鲎, 别名马蹄蟹(horseshoe crab), 是一种非常古老的海洋节肢动物, 其化石可追溯至4.75亿年前的早古生代奥陶纪, 时至今日其形态结构也未发生重大改变, 因此常被称为海洋“活化石”(Rudkin & Young, 2009)。当今世界仅存4种鲎, 即分布在北美洲东岸至墨西哥湾的美洲鲎(Limulus polyphemus)和亚洲地区的中华鲎(Tachypleus tridentatus)、南方鲎(T. gigas)和圆尾鲎(Carcinoscorpius rotundicauda)。世界自然保护联盟(International Union for Conservation of Nature, IUCN)世界自然保护大会(World Conservation Congress)在2012年通过了一项有关保护亚太区三种鲎的提议, 提出“鲎是维持生态系统正常运作的重要生物资源”、“鲎具文化象征的意义”、“鲎是需要持续管理的自然资源”, 并认为亚洲各国正面临鲎种群数量严重下滑的趋势(Jones, 2012)。
鲎的蓝色血液中蕴藏着许多功能特殊的生化活性物质, 目前已发现50多种, 是开发医用药物的宝库(洪水根, 2011)。其中, 鲎血液的变形细胞能与革兰氏阴性细菌内的毒素产生粘连作用, 被广泛用于生产医用检验试剂――鲎试剂(Gauvry, 2015)。鲎栖息于水交换能力较弱的内湾, 其幼体在河口附近潮间带生活8-10年, 高度依赖沿海地区的环境条件, 因此被确定为河口生态系统的标志物种(陈章波等, 2015)和前哨物种(sentinel species, Kwan et al, 2018)。鲎性成熟后会迁移到水深40 m以内的浅海区栖息与觅食, 对维持典型近岸生态和生物多样性结构稳定扮演重要的角色。本文从中华鲎生境破坏和过度捕捞两个主要角度出发, 分析中华鲎全球种群面临的状况, 讨论中华鲎资源保护在海洋保护区划定、增殖放流及科普和野生动物保护法宣传中存在的问题, 并提出相应对策建议与措施。
1 中华鲎全球种群现状
中华鲎的自然地理分布范围相当狭窄, 仅局限于太平洋西岸, 自日本濑户内海开始, 沿我国浙江、福建、广东、广西、海南、香港和台湾沿岸, 南至印度尼西亚爪哇岛北岸以北、苏门答腊岛印度洋东侧的海域(附录1)。相对于东南亚沿岸, 中华鲎在中国东岸和日本南部海域的历史分布较广、产量较高(Laurie et al, 2019; Kwan et al, 2019b)。
早在20世纪70年代, 中华鲎广泛分布于中国的东南部沿海, 是一种“随手可及”的海洋生物, 但随后的系统调查数据显示中华鲎资源出现了显著的衰退(翁朝红等, 2012; Hsieh & Chen, 2015; Liao et al, 2019a)。珠江口以北的海域已多年未见中华鲎上岸产卵; 福建平潭的中华鲎数量由1954年的日产量1,000对下降至2002年的4对(廖永岩和李晓梅, 2001); 中华鲎曾经广泛存在于我国台湾岛西岸、澎湖群岛和金门岛, 种群密度高, 但自20世纪60年代以来, 其在台湾西海岸出现区域性灭绝, 仅在金门岛有数量稀少的幼鲎(Hsieh & Chen, 2015); 广东省仅南部海域存有少量的中华鲎资源, 香港海域潮间带上的中华鲎幼体种群在2002-2009年的7年间数量减少了九成(Shin et al, 2009), 据2012年和2014年的鲎幼体资源分布调查, 目前香港仅有后海湾和东涌湾沿岸存在小而分散的中华鲎幼体种群, 并且低龄的幼鲎相对较少, 表明这些幼鲎种群较脆弱, 局部灭绝的可能性高(Kwan et al, 2016)。
最早研究中华鲎的学者光口晃一曾在他的Biology of Horseshoe Crab专著中提到北部湾是中华鲎最理想的栖息地(Sekiguchi, 1988), 同时国内外专家在20世纪80年代都曾在北部湾的北海沿岸滩涂见到成群的中华鲎上岸产卵, 场面壮观(Brockmann & Smith, 2009; Shuster & Sekiguchi, 2009; 翁朝红等, 2012), 因此北部湾连同周边的雷州湾及海南附近海域被认为是中华鲎在全球种群资源密度较高的“净土”。即便如此, 北部湾的中华鲎数量仍从20世纪90年代约每年60万对骤降为2010年的约30万对(陈章波等, 2015)。据2015年对广西北部湾地区30个沿海乡镇和村庄400名受访者的调查数据表明, 95%的受访者认为北部湾海域在2011-2016年间中华鲎的产量明显比之前少, 相比20世纪90年代的日均捕获量50-1,000只, 2011-2016年间渔民每日仅能捕获0-30只, 且曾见中华鲎上岸产卵的受访者年龄显著高于未见过的(Liao et al, 2019a), 表明近30年来中华鲎资源发生了严重衰退。
20世纪50年代以来, 日本各地的中华鲎种群数量也在急剧减少。濑户内海的所有海岸都曾有丰富的中华鲎资源, 但至2003年几乎已经灭绝(Seino et al, 2003), 2006年中华鲎在日本被评估为极危(CR)。1990-2007年间, 越南的中华鲎数量和分布面积均下降了50%, 收获产量下降了20% (Nguyen, 2007), 2007年中华鲎在越南被评为易危(VU)。马来西亚和印度尼西亚对中华鲎种群的系统研究起步较晚, 尚未对中华鲎进行濒危等级评估。目前马来西亚仅报道了婆罗洲东北部沙巴的中华鲎种群(Manca et al, 2017; Mohamad et al, 2019); 而印度尼西亚爪哇岛北部海岸的渔民走访调查数据表明当地中华鲎的捕获量正在下降(Meilana & Fang, 2017)。2019年3月, 中华鲎在IUCN红色名录中的濒危等级从原来的数据缺乏(DD)变更为濒危(EN; Laurie et al, 2019), 明确了中华鲎资源正呈现全球性衰退的状态。
2 中华鲎种群威胁的识别
目前中华鲎面临全球性资源枯竭, 归结起来主要原因为鲎生境破坏和过度捕捞两个方面。
2.1 生境破坏
中华鲎生活史各阶段(如卵胚胎、幼体和成体)的生长发育都对水文、地貌、水温等环境特征要求较高, 所需生境类型从浅滩高潮线附近的产卵生境、潮间带的幼体栖息地到潮下带浅海区域的觅食生境等(Laurie et al, 2019), 且不同生境的紧密连接对于其不同生命阶段的生长发育至关重要。潮间带沉积物中的叶绿素a与总有机碳含量可能是决定幼鲎密度与生长的重要因素(Hsieh & Chen, 2009; Xie et al, 2020), 而盐度、温度和溶解氧等会直接对鲎胚胎发育和幼体生长产生影响(Jegla & Costlow, 1982; Hong et al, 2009)。从沉积环境而言, 沉积物的颗粒组成会影响中华鲎幼体的分布(Kwan et al, 2016; Xie et al, 2020), 滩涂坡角会影响幼鲎不同生长阶段在滩涂的分布(Rudloe, 1981), 且高密度的鲎幼体集中在红树林外缘、潮沟出水口附近(Chen et al, 2015; Xie et al, 2020)或海草床附近(Morton & Lee, 2011; Kwan et al, 2016)。
海洋经济发展使鲎的栖息地很容易遭到海岸带围垦、沿海基础设施建设、海砂抽取、海水养殖等活动的影响。在日本和中国沿海地区, 填海项目和海岸基础设施的建设都造成其栖息地的直接丧失(Seino et al, 2003; Morton & Lee, 2011; 翁朝红等, 2012; Hsieh & Chen, 2015)。海砂抽取被认为是中国东南沿海与越南之间的中华鲎产卵生境退化的重要原因(Roby & Liu, 2011)。在越南, 海洋和沿海水域支撑着大约2,000万人的生计, 大面积的潮间带被建成蛤蜊池塘, 潮间带红树林和海草生态系统的质量持续下降, 逐步产生“沿海荒漠化”问题(Vietnam NBSAP, 2014)。在日本, 水环境污染直接影响了中华鲎卵和胚胎发育以及间接减少了潮间带底栖生物量(Itow, 1993; Botton, 2001; Botton & Itow, 2009), 导致当地中华鲎种群数量减少或消失。此外, 海平面和水温的升高正逐步地影响中华鲎种群, 海平面的上升将大大减少产卵地的面积, 而沿岸城市化加快了这些栖息地消失的速度(Loveland & Botton, 2015)。例如, 2016年1-8月间在日本北九州滩涂发现了490只中华鲎死亡(
2.2 过度捕捞
中华鲎由于具有重要的医用和食用价值而遭到人类过度捕捞和任意捕杀, 从而导致其种群资源急剧下降。在现代食品药品行业中, 鲎的重要经济价值体现在从其变形细胞提取一种能与内毒素迅速形成凝胶的鲎试剂, 并已被广泛应用于生物学、医学研究、药学及环境卫生学中的痕量内毒素的检测, 具有灵敏、快速、简便、经济、重复性好的特点。目前鲎试剂主要从美洲鲎和中华鲎的血液中提取(Gauvry, 2015), 利用基因工程手段生产鲎试剂还没有实现真正的产业化与普及化。有资料显示在亚洲大部分地区提取鲎血采用的是不可持续的做法, 即杀死中华鲎活体而采集全部血液(洪水根, 2011; Gauvry, 2015), 而在美国仅允许抽取鲎部分血液, 且需在72 h内放归捕捞海域。在我国厦门、湛江等地有一定规模的鲎试剂生产工厂。鲎试剂是《中国药典》中需采用内毒素检测的300多种注射药品的检测试剂。为满足市场需求每年需生产1,000余万支(0.1 mL)鲎试剂, 据此推测中国每年至少需要消耗10万对中华鲎(李裕红等, 2018)。
野生动物的食用与消费一直以来是生物多样性的巨大威胁。在中国、越南、泰国和马来西亚等国的沿海城市, 鲎在历史上是一道很受欢迎的菜肴, 且被普遍认为能提高人体的免疫力(Christianus & Saad, 2007; Shin et al, 2009; Fu et al, 2019)。据统计, 广西沿海城市的海鲜餐厅一只约3 kg的中华鲎价格从1998年的约30元上涨到2018年的300元(Fu et al, 2019)。虽然早在20世纪90年代中国部分沿海省份已将中华鲎列入省级重点保护动物名录, 但在高利润的驱使下, 中华鲎渐渐从渔民饭桌常见的菜肴转变成沿海甚至内陆地区的高价“海鲜”。同时, 鲎的跨国走私贸易活动也开始萌芽, 例如从越南进入我国广西(Liao et al, 2019a)、从马来西亚和印度尼西亚走私到泰国等(Mohamad et al, 2015), 可见东盟各国的鲎资源已无法满足当地市场需求。其中, 为食用鲎卵, 雌鲎常常是重点捕捞对象, 进而导致自然环境中鲎的雌雄比例失调, 总产卵量持续降低。
3 反思与建议
各国在中华鲎资源保护方面已采取了不同的措施, 如中国的浙江、福建、广东和广西等省(区)已将其列为“省级重点保护水生野生动物”, 针对鲎划定的海洋保护区在广东省有6处(中华人民共和国生态环境部, 2013), 香港于2012年底颁布法规禁止底拖捕捞作业。印度尼西亚环境和林业部2018年对政府条例进行了升级并颁布了新法规, 以保护其境内的包括中华鲎在内的3种鲎(Laurie et al, 2019)。马来西亚则对生物资源的收集、研究和出口有非常严格的规定, 每个州都建立了一个生物多样性委员会来履行监督和审核职能(SaBC, 2017)。
作者团队在开展鲎资源保护的多年实践工作中, 感受到中华鲎资源保护中存在以下值得商榷的问题, 并提出相应建议供大家探讨。
3.1 划定保护地不足以逆转中华鲎资源下降的趋势
海洋保护地的划定是为了避免人为因素导致中华鲎繁育与觅食生境的直接丧失。我国的海洋保护地数量从1990年的5处增加至2014年的249处(曾江宁等, 2016), 但由于管理资金投入不足、执法力度低、管理体系复杂等因素的影响, 许多保护地沦为有名无实的“纸上公园”。近15年来在保护地划定和野生动物法规的“护航”下, 中华鲎种群资源仍呈持续下降趋势, 保护效果极不理想。
一个切实有效的珍稀物种保护框架应包括系统评估、规划和公众参与等过程, 可以概括为STEPS的5个方面, 即S (地点、物种、替代物)、T (威胁因素)、E (评估)、P (规划、政策)以及S (公众意识、利益相关方态度) (黄祥麟个人通讯, 2019)。而在中华鲎保护地的划定与实施过程中, STEPS框架中的第一个S即种群具体分布位置、数量及趋势的本底数据几乎空白, 大大阻碍了中华鲎保护规划和行动的有效实施。通过采集中华鲎成年种群数据建立有效的种群增长模型, 是最直接的鲎种群评估方法, 但传统的成鲎种群评估需依赖底拖网完成, 成本高、人力和时间投入大, 对底栖生态环境影响大。因此, 中华鲎种群评估工作多以成鲎重要产卵生境和幼体栖息地替代成年种群数据来评估。我国广西北部湾、香港和台湾地区以及新加坡已开展系统的幼体种群调查, 明确了亚洲3种鲎在该地区幼体栖息地的重要分布位置(Hsieh & Chen, 2015; Kwan et al, 2016; Xie et al, 2020)。Xie等(2020)调查了广西北部湾沿岸适合作为鲎幼体成长生境的18个滩涂, 其中14个是中华鲎的幼体栖息地, 有6个种群密度较香港高。但受限于幼鲎种群调查方法不同, 其研究结果无法与中国台湾地区以及新加坡和菲律宾的幼鲎种群数据进行比对, 因此各地的鲎科学工作者迫切需要制订统一和标准化的种群和环境基线监测指南(Wang et al, 2019)。中华鲎的产卵、觅食生境的核心分布区在哪? 这些生境的环境怎样? 鲎苗是如何从高潮线扩散到红树林外缘, 又如何移动到潮下带觅食? 这些生物学和生态学信息的缺乏是中华鲎资源保护之路上的一大障碍。
3.2 明确增殖放流对鲎资源的修复效果
通过人工大量培育中华鲎苗种并进行野外放流, 是现今对中华鲎种质资源修复最重要且可靠的异地保护(ex-situ conservation)措施(Carmichael & Brush, 2012)。目前最常见的放流方法为乘船到较深海域或在码头将中华鲎成体或苗种投入海中, 苗种多为刚从胚胎孵化出来的1龄鲎, 每只1龄鲎的竞标价从0.8元至1.5元不等(表1)。1龄中华鲎体长约10 mm (Sekiguchi, 1988), 个体太小导致标记难度过大, 放流效果评估难以开展。胡梦红等(2013)初步尝试采用可视嵌入性荧光标记技术对2龄幼鲎进行标记放流, 而Kwan等(2015)则将被动式追踪芯片(passive integrated transponder)嵌入7-10龄幼鲎以量化其在滩涂上的活动面积, 但仍无法解决标记物的稳定性(易回捕)、持久性(不易脱落)和适应性(不对个体造成影响、能标记小个体)三大难题。此外, 幼鲎在6龄以前在自然环境下的自然死亡率高达80%(Carmichael et al, 2003), 再加上组织放流的单位对鲎幼体栖息地的位置与环境特征的认知有限, 常导致放流成功但成效难以保障的现状。放流鲎成体也是不明智的, 因为鲎生长周期8-10年, 而目前人工养殖技术无法将苗种饲养至成体, 所以成体都是从自然种群获取, 除了非法贸易被收缴的或生产鲎试剂被抽血的鲎以外, 不建议捕捞野生的鲎成体用作放流。
表1 近十年在中国沿岸放流中华鲎的报道
Table 1
| 放流日期 Date of release | 放流选址 Site selection for release | 鲎生长阶段 Growth stage of horseshoe crab | 数量 Number | 组织单位 Organization(s) |
|---|---|---|---|---|
| 2019.11 | 广东湛江遂溪县 Suixi County, Zhanjiang City, Guangdong | 1龄 1st instar | 460,000 | 北部湾大学、广西大学 Beibu Gulf University and Guangxi University |
| 2019.08 | 福建泉州丰泽滨海公园 Fengze Seaside Park, Quanzhou City, Fujian | 1龄 1st instar | 5,000 | 福建省淡水水产研究所、华侨大学、集美大学等 Freshwater Fisheries Research Institute of Fujian, Huaqiao University, Jimei University etc. |
| 2019.08 | 广东汕头潮南区田心湾 Tianxin Bay, Chaonan District, Shantou City, Guangdong | 成年 Adult | 315 | 广东省汕头市潮南区海洋与渔业局 Marine and Fishery Bureau of Chaonan District, Shantou City, Guangdong Province |
| 2019.06 | 广西防城港 Fangchenggang City, Guangxi | 1龄 1st instar | 60,000 | 中国水产科学研究院南海水产研究所 South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences |
| 2018.11 | 广西北海 Beihai City, Guangxi | 成年 Adult | 2,000 | 湛江博康海洋生物有限公司北海分公司 Zhanjiang Bokang Marine Biotechnology Co. Ltd., Beihai |
| 2018.10 | 广西钦州三娘湾 Sanniang Bay, Qinzhou City, Guangxi | 1龄 1st instar | 5,170,000 | 北部湾大学、广西大学 Beibu Gulf University and Guangxi University |
| 2018.08 | 广西防城港 Fangchenggang City, Guangxi | 1龄 1st instar | 30,000 | 中国水产科学研究院南海水产研究所 South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences |
| 2018.06 | 福建平潭敖东镇山歧澳中华鲎保护区 Shanqi’ao Chinese Horseshoe Crab Protected Area, Aodong Town, Pingtan City, Fujian | 1龄 1st instar | 215,730 | 平潭综合实验区农村发展局 Rural Development Bureau of Pingtan Comprehensive Experimental Area |
| 2018.05 | 广东湛江东海岛东南码头 Dongnan Pier, Donghai Island, Zhanjiang City, Guangdong | 1龄 1st instar | 77,000 | 湛江市海洋与渔业局 Zhanjiang Marine and Fishery Bureau |
| 2017.07 | 广西北海侨港镇南部水域 Southern waters of Qiaogang Town , Beihai City, Guangxi | 1龄 1st instar | 550,000 | 北海市海洋与渔业局 Beihai Marine and Fishery Bureau |
| 2017.07 | 福建福州市海域 Fuzhou City waters, Fujian | 1龄 1st instar | 134,600 | 福州市海洋与渔业局 Fuzhou Marine and Fishery Bureau |
| 2017.06 | 广东阳江东平镇南鹏岛海域 Nanpeng Island waters, Dongping Town, Yangjiang City, Guangdong | 1龄 1st instar | 1,000 | 阳江市海洋与渔业局 Yangjiang Marine and Fishery Bureau |
| 2017.02 | 广东湛江东海岛东南码头 Dongnan Pier, Donghai Island, Zhanjiang City, Guangdong | 1龄 1st instar | 100,000 | 湛江市海洋与渔业局 Zhanjiang Marine and Fishery Bureau |
| 2015.06 | 福建福州长乐区五显鼻渔港码头外海域 Coastal waters outside Wuxianbi Fish Pier, Changle District, Fuzhou City, Fujian | 1龄 1st instar | 81,000 | 福州市海洋与渔业局 Fuzhou Marine and Fishery Bureau |
| 2015.06 | 福建闽江口川石岛海域 Chuanshi Island waters, Minjiang Estuary, Fujian | 1龄 1st instar | 110,000 | 福建省海洋渔业厅 Administration of Ocean and Fisheries of Fujian Province |
| 2015.05 | 广东湛江东海岛东南码头 Southeast Pier, Donghai Island, Zhanjiang City, Guangdong | 1龄 1st instar | 200,000 | 湛江市相关政府部门 Zhanjiang local governments |
| 2014.05 | 福建金门烈屿乡埔头 Putou, Lieyuxiang Village, Kinmen County, Fujian | 1龄 1st instar | 20,000 | 金门县政府 Kinmen County Government |
| 2013.05 | 广东汕头南澳县前江港务码头 Qianjiang Wharf, Nan’ao County, Shantou City, Guangdong | 不明 Unknown | 不明 Unknown | 不明 Unknown |
| 2010.06 | 广东阳江海陵岛大角湾畔 Dajiao Bayside, Hailing Island, Yangjiang City, Guangdong | 成年 Adult | 200 | 国家农业农村部、广东省政府 Ministry of Agriculture and Rural Affairs of the P. R. China and People’s Government of Guangdong Province |
一个严谨的放流方案应包括合理的放流地点、时间及放流效果评估。我们建议在水温高于20℃的海水退潮期间, 在已知的中华鲎幼体生境滩涂上进行鲎苗放流; 若附近海岸线已被破坏, 可选择与已识别的中华鲎幼体栖息地特征(Xie et al, 2020)类似的潮间带。高密度的幼鲎种群多集中在红树林外缘、潮沟出水口附近或海草床附近(Chen et al, 2015; Kwan et al, 2016; Xie et al, 2020)。若条件允许, 建议选择自然死亡率较低的6-7龄幼鲎(头胸甲宽度8-10 mm)进行放流。同时也应考虑因长期养殖对种质的影响(Kwan et al, 2014)。有关政府部门可组织科研单位、公益组织和其他有关利益方一同开展放流活动, 严格采取科学把关以保障放流效果, 也可进行社区共管的方式改善市民的保护意识, 以形成政府主导、公众参与的生动局面。
3.3 加强科普教育和海洋野生动物保护法的宣传
珍稀物种保护的成败很大程度上取决于公众意识(social awareness)和利益相关方的态度(stakeholders’ perception), 即STEPS框架最后的S。我国福建、广东、广西、香港和台湾地区开展了一系列科普宣教活动, 如“七夕海峡两岸中华鲎保育日”、“马蹄蟹校园保姆计划”、“不吃鲎消费倡导活动”、“水生野生动物保护宣传月活动”、“北部湾滨海湿地和鲎野外种群调查”等。2019年第四届国际鲎科学与保护研讨会首次在我国广西召开, 来自全球的100多名专家学者共同发布《全球鲎保护北部湾宣言》, 并将每年的6月20日正式确定为“国际鲎保育日”, 呼吁社会各界联动保护鲎资源。
科普活动需要公众长期参与, 在情感上建立联系, 才能有效改变环保态度、提升环保意识。例如, 在“马蹄蟹校园保姆计划”中, 参与的中学生负责幼鲎的饲养、为低年级学生或家长讲解鲎知识, 夏天将幼鲎放归野外; 在长时间的接触中与鲎建立了感情, 能显著提升中学生的野生动物保护意识与行动力(Kwan et al, 2017)。在开展长期自然教育活动的同时, 也应探索社区共管方式的可能性, 如建立一套举报奖励机制, 鼓励公众协助巡逻监管、渔民有偿参与鲎捕捞数据的收集、组建志愿队伍定期在邻近海域开展幼鲎资源调查、提供社区鲎苗养殖技术支撑并回购幼鲎进行增殖放流等。Plummer和Taylor (2004)发现, 公众参与环境保护工作的主动性和积极性, 很大程度上取决于有关政府部门是否提供社区参与的机会与机制。
鲎保护也可从食物安全的角度切入。据2017年广西北部湾沿岸市民的访谈资料可知, 40%的受访者知道圆尾鲎含河豚毒素(Fu et al, 2019), 食用后可引发急性食物中毒, 但只有59%的受访者能正确辨认不同种类的鲎。2018年7月广西防城港发生食鲎集体中毒事件后, 有关部门加强了市场巡查, 严禁售卖鲎(
通过自然教育或公众科普宣教方式提升鲎保护意识虽然取得了一定进展, 但要改变整个社会的公众环境保护意识和态度所需周期很长, 在中华鲎资源急速下降的情况下应采用多种方式大力加强科普和海洋野生动物保护法宣传。海洋珍稀物种保护的成效, 尤其是关注度不高的无脊椎生物, 不仅需要扎实的种群生态学研究支撑, 更取决于长期有效的管理规划以及公众积极正向的态度。政府部门、科研机构、环境宣教组织和其他利益相关方应加强联系、开展合作, 多方位、多角度保障中华鲎资源的可持续利用与发展。
附录 Supplementary Material
附录1 中华鲎在世界海域的分布记录
Appendix 1 Global distribution records of Tachypleus tridentatus
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