張云龍 賀亞蒙 袁 娟 張子涵 丁淑荃 楊啟超 張志強(qiáng)鮑傳和 萬 全 沈志剛
(1. 安徽農(nóng)業(yè)大學(xué)動物科技學(xué)院, 合肥 230036; 2. 肥東縣水產(chǎn)技術(shù)推廣站, 肥東 231600;3. 華中農(nóng)業(yè)大學(xué)水產(chǎn)學(xué)院, 武漢 430070)
在水產(chǎn)養(yǎng)殖中, 活體運(yùn)輸是魚類經(jīng)常需要面臨的應(yīng)激之一[1]。魚類運(yùn)輸具有重要的意義, 主要應(yīng)用于食用魚銷售[2,3]; 苗種轉(zhuǎn)運(yùn)至成魚養(yǎng)殖設(shè)施[4];野生魚類人工馴養(yǎng)[2,5]以及運(yùn)輸至研究機(jī)構(gòu)中[6]等。出于不同的目的, 養(yǎng)殖魚類在生活史的不同階段均可能面臨應(yīng)激脅迫[7], 然而, 應(yīng)激會導(dǎo)致魚類氧化應(yīng)激、細(xì)胞凋亡和免疫力下降[8—10], 進(jìn)而導(dǎo)致病原感染和死亡。因此, 制定標(biāo)準(zhǔn)的魚類運(yùn)輸操作規(guī)程以最小化應(yīng)激反應(yīng)對水產(chǎn)養(yǎng)殖業(yè)是至關(guān)重要的,可降低魚類運(yùn)輸后的死亡率、提高經(jīng)濟(jì)效益。相關(guān)的科學(xué)研究也是以此為主要目的而進(jìn)行的。本文綜述了已有的相關(guān)資料, 旨在整合相關(guān)的文獻(xiàn)信息, 分析魚類運(yùn)輸過程中生理指標(biāo)及水質(zhì)的變化,并為魚類運(yùn)輸操作標(biāo)準(zhǔn)規(guī)程的制定提供理論依據(jù)。
魚類的運(yùn)輸方式各不相同, 最主要的表現(xiàn)為運(yùn)輸時使用的容器, 其主要取決于魚的種類、個體規(guī)格以及生態(tài)習(xí)性等。幼魚以及個體規(guī)格較小的魚類常用的運(yùn)輸容器為塑料袋, 如黑點(diǎn)無須■(Puntius filamentosus)[11]、翹嘴鲌(Culter alburnus)幼魚[12,13]、斑點(diǎn)叉尾鲙(Ictalurus punctatus)幼魚[14]等, 而一些規(guī)格較大的魚則使用盒、箱子或桶等較大的容器, 如大西洋鱈(Gadus morhua)[15]、庫魯魯■(Potamotrygon cf. histrix)[16]、刀鱭(Coilia nasus)[4]、巴鰹(Euthynnus affinis)[17]等, 甚至有使用網(wǎng)箱直接在海水中運(yùn)輸規(guī)格特別大的魚類, 如藍(lán)鰭金槍魚(Thunnus thynnus)[3]。由此可見, 運(yùn)輸會將魚類限制在一個小環(huán)境水體中, 因而水質(zhì)對運(yùn)輸?shù)某苫盥?、效率以及魚類福利起到了至關(guān)重要的作用。對黃尾■(Seriola lalandi)[18]和大西洋鮭(Salmo salar)[19]的研究中發(fā)現(xiàn), 運(yùn)輸會造成魚類代謝速率明顯加快, 繼而引起耗氧率增加和代謝廢物排泄量的(CO2和氨氮)增加, CO2溶于水中又會導(dǎo)致水體pH降低。CO2作為一種代謝產(chǎn)物, 對魚類具有很大的生理副作用。CO2在血漿中富集會導(dǎo)致血漿酸化, 降低血液的攜氧能力和血紅蛋白對氧氣的親和力[20]。運(yùn)輸過程中水質(zhì)的惡化很大程度上受到運(yùn)輸密度、魚的規(guī)格以及運(yùn)輸時間的影響。表 1中列出了一些魚類運(yùn)輸過程對運(yùn)輸容器中水質(zhì)的影響狀況, 可見運(yùn)輸對容器中水質(zhì)具有顯著的影響, 常用評價指標(biāo)有溶解氧、總氨氮及pH。
由于運(yùn)輸過程中水質(zhì)的惡化會影響運(yùn)輸魚類的成活率, 如何維持運(yùn)輸過程中的水質(zhì)是急需解決的問題, 具有十分重要的應(yīng)用價值。溶解氧是相對較為容易控制的水質(zhì)指標(biāo)之一, 開放容器中常用持續(xù)曝氣的方法維持水體溶解氧[21], 密閉容器(如氧氣袋)常用充純氧來維持[12,13]。水體pH的降低則可通過添加緩沖劑來維持[22]。而魚類運(yùn)輸過程中水體氨氮和CO2則并不是那么容易控制的, 且氨氮對魚類具有較強(qiáng)的毒理作用[23,24], CO2對魚類也具有明顯的副作用[20]。運(yùn)輸過程中如果溶氧充足, 魚類會進(jìn)行有氧代謝消耗機(jī)體儲存的糖原, 產(chǎn)生大量的CO2, 致使周圍水體和血漿酸化; 如果溶氧不足, 魚類則會進(jìn)行無氧代謝, 產(chǎn)生大量的乳酸, 致使機(jī)體酸中毒。待機(jī)體儲存糖原耗盡后, 魚類則會代謝機(jī)體蛋白或氨基酸以維持能量供應(yīng), 這一過程則會產(chǎn)生氨。就此來說, CO2的大量釋放以及水體的酸化在短途運(yùn)輸中會經(jīng)常出現(xiàn), 而氨氮的大量排泄則常出現(xiàn)在長途運(yùn)輸中。CO2會在魚類運(yùn)輸開始后的24h內(nèi)迅速上升, 隨著運(yùn)輸時間的增加, CO2的排泄速率則明顯下降, 而氨排泄速率則明顯上升[25]。由此可見, CO2排泄速率這一指標(biāo)在魚類運(yùn)輸中具有非常重要的意義, 然而這一指標(biāo)在現(xiàn)有研究中的應(yīng)用卻并不多[22]。此外, 隨著運(yùn)輸時間的增加, 氨排泄速率會明顯上升, 說明了長途運(yùn)輸中容器內(nèi)水質(zhì)調(diào)控的難度更大[26]。
運(yùn)輸會對魚類造成應(yīng)激反應(yīng), 首先表現(xiàn)在生理指標(biāo)的變化上, 造成魚體外與體內(nèi)穩(wěn)態(tài)的變化[22]。魚類對應(yīng)激的生理反應(yīng)可分為3個階段, 第一階段是神經(jīng)系統(tǒng)接受外界應(yīng)激信號并迅速分泌腎上腺素, 刺激魚類頭腎組織釋放皮質(zhì)醇[22], 隨后皮質(zhì)醇會調(diào)動體內(nèi)儲備的能量以提供調(diào)節(jié)滲透壓所需的氧氣和能量[38,39], 因此血漿皮質(zhì)醇水平也成為了評價魚類應(yīng)激第一階段最重要的指標(biāo)之一。第二階段主要涉及生理生化方面的代謝反應(yīng), 包括血漿和組織中代謝產(chǎn)物、血液學(xué)指標(biāo)的變化以及應(yīng)激蛋白(如熱休克蛋白)的激活等[37,40—42], 研究魚類應(yīng)激第二階段最常用的指標(biāo)則是血糖含量。第三階段主要涉及應(yīng)激對魚類造成的生長、繁殖、抗病力、代謝能力以及存活的影響[43,44], 然而有關(guān)魚類運(yùn)輸應(yīng)激第三階段的研究卻相當(dāng)缺乏。表 2總結(jié)了運(yùn)輸應(yīng)激對魚類血漿中生理指標(biāo)的影響。從表中可看出, 現(xiàn)有的研究主要針對于淡水魚類的短途運(yùn)輸(運(yùn)輸時間<8h), 而有關(guān)海水魚類以及長途運(yùn)輸(運(yùn)輸時間>8h)的報道則相對較少, 而且評價指標(biāo)也以血漿皮質(zhì)醇和血糖含量為主、缺乏應(yīng)激第三階段的研究。
表 1 魚類運(yùn)輸后水體中溶氧、pH及總氨氮的增加倍數(shù)Tab. 1 Fold changes in dissolved oxygen, pH and total ammonia nitrogen after live fish transport
從表 2可看出, 血漿皮質(zhì)醇是最經(jīng)典、最常用的魚類應(yīng)激評價指標(biāo)之一, 由魚類頭腎組織分泌,受下丘腦-垂體-腎間軸調(diào)控[58]。魚類在應(yīng)激的初期階段會迅速分泌皮質(zhì)類固醇和兒茶酚胺, 尤其是腎上腺素[59], 但腎上腺素增加的持續(xù)時間卻遠(yuǎn)比皮質(zhì)醇增加的時間短, 因此血漿皮質(zhì)醇被廣泛用于魚類應(yīng)激的評價中。皮質(zhì)醇的釋放會對魚類造成多種不利的生理影響, 如降低免疫力、刺激蛋白分解及引起糖異生反應(yīng)等(圖 1)[22,60,61]。然而, 運(yùn)輸應(yīng)激會誘導(dǎo)魚類血漿皮質(zhì)醇含量顯著增加, 如細(xì)鱗肥脂鯉(Piaractus mesopotamicus)[55]、斑點(diǎn)叉尾鲙[14]、大口黑鱸(Micropterus salmoides)[51]及翹嘴鲌[12]等(表2)。血漿皮質(zhì)醇對外界應(yīng)激因子的反應(yīng)是非常迅速的, 一般10min之內(nèi)即可發(fā)現(xiàn)顯著的上升, 反應(yīng)較慢的也在2—24h內(nèi)迅速釋放, 而且一旦外界應(yīng)激因子消失, 血漿皮質(zhì)醇會在24h內(nèi)恢復(fù)到基礎(chǔ)水平[22]。魚體內(nèi)皮質(zhì)醇的消除主要在肝臟中進(jìn)行[62], 皮質(zhì)醇的消除速度可能與環(huán)境因子相關(guān), 但應(yīng)激、鹽度、性成熟度、營養(yǎng)狀況等因子會抑制魚體內(nèi)皮質(zhì)醇的消除[63]。然而, 魚類處于運(yùn)輸脅迫時, 其血漿皮質(zhì)醇會迅速上升并持續(xù)相當(dāng)長的時間(表 2), 甚至在整個運(yùn)輸過程中均保持非常高的水平[64]。從這個角度來看, 血漿皮質(zhì)醇指標(biāo)適用于魚類短途(<8h)和長途(>8h)運(yùn)輸應(yīng)激的評價。值得一提的是, 一些仔魚及小型觀賞魚的血液采集是非常困難的, 通常的解決辦法是通過合并血液樣本或者測定全魚皮質(zhì)醇水平[12,29,49,65]。此外, 魚類在運(yùn)輸之前通常需要手工搬運(yùn), 這也會對魚類造成應(yīng)激反應(yīng), 此時盡管還未開始運(yùn)輸, 其血液皮質(zhì)醇水平仍然不能作為正常的基礎(chǔ)水平。為了解決這些問題, 一些報道提供了許多措施, 主要可分為2大類, 一類是使用麻醉劑, 如間氨基苯甲酸乙酯甲磺酸鹽(Tricaine methanesulphonate, MS-222)[13,28,51]、植物精油[56,66]、丁香提取物[67]等; 另一類是使用非應(yīng)激手段(Non invasive methods), 其目的在于去除或最小化取樣造成的應(yīng)激脅迫, 如檢測魚類釋放到水體中的皮質(zhì)醇[15,68]及檢測糞便中的皮質(zhì)醇[69]等。然而, 魚體內(nèi)皮質(zhì)醇的排出主要通過鰓組織[70], 因而檢測糞便中的皮質(zhì)醇含量的準(zhǔn)確度并不夠??傮w來看, 皮質(zhì)醇作為評價魚類運(yùn)輸應(yīng)激指標(biāo)適用于短途和長途運(yùn)輸, 但其檢測方法仍需進(jìn)行改善, 檢測魚類釋放至水體中的皮質(zhì)醇是非常有效的非應(yīng)激手段之一, 對采血不便的小型魚類同樣適用且可消除取樣引起的應(yīng)激反應(yīng)。
魚類在應(yīng)激第一階段會釋放大量皮質(zhì)醇, 誘導(dǎo)機(jī)體糖原分解和糖異生作用以應(yīng)對應(yīng)激脅迫(圖 1),血糖含量及糖原分解的增加被認(rèn)為是應(yīng)激的第二階段。肝臟和肌肉糖原含量作為應(yīng)激評價指標(biāo)的原理與血糖基本是一致的, 近年的報道也較多[52,56,65,71]。在通常情況下, 運(yùn)輸應(yīng)激會導(dǎo)致魚類血糖含量上升、糖原含量降低(表 2), 其原因主要是魚類在應(yīng)激狀態(tài)下會消耗更多的能量。魚類應(yīng)激狀態(tài)下血糖含量也受到多種因素的影響, 包括種類、生長階段、營養(yǎng)狀況等, 如營養(yǎng)狀況會影響點(diǎn)帶石斑魚(Epinephelus coioides)寒冷脅迫下血糖的含量[72]。皮質(zhì)醇的釋放已被證實會刺激魚類血糖含量的上升, 但塞內(nèi)加爾鰨(Solea senegalensis)在中密度擁擠脅迫下血糖含量顯著高于低密度組, 盡管其血漿皮質(zhì)醇含量與低密度組無差異[73]。這種增加顯然不是皮質(zhì)醇誘導(dǎo)的, 可能是由于兒茶酚胺的釋放引起的(圖 1)[74,75]。因此, 要確認(rèn)魚類在運(yùn)輸應(yīng)激下血糖增加是否由皮質(zhì)醇誘導(dǎo), 研究魚類皮質(zhì)醇的基礎(chǔ)水平是非常重要的。從表 2中可看出, 隨著運(yùn)輸時間的延長, 魚類血糖含量呈現(xiàn)下降的趨勢, 說明魚體內(nèi)貯存能量的耗盡。也就是說血糖作為魚類長途運(yùn)輸應(yīng)激評價指標(biāo)的適用性并不如短途運(yùn)輸, 而且長途運(yùn)輸時血糖測定的間隔時間也應(yīng)該更短, 有助于更直觀的評價魚類在應(yīng)激時的生理應(yīng)答。
乳酸是魚體肌肉無氧代謝產(chǎn)物, 魚類在應(yīng)激條件下肌肉無氧代謝會顯著增加, 導(dǎo)致血漿乳酸濃度升高, 如圓口銅魚(Coreius guichenoti)[28]、大黃魚(Larimichthys crocea)[65]、長吻裸頰鯛(Lethrinus miniatus)[76]等。魚類運(yùn)輸后血漿乳酸濃度的升高說明了運(yùn)輸后機(jī)體供氧的不足, 如何保證運(yùn)輸期間充足的機(jī)體供氧是急需解決的關(guān)鍵問題之一。溫度對魚類應(yīng)激條件下血漿乳酸含量可能起到某些特殊的作用[77,78], 溫度對魚類血漿乳酸含量的影響也是后續(xù)研究需要關(guān)注的方向之一。乳酸作為一種無氧代謝產(chǎn)物, 對機(jī)體是有危害的, 但對斑馬魚(Danio rerio)的研究發(fā)現(xiàn)乳酸在寒冷脅迫下可作為能量以維持機(jī)體生理穩(wěn)態(tài)[79]。后續(xù)的研究可以關(guān)注乳酸在魚類運(yùn)輸應(yīng)激下是否具有類似的作用及其調(diào)節(jié)機(jī)制。
表 2 運(yùn)輸后魚類血漿中皮質(zhì)醇、葡萄糖及乳酸含量的增加倍數(shù)Tab. 2 Fold changes in plasma cortisol, glucose and lactate after transport
由于外界環(huán)境的可變性, 魚類血漿的滲透壓也是可調(diào)節(jié)的, 但相對是較為穩(wěn)定的。淡水魚類的血漿滲透壓為230—280 mOsmol/kg, 海水魚類為300—350 mOsmol/kg, 兩者相差并不大[22]。血漿滲透壓作為魚類運(yùn)輸應(yīng)激評價指標(biāo)的報道并不多, 可見維多利亞野鯪(Labeo victorianus)[5]、圓口銅魚[28]、刀鱭[4]及庫魯魯■[16]等。但其是反映魚類自身內(nèi)穩(wěn)態(tài)的重要指標(biāo)之一, 而且很大程度上受到魚體內(nèi)皮質(zhì)醇的影響[80], 因此將血漿滲透壓作為魚類運(yùn)輸應(yīng)激的評價指標(biāo)是非常必要的。除此之外, 也有研究將血漿無機(jī)鹽離子作為魚類運(yùn)輸應(yīng)激的評價指標(biāo)之一, 報道較多的是血漿Na+離子、Cl-離子和K+離子, 因為這些離子對魚類滲透壓影響較大, 如維多利亞野鯪[5]、庫魯魯■[16]、三帶鉛筆魚(Nannostomus trifasciatus)[31]、細(xì)鱗肥脂鯉[55]、小鋸蓋魚(Centropomus parallelus)[81]、克林雷氏鯰(Rhamdia quelen)[36]等。
血液學(xué)指標(biāo)也被用于評價魚類運(yùn)輸應(yīng)激[27,37,41],但其對應(yīng)激的響應(yīng)時間較長, 用于評價魚類長途運(yùn)輸應(yīng)激和魚類應(yīng)激第二階段可能更加合適。近年來, 隨著研究手段的進(jìn)步, 應(yīng)用于評價魚類運(yùn)輸應(yīng)激的生理指標(biāo)也越來越多, 如肌肉品質(zhì)[14,82]、核苷酸含量[21]、性激素[57]、對氧磷酶活性[33,66]、血氨濃度[5]、耗氧率[12]、基因表達(dá)水平[9,83,84]、轉(zhuǎn)錄組學(xué)[49]等。這些指標(biāo)在特定的研究條件和研究目的下, 可作為經(jīng)典應(yīng)激評價指標(biāo)(皮質(zhì)醇和血糖)的補(bǔ)充資料, 對真實反映特定狀態(tài)下魚類運(yùn)輸后的應(yīng)激生理具有重要意義。
魚類運(yùn)輸在生產(chǎn)中應(yīng)用十分廣泛, 如何在運(yùn)輸過程中維持水質(zhì)指標(biāo)、盡可能的降低應(yīng)激反應(yīng)是相關(guān)研究重點(diǎn)關(guān)注的方向之一, 對提高運(yùn)輸成活率起到關(guān)鍵的作用, 常用的措施主要有i)增加水體溶解氧, ii)調(diào)整水體鹽度, iii)添加緩沖劑, iv)氨處理措施, v)降低水溫, vi)麻醉運(yùn)輸, vii)添加益生菌,viii)合理的運(yùn)輸密度。
溶解氧是魚類運(yùn)輸過程中的重要限制因子之一, 理想的運(yùn)輸水體中溶解氧飽和度應(yīng)為100%。氧氣在水體中的溶解度受水溫、氣體組成、鹽度、氣壓等因素的影響[25]。充氧是魚類運(yùn)輸過程中最常用的措施之一, 常用的設(shè)備有氧氣罐、增氧機(jī)及液氧等, 有關(guān)這幾種充氧方式的優(yōu)劣點(diǎn)可見Harmon的綜述[25]。運(yùn)輸開始的0.5—1h內(nèi)魚類活動會異常激烈, 對氧氣的消耗也非常大, 在此期間必須保證足夠的水體溶解氧。然而, 水體中過飽和的溶解氧也存在一定的危害性, 如過高的溶解氧會以氣泡的形式存在并導(dǎo)致魚類的氣泡病[85], 而且過飽和的溶解氧是否會對魚類造成氧化應(yīng)激反應(yīng)也是需要關(guān)注的問題之一。
魚類處于運(yùn)輸應(yīng)激狀態(tài)下, 由于腎上腺素的釋放會導(dǎo)致魚類鰓上皮的水通透性改變。在這種情況下會引起魚類滲透壓內(nèi)穩(wěn)態(tài)失調(diào)[86], 進(jìn)而引起運(yùn)輸魚類的死亡。淡水魚類為高滲型, 需要抑制外源水的進(jìn)入和體內(nèi)無機(jī)鹽離子的流失, 因此在淡水魚類運(yùn)輸中通常選擇加入適量的鹽(如NaCl、NaHCO3和海水)以形成體內(nèi)外等滲的環(huán)境, 有助于降低淡水魚類運(yùn)輸過程中的應(yīng)激反應(yīng), 對維多利亞野鯪[5]、刀鱭[4]、虹鱒[84]等淡水魚類的研究結(jié)果均證實了這一結(jié)論。但在巨骨舌魚(Arapaima gigas)幼魚運(yùn)輸過程中加入NaCl不僅不會降低其運(yùn)輸應(yīng)激反應(yīng), 還會導(dǎo)致其滲透壓調(diào)節(jié)紊亂[45], 說明適量添加鹽可降低淡水魚類運(yùn)輸應(yīng)激似乎是因種而異的, 針對不同種類的研究仍然是必要的。有關(guān)水體鹽度對海水魚類運(yùn)輸應(yīng)激的研究并不多, 僅見軍曹魚(Rachycentron canadum)[7]和大黃魚[65], 后續(xù)研究應(yīng)關(guān)注這一問題。
運(yùn)輸會造成魚類代謝速率明顯加快, 繼而排泄大量的CO2溶于水中, 導(dǎo)致水體pH降低。由于運(yùn)輸用水體積有限, 添加緩沖劑可有效的緩解水體酸化。研究表明在運(yùn)輸容器中加入小蘇打[87]、三甲醇氨基甲烷(Tris)[88]以及氫氧化鎂微粒[15]可有效緩解運(yùn)輸導(dǎo)致的水體pH降低。由于運(yùn)輸過程中水體中氨的含量會顯著增加(表 2), 而氨的存在形式很大程度上又取決于水體pH[89]。在偏堿性環(huán)境中氨主要以分子態(tài)(NH3)形式存在, 對水生動物毒性較大; 在偏酸性環(huán)境中主要以離子態(tài)()形式存在,毒性較小。因此, 在使用緩沖劑時還應(yīng)考慮到水體中氨氮的含量, 以免造成氨中毒。
氨是水體中常見的環(huán)境毒素之一, 可造成魚類鰓結(jié)構(gòu)異化[90]、腦組織損傷[91]、氧化應(yīng)激[92]等毒理作用。因此, 運(yùn)輸導(dǎo)致的水體氨增加也是急需處理的問題之一。甲醛亞硫酸氫鈉[87]及沸石[88]等氨還原劑均被證實可有效降低運(yùn)輸導(dǎo)致的水體氨累積??紤]到水體pH對氨毒性的影響, 氨還原劑和pH緩沖劑的同時使用更利于魚類運(yùn)輸過程中水質(zhì)的調(diào)控。
對于魚類這種變溫動物來說, 降低水溫可降低其消化酶活性、降低新陳代謝速率[93], 也就降低了魚類的耗氧率、CO2和氨的排泄率, 還可增加氧氣在水體中的溶解率[25]。在對圓口銅魚的研究中發(fā)現(xiàn)降低水溫可降低運(yùn)輸應(yīng)激并提高水質(zhì)[28]。但水溫降低需要精細(xì)的操作規(guī)程, 并不能驟降, 而且各種魚類對溫度的適應(yīng)性也各不相同, 基礎(chǔ)資料的累積和規(guī)范化操作規(guī)程的制定是必要的。
麻醉是魚類運(yùn)輸中常用的降低應(yīng)激措施之一,麻醉后魚類的代謝速率、耗氧率、CO2和氨排泄率均有明顯的降低趨勢。MS-222[11,13,28,51]、丁香酚[35,94,95]、苯佐卡因[11,96]、喹哪啶[96]及2-苯氧乙醇[88]等麻醉劑已廣泛應(yīng)用于魚類運(yùn)輸中, 并取得了良好的抗應(yīng)激效果。某些植物提取的精油同樣對魚類具有一定的麻醉效果, 也可用于魚類運(yùn)輸抗應(yīng)激, 如丁香屬[67]、過江藤屬[33,35,66]、阿萊藤屬[36,56,97]以及羅勒屬[97]等屬的植物。然而, 對小鋸蓋魚的研究中發(fā)現(xiàn)植物精油盡管可以起到麻醉的作用, 但并不能降低其運(yùn)輸過程中的應(yīng)激反應(yīng)[81]??梢娐樽碓隰~類運(yùn)輸中的作用并不都是積極的, 針對特定種類的研究仍然是必要的。此外, 麻醉劑對魚類也是具有一定的生理副作用的, 如常用的MS-222可造成魚類血液學(xué)指標(biāo)的變化[98]。因此, 在魚類運(yùn)輸過程中使用麻醉劑是需要充分的科研理論作為支撐的, 相關(guān)的理論研究也需要加強(qiáng)。
近年來, 一些益生菌和免疫多糖類被應(yīng)用于水產(chǎn)養(yǎng)殖中, 用于提高養(yǎng)殖魚類的生長速度和免疫機(jī)能[99,100]。也有將益生菌和免疫多糖應(yīng)用于魚類運(yùn)輸中以維持機(jī)體內(nèi)穩(wěn)態(tài)、提高運(yùn)輸成活率, 如益生菌Efinol?L可降低阿氏霓虹脂鯉(Paracheirodon axelrodi)運(yùn)輸過程中代謝廢物的排放量, 從而維持水質(zhì)、提高運(yùn)輸成活率[53]; 飼料中添加β-1, 3葡聚糖投喂三帶鉛筆魚7d可維持其運(yùn)輸過程中Na+和K+內(nèi)平衡和水質(zhì)指標(biāo)[31]。然而, 添加益生菌對庫魯魯魟等[16]運(yùn)輸并沒有明顯的有益作用。因此, 特定益生菌和免疫多糖對特定魚類運(yùn)輸過程中的作用仍需要持續(xù)研究。
魚類運(yùn)輸過程中的裝載密度受到水質(zhì)、運(yùn)輸時間、水溫、規(guī)格和魚的種類等因素影響[25], 運(yùn)輸密度反之又會影響魚類運(yùn)輸過程中的應(yīng)激反應(yīng)和水質(zhì)[12,34]。過高的運(yùn)輸密度會導(dǎo)致應(yīng)激更強(qiáng)、代謝廢物排放更多、水質(zhì)惡化更快, 運(yùn)輸成活率顯著降低; 過低的運(yùn)輸密度則意味著更高的運(yùn)輸成本。合理的運(yùn)輸密度是魚類運(yùn)輸操作規(guī)程中重要的組成部分之一, 隨著運(yùn)輸時間的延長, 運(yùn)輸密度應(yīng)相應(yīng)降低。
運(yùn)輸在水產(chǎn)養(yǎng)殖業(yè)中是非常重要的, 魚類標(biāo)準(zhǔn)化運(yùn)輸操作規(guī)程的制定有利于水產(chǎn)養(yǎng)殖業(yè)的健康發(fā)展。運(yùn)輸會對魚類造成應(yīng)激反應(yīng), 引起生理指標(biāo)和水質(zhì)的變化。盡管近年來魚類運(yùn)輸越來越受到關(guān)注, 相關(guān)研究報道也逐年增加, 但與世界上種類繁多的經(jīng)濟(jì)魚類和觀賞魚類相比, 這些研究的量仍明顯不足。水質(zhì)惡化(尤其是pH降低和氨累積)是魚類運(yùn)輸中急需解決的關(guān)鍵問題, 現(xiàn)有的解決措施主要是通過添加緩沖劑、氨還原劑以及降低運(yùn)輸魚類代謝來實現(xiàn)水質(zhì)調(diào)控, 然而這些解決措施尚存在一定的不穩(wěn)定性。運(yùn)輸引起的魚類應(yīng)激反應(yīng), 常用的評價指標(biāo)有皮質(zhì)醇和血糖。皮質(zhì)醇仍然是經(jīng)典的應(yīng)激評價指標(biāo), 適用于魚類短途和長途運(yùn)輸,而血糖則更加適用于評價短途運(yùn)輸應(yīng)激。某些特殊的生理指標(biāo)在特定的研究條件和研究目的下, 可作為皮質(zhì)醇和血糖的補(bǔ)充資料, 對真實反映特定狀態(tài)下魚類運(yùn)輸后的應(yīng)激生理具有重要意義。生理影響和水質(zhì)影響在魚類運(yùn)輸中是相互影響的, 水質(zhì)惡化會導(dǎo)致魚類生理學(xué)變化, 魚類生理學(xué)活動又會影響水質(zhì)。因此, 在魚類運(yùn)輸相關(guān)研究中, 水質(zhì)和生理指標(biāo)都是需要評價的。
魚類標(biāo)準(zhǔn)化運(yùn)輸操作規(guī)程的制定是魚類運(yùn)輸相關(guān)研究的核心任務(wù), 后續(xù)的研究應(yīng)圍繞這一主題進(jìn)行。以下幾個方面應(yīng)該是后續(xù)研究應(yīng)該重點(diǎn)關(guān)注的: 第一, 不同魚類的生態(tài)習(xí)性和不同生活史階段對運(yùn)輸?shù)捻憫?yīng); 第二, 魚類運(yùn)輸過程中CO2的排泄率和水體中CO2含量應(yīng)該被充分認(rèn)識和重視; 第三, 針對特定魚類其適宜的運(yùn)輸密度、運(yùn)輸時間、水溫、鹽度、麻醉劑種類和麻醉方式等抗應(yīng)激措施應(yīng)被充分評估; 第四, 更加多樣化的應(yīng)激評價指標(biāo)(如抗氧化酶等)和更加科學(xué)的測定手段應(yīng)被應(yīng)用到后續(xù)研究中; 第五, 干法運(yùn)輸可以避免運(yùn)輸過程中水質(zhì)的惡化, 且很大程度上消除了運(yùn)輸密度的限制, 某些氣呼吸型魚類如大鱗副泥鰍(Paramisgurnus dabryanus)[101]等, 可在空氣中(需保持體表濕潤)存活相當(dāng)長的時間, 其干法運(yùn)輸?shù)目尚行灾档蒙罹俊?/p>
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