟東嘴與濟(jì)州島西南角連線為界,與東海相連,平均深度44 m[29-30];東海北與黃海相連,南以廣東南澳島與臺(tái)灣島南端的鵝鑾鼻連線為界,與南海相連,平均深度370 m[30-31]。黃海海底輻射沙脊群規(guī)模龐大、形狀奇特,構(gòu)成了一種獨(dú)特的地貌形態(tài)[32]。東海陸坡廣泛發(fā)育海底峽谷—扇體系的沉積地層結(jié)構(gòu)[33],東海中南部海底分布著廣泛的脊槽狀地形,被冰后期陸架席狀砂所覆蓋[34],海底沉積物分布有中砂、細(xì)砂、粉砂、粉土、淤泥質(zhì)黏土、淤泥6種巖性,均屬全新世淺海相松散沉積物,以粉砂為主,表層土屬現(xiàn)代沉積,抗剪強(qiáng)度小[35],在受到拖網(wǎng)拖曳網(wǎng)具擾動(dòng)時(shí),極易改變海底環(huán)境。
據(jù)《中國(guó)漁業(yè)統(tǒng)計(jì)年鑒》,2016年國(guó)內(nèi)海洋捕撈拖網(wǎng)漁船34 141艘,總計(jì)功率65.9×104 kW,雖然拖網(wǎng)漁船數(shù)量只占19%,但總計(jì)功率占55%[36],其中60%以上的拖網(wǎng)船分布在東海和黃海。拖網(wǎng)漁船噸位、功率都比較大,船上硬件基礎(chǔ)好,因此安裝北斗終端的漁船80%以上是拖網(wǎng)漁船。北斗衛(wèi)星導(dǎo)航系統(tǒng)2012年12月就完成了亞太區(qū)域組網(wǎng),系統(tǒng)定位精度水平10 m、高程10 m、測(cè)速精度0.2 m/s,很好地覆蓋了東黃海。中國(guó)東黃海拖網(wǎng)作業(yè)漁船多,對(duì)于砂質(zhì)沉積的海底環(huán)境影響大,北斗VMS在中國(guó)應(yīng)用時(shí)間短[37],基于船位的海底環(huán)境影響研究鮮見(jiàn),北斗船位數(shù)據(jù)時(shí)間分辨率3 min,拖網(wǎng)捕撈狀態(tài)一般持續(xù)幾個(gè)小時(shí)[38],按照奈奎斯特采樣定理(一般實(shí)際應(yīng)用中保證采樣頻率為信號(hào)最高頻率的5~10倍)可以滿足信息提取的需要。北斗船位數(shù)據(jù)與國(guó)外數(shù)據(jù)相比時(shí)間分辨率更高,如利用北斗數(shù)據(jù)在沒(méi)有插值處理的情況下,提取到象山漁船在近海拖網(wǎng)累計(jì)捕撈時(shí)間與捕撈努力量(kW·h)[39]。張勝茂等[40]利用象山港1 403只拖網(wǎng)漁船VMS數(shù)據(jù)對(duì)2013年的拖網(wǎng)作業(yè)進(jìn)行分析,采用累計(jì)距離、累計(jì)功率距離和拖網(wǎng)強(qiáng)度作為反映黃海和東海拖網(wǎng)作業(yè)影響的評(píng)價(jià)指標(biāo),這3個(gè)指標(biāo)在黃海和東海中具有相似的模式,表明靠近港口的漁場(chǎng)具有較高的捕撈努力量,整個(gè)漁區(qū)海底平均被拖網(wǎng)0.73次,51.38%的漁場(chǎng)沒(méi)有捕撈活動(dòng)。
4?結(jié)語(yǔ)
聲納與視頻輔助評(píng)價(jià)方式適合小區(qū)域的量化分析,重復(fù)調(diào)查的時(shí)間周期長(zhǎng),在水體較渾濁的情況下視頻方式不能使用。遙感數(shù)據(jù)輔助評(píng)價(jià)方式與聲納視頻輔助評(píng)價(jià)方式相比適用范圍較大,但在水體渾濁或水較深的情況下不能獲取到海底影像,在有云的情況下也不能獲取到影像,數(shù)據(jù)獲取周期一般2 d以上。VMS輔助評(píng)價(jià)方式,時(shí)間頻率與VMS采樣頻率有關(guān),空間頻率與定位精度有關(guān),北斗船位數(shù)據(jù)時(shí)間分辨率3 min,空間分辨率10 m,因此北斗船位數(shù)據(jù)輔助拖網(wǎng)海底影響量化評(píng)價(jià),理論上也可以達(dá)到較高的分辨率。
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