• ISSN 1000-0615
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Volume 43 Issue 9
Sep.  2019
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Strategy optimization of stock enhancement of golden cuttlefish, (Sepia esculenta) based on structural characteristics of reproductive and recruitment populations

  • Corresponding author: Xiumei ZHANG, xiumei1227@163.com
  • Received Date: 2019-07-15
    Accepted Date: 2019-07-28
  • Golden cuttlefish, Sepia esculenta, was one of the largest economic cephalopods in Chinese northern seas, with an annual yield of more than 1 000 tons until the 1970 s. Since then, with the continuous expansion of fishing activities and dwindling of spawning field, this species has greatly reduced its local abundance compared with the previous condition due to its biological sensitivity to the environment (one-year life cycle and low fecundity). For this situation, many measures have been adopted to boost the population to meet the needs of human consumption, such as the release of artificially hatched juveniles and restoration of spawning habitats. The primary objective of this study is to optimize the existing conservation strategies and methods of cuttlefish by exploring the structural characteristics of the cuttlefish population in Qingdao coastal waters. The brood amount and size of egg cells of adult cuttlefish in 2017 were calculated. The distribution of body length and spatiotemporal migration process of juveniles were analyzed by using continuous monitoring methods. Results showed a decreasing trend in the specification of golden cuttlefish that arrived at different stages during the grouping period (May- July). The brood amount and size of egg cells of breeding cuttlefish in the early-stage were significantly higher than those of middle- and later-stages. The fertilized eggs of cuttlefish are characterized by sinking and stickiness, thus attaching to the attachment, such as algae or reefs. For juvenile cuttlefish, they grow rapidly, and the dispersion level of body specifications increases with sampling time due to the long grouping period and spawning in batches habits of breeding cuttlefish. This behavior might be a reproductive strategy for avoiding intraspecific feeding competition and promoting the differentiation of spatial niche. Therefore, according to the actual brood amount and the quality of eggs in the collection periods of breeding cuttlefish, it is suggested that the utilization rate of the early-stage population should be increased to improve breeding efficiency and guarantee the supply of large-size juveniles. The placement of spawning substrates in the spawning ground (slow-flowing area, 15-20 m) of adult cuttlefish from May to July is an important measure of resource restoration. Based on the reproductive habits of breeding cuttlefish, background survey in food abundance of release’s water for juvenile cuttlefish is indispensable. To reduce the intraspecific predation pressure and improve the survival rate of released population during large-scale juveniles’ release, juveniles’ release in different periods should be taken into consideration. In order to further optimize the mode of resource restoration and reduce the cost of the proliferation of cuttlefish, we also suggest exploring the technology of releasing fertilized eggs.
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Strategy optimization of stock enhancement of golden cuttlefish, (Sepia esculenta) based on structural characteristics of reproductive and recruitment populations

    Corresponding author: Xiumei ZHANG, xiumei1227@163.com
  • 1. Key Laboratory of Mariculture, Ministry of Education, Ocean University of China, Qingdao    266003, China
  • 2. Function Laboratory for Marine Fisheries Science and Food Production Processes, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266072, China
  • 3. National Fisheries Technology Extension Center, China Society of Fisheries, Beijing    100125, China

Abstract: Golden cuttlefish, Sepia esculenta, was one of the largest economic cephalopods in Chinese northern seas, with an annual yield of more than 1 000 tons until the 1970 s. Since then, with the continuous expansion of fishing activities and dwindling of spawning field, this species has greatly reduced its local abundance compared with the previous condition due to its biological sensitivity to the environment (one-year life cycle and low fecundity). For this situation, many measures have been adopted to boost the population to meet the needs of human consumption, such as the release of artificially hatched juveniles and restoration of spawning habitats. The primary objective of this study is to optimize the existing conservation strategies and methods of cuttlefish by exploring the structural characteristics of the cuttlefish population in Qingdao coastal waters. The brood amount and size of egg cells of adult cuttlefish in 2017 were calculated. The distribution of body length and spatiotemporal migration process of juveniles were analyzed by using continuous monitoring methods. Results showed a decreasing trend in the specification of golden cuttlefish that arrived at different stages during the grouping period (May- July). The brood amount and size of egg cells of breeding cuttlefish in the early-stage were significantly higher than those of middle- and later-stages. The fertilized eggs of cuttlefish are characterized by sinking and stickiness, thus attaching to the attachment, such as algae or reefs. For juvenile cuttlefish, they grow rapidly, and the dispersion level of body specifications increases with sampling time due to the long grouping period and spawning in batches habits of breeding cuttlefish. This behavior might be a reproductive strategy for avoiding intraspecific feeding competition and promoting the differentiation of spatial niche. Therefore, according to the actual brood amount and the quality of eggs in the collection periods of breeding cuttlefish, it is suggested that the utilization rate of the early-stage population should be increased to improve breeding efficiency and guarantee the supply of large-size juveniles. The placement of spawning substrates in the spawning ground (slow-flowing area, 15-20 m) of adult cuttlefish from May to July is an important measure of resource restoration. Based on the reproductive habits of breeding cuttlefish, background survey in food abundance of release’s water for juvenile cuttlefish is indispensable. To reduce the intraspecific predation pressure and improve the survival rate of released population during large-scale juveniles’ release, juveniles’ release in different periods should be taken into consideration. In order to further optimize the mode of resource restoration and reduce the cost of the proliferation of cuttlefish, we also suggest exploring the technology of releasing fertilized eggs.

  • 金乌贼(Sepia esculenta)广泛分布于我国近海、朝鲜半岛海域及日本海[1-2],曾是我国北方海域经济价值最大的乌贼[3-4]。自20世纪70年代以来,由于受过度捕捞与产卵场环境破坏等影响,其资源量明显衰退[5],加之其较高的环境敏感性(一年生种类、低生殖力)[6-7]和特殊的繁殖习性(雌雄交配,产黏性卵)[8],导致金乌贼渔业产业前景不容乐观。1991年,金乌贼作为一种珍贵的地方渔业种类,被列入山东省资源增殖计划,通过向产卵水域投放人工附着基为繁殖群体提供受精卵附着载体。2010年和2014年起,青岛市与山东省分别开展金乌贼幼体增殖放流,放流数量逐年增加,2018年全省放流数量达240万只(山东省水生生物资源养护管理中心)。

    资源繁盛时期,北起鸭绿江口南至舟山近岸水域,春季均广布金乌贼繁殖群体,不同产卵场的结群时间也有差异。日照近海的结群时间为4月中旬至5月底,而于青岛近岸,则变为5月初到7月中旬[4, 9]。亲体生殖结束后即死亡,产卵场转为子代育幼场,秋季11月,随水温逐渐降低,补充群体开始向黄海中南部越冬迁徙。因此,金乌贼亲体近岸结群生殖(5—7月)、幼体孵化(6—8月)及近海生长发育(7—10月)阶段是进行其资源养护、苗种放流和补充群体监测管理的主要时期。金乌贼产黏性卵,需在附着物上产卵固定,九十年代初期,山东省主要在海上投放乌贼笼、黄花蒿和柽柳[10]等作为附卵基质,以营造人工产卵场。2017年,牛超等[11]研制了十字型可折叠产卵附着基,进一步改进了金乌贼资源增殖方式。一年生的生活史特征决定了金乌贼资源补充量主要取决于受精卵数量及子代成活率,幼体数量决定了翌年亲体资源量和渔获量[4],因此,苗种放流也是其资源增殖的有效方式之一[12]。近年来,虽然金乌贼苗种放流规模不断增大,但关于放流模式、放流时期、放流海域合理规划等研究尚未开展,因此,完善放流策略、提高放流群体存活比例是亟待解决的瓶颈问题。

    开展渔业资源种群结构的调查研究,是资源增殖修复的重要生态学基础[13],也是了解水生生物种群动态和结构特征的关键手段。本研究依据相关文献及海域连续监测数据,对青岛近岸金乌贼产卵繁殖亲体和补充群体的结构特征及时空迁移过程等进行探讨,并基于上述结果提出其资源增殖修复策略优化措施,以为近海头足类资源的可持续利用提供借鉴。

1.   材料与方法
  • 金乌贼繁殖群体调查海域位于青岛薛家岛近岸(35°55′30″N, 120°15′E),水深15~21 m, 泥沙底质(图1)。分别于2014年、2015年和2017年繁殖亲体结群期内(5月中旬至7月上旬),以约20 d为间隔进行洄游前(5月中旬至6月上旬)、中(6月上旬至6月下旬)、后期(6月下旬至7月上旬)划分,逐日统计地笼网渔获物信息,明确不同洄游时期繁殖亲体平均体质量变化。针对2017年不同洄游时期繁殖亲体的绝对生殖力,采用质量比例法进行统计,随机混合卵巢前、中、后部卵粒3~4 g(约占卵巢总重的10%),记录其数量并结合卵巢总重计算怀卵量。统计雌性亲体输卵管中成熟卵细胞的卵径和卵重,以比较不同洄游时期繁殖亲体卵子规格的差异。

    Figure 1.  The sampling sites of S. esculenta

  • 金乌贼补充群体调查水域位于青岛灵山岛及其周边海域(图1),共计布设12个站位,分别于2016年9—11月(4航次)、2017年9—11月(4航次)和2018年9—11月(3航次)进行了11航次的调查。调查方法参照《海洋调查规范》(GB/T 12763.6-2007)进行,每一站位拖网时长1 h,拖网时速2 kn/h,调查采用底拖网,网口周长为800目×40 mm,扩张网口高度约为2 m,囊网网目尺寸为16 mm。所获金乌贼样品按站位分别冰冻保存,并于实验室内统计乌贼幼体数量、质量(精确至0.1 g)和胴长(精确至0.1 mm)等信息。此外,同步收集各站位的水温、水深、盐度等水文信息。调查站位的资源量以单位时间尾数密度(尾/h)表示,利用surfer12.0 绘制时空分布图,使用直方图描述乌贼幼体的胴长、体质量分布特征,运用箱线图和正态分布图表示其群体规格的离散程度。

2.   结果
  • 2014、2015和2017年青岛近岸金乌贼繁殖群体平均体质量均随结群时间推移呈逐渐下降趋势,洄游前期亲体平均体质量可达后期的2倍之余(图2)。同时,绝对生殖力亦随胴长和体质量的降低逐渐下降,前期群体的绝对生殖力(2 940.6粒)显著高于中期(1 802.1粒)和后期群体(1 593.6粒)(P<0.05)。此外,前期大规格繁殖亲体的卵径和卵重显著大于中期和后期群体(P<0.05)(表1)。

    Figure 2.  Change trend of average body weight of S. esculenta spawning stock in coastal waters of Xuejiadao in 2014, 2015 and 2017

    洄游时期
    period
    数量/只
    number
    平均胴长/mm
    average mantle length
    平均体质量/g
    average body weight
    绝对繁殖力/粒
    absolute fecundity
    卵径/mm
    egg diameter
    卵重/mg
    egg weight
    前期
    early stage
    28(♀: 16) 167.0±14.6a 734.5±178.4a 2 940.6±648.9a 6.00±0.16a 65.1±5.4a
    中期
    middle stage
    27(♀: 13) 158.4±20.3a 575.5±184.8b 1 802.1±386.9b 5.13±0.04b 45.7±1.8b
    后期
    later stage
    33(♀: 14) 145.3±16.8b 455.2±148.5c 1 593.6±476.0b 5.08±0.07b 48.0±1.2b
    注:同列肩标不同小写字母表示差异显著(P<0.05)
    Notes: in the same column, different letter superscripts mean significant differences (P<0.05)

    Table 1.  Information of reproductive population samples of S. esculenta in different periods

  • 2016年、2017年和2018年共11航次的调查中,采集金乌贼幼体样本总计3 710只(表2)。统计数据显示,金乌贼补充群体生长迅速,其平均胴长和体质量随时间推移快速增长,以2017年样本数据为例,平均体质量在3个月内可增加约100 g。其体质量范围由1.2~100 g (9月1日采集的样本)逐渐增至11.3~198.5 g (11月18日采集的样本),优势胴长范围和优势体质量范围亦呈相似趋势。从胴长、体质量分布箱线图可知,9—11月期间,青岛近岸金乌贼补充群体的平均胴长和体质量迅速增加,箱体宽度和内限宽度亦呈逐渐升高趋势,表明补充群体胴长和体质量的整体离散水平随采样时间推移不断升高。胴长、体质量分布直方图及胴长正态分布图显示,胴长和体质量分布频率的“峰值”坡度随时间推移逐渐变“缓”,整体离散程度逐渐增大(图34)。

    采样时间
    sampling time
    样本数量
    sample size
    胴长/mm mantle length胴长分布情况/mm
    distribution of mantle length
    范围
    range
    优势胴长(%)
    dominant mantle length
    平均值
    mean
    2016-09-03 259 17~79 21~50(91.1%) 37.2
    2016-09-20 587 19~100 31~60(81.4%) 45.9
    2016-10-25 471 24~123 41~80(77.5%) 69.3
    2016-11-16 51 31~142 51~100(80.0%) 76.8
    2017-09-01 498 15~80 21~50(88.4%) 33.2
    2017-09-25 1 095 16~100 21~60(91.4%) 47.5
    2017-10-18 158 35~96 31~80(92.8%) 64.8
    2017-11-18 87 31~120 51~100(81.0%) 78.2
    2018-09-19 316 24~107 41~70(69.2%) 55.7
    2018-10-10 60 16~100 41~80(80.0%) 59.5
    2018-11-02 128 46~120 51~80(69.5%) 73.3
    采样时间
    sampling time
    样本数量
    sample size
    体质量/g body weight 体质量分布情况/g
    distribution of body weight
    范围
    range
    优势体质量(%)
    dominant body weight
    平均值
    mean
    2016-09-03 259 1.4~84.3 0.1~20.0(86.5%) 13.0
    2016-09-20 587 2.2~173.3 0.1~40.0(89.3%) 22.2
    2016-10-25 471 7.4~318.2 20.1~100.0(69.0%) 81.6
    2016-11-16 51 10.2~382.6 40.1~140.0(72.0%) 96.3
    2017-09-01 498 1.2~100.0 0.1~40.0(97.2%) 13.3
    2017-09-25 1 095 3.3~185.0 0.1~60.0(93.7%) 26.8
    2017-10-18 158 12~186.6 20.1~100.0(76.6%) 63.3
    2017-11-18 87 11.3~198.5 40.1~140.0(72.0%) 106.3
    2018-09-19 316 3.2~165.2 0.1~60.0(83.5%) 37.5
    2018-10-10 60 5.5~173.7 0.1~40.0(83.7%) 46.1
    2018-11-02 128 34.2~280.2 20.1~100.0(70.3%) 70.1
    注:箱线图中,箱体宽度为中间50%数据分布区间,x为数据平均值,箱体中横线为中位数值,T形线宽度为整体数据分布区间(内限)
    Notes: the width of the box is the middle 50% data distribution interval, x is the average value of data, the transverse line in the box is the median value, and the width of the T-shaped line is the whole data distribution interval

    Table 2.  Distribution characteristics of mantle length and body weight of S. esculenta in 2016, 2017 and 2018

    Figure 3.  Mantle-length and body-weight frequency distribution histogram of S. esculenta in 2016, 2017 and 2018

    Figure 4.  Normal distribution of mantle length of S. esculenta in 2017

  • 金乌贼补充群体的时空分布格局以2017年统计结果为例,9月和10月近岸水域幼体密度较高且分布较随机,11月中旬,表层水温由9月初的26 °C降至14 °C左右,近岸水域资源密度明显减少,仅在灵山岛东南海域的个别站位捕到极少量金乌贼幼体(图5)。综合3年的调查数据分析认为,金乌贼补充群体广泛分布于青岛近岸海域,随幼体生长,活动能力增强及水温变化逐步开始由北向南,由近海向远海移动扩散;其密度在经度上的分布呈现出由西向东先增加再减小的趋势,在纬度上的分布呈现出由北向南先增加再减小的趋势。

    Figure 5.  The quantitative distribution of S. esculenta in the coastal waters of Qingdao in 2017

3.   讨论
  • 研究水生动物繁殖生态和种群结构,不仅涉及遗传进化、种群动态等诸多问题,还可为其人工繁育和资源保护提供充足的依据。通常生殖洄游期鱼类的分群现象最为明显,常以年龄或胴长分组循序进行[14],这主要与其性腺发育程度的差异有关。金乌贼虽为1年生头足类,但在青岛近岸水域产卵期长达2个月,幼体孵化期的差异是引起生长离散的主要原因,亦是导致繁殖亲体性腺发育不同步的重要因素;同时,营养水平是影响性腺发育的关键因子[15],不同规格金乌贼对于饵料的摄食能力及食物竞争能力的差异,亦可进一步导致性腺发育速率的差异。目前,金乌贼放流苗种主要通过捕捞野生亲体进行人工繁育获取。研究表明,虽青岛近岸金乌贼繁殖群体的规格相差较大,但遗传多样性水平及遗传结构相仿,可视为一个管理单元[16]。由于前期繁殖亲体较中、后期繁殖亲体具更多的卵子且卵径较大,其所育仔乌亦具更大的规格和更高的活力。因此,建议在人工苗种繁育时,可倾向对洄游前期大规格亲体的采集,在保证放流群体遗传多样性的同时亦能提高繁育效率,降低繁育成本。

  • 通过对3年补充群体监测数据的箱型图(boxplot)分析发现,金乌贼补充群体胴长和体质量的中间50%数据区间范围(箱体宽度)和数据整体区间范围(T形线宽度)均随取样时间的推移不断增大,表明补充群体胴长和体质量的离散水平随时间推移逐渐升高。由于2个月结群期内陆续洄游至近岸的金乌贼亲体胴长和体质量呈逐渐下降趋势,且其所育子代的规格亦呈现相似的变化规律。同时,雌性亲体多次交配、分批产卵,仔乌分批孵化的繁殖习性[17],共同导致补充群体规格具较高的离散水平,胴长和体质量分布频率的“峰值”坡度随时间推移逐渐变“缓”。而补充群体规格不一致导致翌年繁殖亲体性腺发育的不同步及游泳能力的差异,因此形成繁殖群体在近岸结群周期长,产卵繁殖期分散的交互循环过程。

    通常,生物为减少种内摄食压力会通过生长离散以谋求增大对饵料生物种类的捕食范围。金乌贼为一年生头足类,种群的胴长和体质量结构组成理应相对简单,但其群体组成却大小不一,这可能是金乌贼长期适应生存环境所形成的生活史策略。头足类摄食广泛且“投机主义”较强[18],随身体不断生长、捕食能力逐渐增强,其饵料组成亦发生相应变化,在其生活史过程中存在明显的食谱转换过程[19-21],符合最佳摄食理论。如张宇美[22]在研究南海鸢乌贼(Sthenoeuthis oualaniensis)食性组成时发现,胃含物中鱼类比例随胴长增加逐渐减少,而头足类比例不断升高;贾涛等[18]研究表明,茎柔鱼(Dosidicus gigas)幼体主要捕食小型甲壳类和头足类,而成体茎柔鱼则以鱼类作为主要食物源。由此分析,金乌贼较长的繁殖结群期及多次交配、产卵的繁殖习性,既能较好地避免不同规格亲体间的捕食、交配竞争,亦使得补充群体产生较大的胴长离散水平,通过不同发育阶段的食谱转换,从而有效降低种内摄食压力。由于金乌贼通过其发达的喙对食物切割,致使对其胃含物分析比较困难。深入探究金乌贼食性组成及不同发育阶段食谱转换过程,并进一步分析其种内食物竞争的缓解机制,对丰富金乌贼繁殖生态学认知及优化苗种放流模式均具有重要作用。今后相关研究可考虑运用DNA条形码技术定性鉴定饵料物种组成,并结合稳定同位素技术量化不同饵料生物的贡献比,以便全面了解金乌贼的摄食生态习性,并应用于其资源保护与修复。

  • 研究发现,金乌贼孵化后5个月胴长便可增长至120~140 mm,达最大规格的60%~70%,与董正之[23]报道的结果一致,均表明金乌贼幼体阶段生长迅速。大多头足类均具较快的生长速度[24-26],这主要可能与其生命周期较短有关。同时,头足类食性广泛[27],通过对食物较强的消化、吸收及转换能力亦为快速生长提供了有力保障[28]。仔乌孵化后,亲体产卵场成为仔乌育幼场,10月之前,幼体停留于青岛近海继续生长发育以储备越冬洄游所需的能量。而后,随时间推移、水温降低和运动能力增强,补充群体逐渐出现由青岛近岸向东、南深水区扩散的趋势,至11月中旬(水温约15 °C),灵山湾及其邻近海域已基本无金乌贼分布。孙峰德[29]在对青岛胶州湾金乌贼的研究中亦发现,10月湾内乌贼资源开始逐渐减少、转移并于11月离开胶州湾进行越冬,李嘉泳[9]的研究亦显示相近结果。

  • 根据金乌贼洄游习性,可分为离岸期和近岸人工养护期。5—7月为亲体近岸结群洄游期,此时可选用前期大规格亲体进行繁育,因其具更高的怀卵量、子代亦有较大的规格和较好的活力,既可有效提高亲体产卵率、降低繁育成本,亦能使仔乌提前适应海域环境;金乌贼产黏性卵,需在附着物(近岸海藻、礁石等)上产卵固定,但由于近岸产卵生境的丧失,人工投放附着基、营造产卵环境已成为一种重要的资源原位修复手段。在近岸水深15~20 m的缓流区,底层设置产卵附着基,可有效进行资源增殖[11];6—7月是苗种放流的主要时期,放流海域的饵料丰度是决定放流效果的重要保障,根据其繁殖习性和补充群体结构特征,建议在大规模苗种放流时开展分批放流,以减小同一放流水域的摄食压力;研究显示,饥饿对仔乌的形态、行为、活力等影响较大,且沉积物粒径较小的底质便于仔乌贴底潜伏,对于缓解仔乌饥饿也能发挥积极作用[7],因此,对放流海域的择优筛选也是提高放流群体成活率的有效手段(图6)。

    Figure 6.  Measures and suggestions for conservation of S. esculenta resources

    目前,浙北近海将受精卵放流作为曼氏无针乌贼(Sepiella maindroni)规模化增殖的主要方式[30],山东省金乌贼受精卵放流刚刚起步。与投放金乌贼产卵附着基相比,受精卵放流具有可调控附卵数量,提高附卵装置使用效率等优点,与苗种放流对比,受精卵放流能有效降低增殖成本并使幼体提前适应水域环境。因此,今后需对金乌贼受精卵放流的时间节点、放流装置和放流效果进行评估和验证,以便进一步优化金乌贼资源修复手段,增加其增殖方式的多样性。

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