Late Pleistocene divergence and subsequent population expansion of two closely related fish species, Japanese anchovy (Engraulis japonicus) and Australian anchovy (Engraulis australis)

Climatic oscillations during the Pleistocene ice ages produced great changes in species' geographical distribution and abundance, which could be expected to have genetic consequences. Living in the temperate upwelling zones of the northwestern Pacific, Ja

Spatial distribution of ciliates, copepod nauplii and eggs, Engraulis japonicus post-larvae and microzooplankton herbivorous activity in the Yellow Sea, China

The abundance of anchovy Engraulis japonicus larvae, >20 mum ciliates, copepod eggs and nauplii, and microzooplankton herbivorous activity were studied in the Yellow Sea in June 2000. Anchovy juveniles and larvae were found in only 6 of the 19 stations sampled. The ciliate communities were dominated by 2 species: Laboea strobila and Strombidium compressum. In the surface waters, the abundance of L. strobila ranged between 0 and 560 ind. l(-1). S. compressum only appeared at Stns 15 to 18 (20 to 3300 ind. l(-1)). L. strobila was found mainly in the top 20 m. The abundance of L. strobila was less than 50 ind, l(-1) in waters deeper than 25 m. S, compressum showed subsurface abundance peaks at the salinity abnormality. Tintinnids occurred occasionally with abundance lower than 100 ind. l(-1), The total ciliate abundance fell in the range of 40 to 3420 ind. l(-1). The ciliate biomass in the surface water and the water column ranged between 0,15 and 6.76 mug C l(-1) and 0.4 and 134.4 mg C m(-2), respectively, In the surface waters, the abundance of copepod eggs and nauplii ranged from 0,3 to 3.1 and 1,1 to 15.6 ind, l(-1), respectively. The average abundance of copepod eggs and nauplii in 4 depth (0, 5, 10 and 20 m) fell in the range of 0.2 to 2.8 and 1.0 to 29.4 ind. l(-1), respectively. As a food item of the E. japonicus post-larvae, the abundance of copepod nauplii and eggs appeared to be low. The abundance peaks of ciliate and E, japonicus post-larvae coincided. Although not found in the gut of E, japonicus post-larvae, aloricate ciliates might be ingested by first-feeding anchovy larvae, preventing initial starvation and prolonging the time to irreversible starvation. On the basis of dilution experiments with positive microzooplankton grazing rates, microzooplankton grazed at rates of 0 to 0.61 d(-1). Grazing pressure of microzooplankton on chlorophyll a standing stock (P-i) and potential chlorophyll a primary production (P-p) were 17 to 46% and 35 to 109% d(-1), respectively.

长江口及其邻近海域鱼类浮游生物群落结构特征研究

本文根据1999年－2007年间7个航次的长江口及其邻近海域的鱼类浮游生物和环境因子的调查资料，探讨了三峡工程蓄水前后长江口鱼类浮游生物群落特征，主要包括长江口鱼类浮游生物的种类组成、数量变动、群落多样性、空间分布格局等，分析了三峡工程蓄水前后长江口鱼类浮游生物群落结构的动态变化，为评价三峡工程对河口生态环境及鱼类补充资源的影响提供科学依据，研究结果如下： 1．三峡工程蓄水前春季（1999年5月和2001年5月）共捕获11540个鱼类浮游生物隶属11目18科32种，其中凤鲚(Coilia mystus)、鳀(Engraulis japonicus)、六丝矛尾鰕虎鱼(Chaeturichthys hexanema)、白氏银汉鱼(Allanetta bleekeri)、松江鲈(Trachidermis fasciatus)为优势种；鱼类浮游生物群落可划分为三个类群：①以凤鲚为优势种的河口群组，②以松江鲈和白氏银汉鱼为优势种的沿岸群组，③以鳀和六丝矛尾鰕虎鱼为优势种的近海群组；盐度、深度、溶解氧和悬浮体是影响这一海域鱼类浮游生物分布的主要环境因子。 2．三峡工程蓄水后春季长江口的环境因子总体分布格局没有改变，部分环境因子表现出数量上的差异：溶解氧含量偏低，COD平均值下降，pH值显著上升，盐度升高，河道水域和口门附近悬浮体下降，长江口鱼类生物群聚栖息环境发生了变化。三峡工程蓄水后春季（2004年5月和2007年5月）共采获的25种鱼类浮游生物，总丰度(1 070个)仅为蓄水前(11 540)的10%，4种优势种白氏银汉鱼、凤鲚、松江鲈和鳀贡献了总丰度的94.57％；蓄水后影响长江口鱼类浮游生物格局的主要环境因子是盐度和悬浮体，蓄水后长江口鱼类浮游生物群聚类型(河口型、沿岸型和近海型)与蓄水前一致，但沿岸型空间分布区域明显减少，近海型分布区域向河口方向扩展。长江口环境因子中，盐度和悬浮体是蓄水前后鱼类浮游生物群聚变异的主要影响因素。 3．三峡工程蓄水前后春季四次调查共记录鱼类浮游生物39种，生态类型包括淡水、半咸水、沿岸和近海。春季长江口鱼类浮游生物优势种包括鯷、凤鲚、白氏银汉鱼、松江鲈和六丝矛尾鰕虎鱼，2004年和2007年鯷、凤鲚优势地位有所下降。长江口鱼类浮游生物群落结构年度间差异显著，以2004年长江口鱼类浮游生物群落结构与1999和2001年相异性最高，鯷和凤鲚种群数量变动对其群落变异的贡献率最大，群落相似性以2004和2007年最高，达71.17%。与蓄水前相比，三峡水库蓄水后的长江口鱼类浮游生物群落多样性显著下降。1999—2007年间，长江口发生了一系列环境环境条件变化，直接或间接的环境干扰带来了长江口鱼类生物群落结构变异。 4．2007年四个航次（春、夏、秋、冬）共捕获鱼类浮游生物6,463个, 隶属12目28科45种, 以鳀科丰度最高, 占总丰度的76.5%。长江口鱼类浮游生物包括4种生态类型: 淡水型(2种), 半咸水型(14种), 沿岸型(10种)和近海型(16种)。其中, 近海型物种个体数量最多, 占2007年长江口鱼类浮游生物总丰度的74.55%。鳀在4个季节均有出现, 对长江口鱼类浮游生物丰度贡献最大。鱼类浮游生物在物种组成上存在明显的季节变化: 白氏银汉鱼在春季占绝对优势, 鳀是夏季优势度最高的种类, 秋季除鳀外, 前颌间银鱼(Hemisalanx prognathus)成为优势种; 鱼类浮游生物丰度高度集中在优势种上, 各季节优势种对总丰度贡献超过89%。群落物种数量、丰度、丰富度指数、均匀度指数和多样性指数, 在夏季最高, 春季和秋季次之, 冬季最低。2007年长江口鱼类浮游生物的群落格局在空间上分为三个不同的群组：河口、沿岸、外海，并且这种分布随长江冲淡水的季节变化而变化。盐度是影响决定长江口鱼类浮游生物群落空间分布的最重要因子，而温度则是影响其季节性结构变化的主要因子。此外，生殖洄游和索饵洄游也是影响鱼类浮游生物群落结构时间变化的重要因子。