长江口两种仔稚鱼网具的采集效率比较

Comparative analysis of ichthyoplankton samping gear for early life stages of fish in the Yangtze River estuary and adjacent areas

为获得更为准确的数据,提高对长江口鱼类早期资源评估的准确度,本研究同时采用方形网(网口为1.0 m×2.0 m,网目1.0 mm,网衣长5.5 m)和标准网(网口为直径0.8 m圆形,网目0.5 mm,网衣长3.5 m)于2014年4—7月在长江口邻近海域采集仔稚鱼样本,分析比较这2种网具所采集仔稚鱼种类和数量差异,从而选择更适宜采集长江口水域仔稚鱼的采样网具。结果表明:1方形网采集到51种仔稚鱼,多于标准网的37种,两种网具的共有种类有34种,主要优势种组成没有明显差异;24月,前弯曲期仔鱼较多,它们个体小,容易从网眼滤出,导致网目较小的标准网的样品平均密度高于方形网;35—7月,方形网的样本平均密度高于标准网,但样本中仔稚鱼个体发育阶段组成有明显不同。标准网样本中前弯曲期仔鱼占多数,体形为细长形的日本鳀和鳅虎鱼的前弯曲期仔鱼平均密度高于方形网;方形网样品中后弯曲期仔鱼占优势,小黄鱼、日本鳀和鳅虎鱼等优势种的后弯曲期仔鱼平均密度均高于标准网。总体而言,方形网所采集仔稚鱼种类和数量的效果优于标准网;对于细长体形种类(如日本鳀),需结合不同网具以便采集到全部发育阶段个体,而体形较宽种类(如小黄鱼)可仅以方形网采集。

Although it is common sense that the accurate sampling and unbiased data acquired are essential bases for further evaluation, during the field work of ichthyoplankton study, samples from conventional single gear are highly possible to deviate from the population greatly. To choose the more efficient sampling gear, comparative gear tows were conducted from April to July of 2014 in the Yangtze River estuary and adjacent areas with focus on species composition, density estimates（ind./100 m3）, developmental stage composition of larval fish assemblages.Larval fish were collected by the plankton net（0.5 mm mesh） and frame net（1.0 mm mesh）, respectively. Results indicated that the species richness was higher in the catches of frame net（51 species） than that in plankton net（37species）, along with 34 mutual species between them, while the composition of dominant species showed no significant difference. There was an unpredicted bias in average density of catches between two gears. In April, the average density was higher in catches of plankton net, due to the abundant occurrence of pre-flexion larvae which were too tiny to be efficiently caught by large mesh. In contrast, the average density was higher in frame net than those in plankton net from May to July. The plankton net samples were dominated by pre-flexion larvae, and the average density of pre-flexion larvae of anchovy（Engraulis japonicus） and gobies（Gobiidae spp.） was higher in catches of plankton net. While post-flexion larvae were dominated in frame net, and the average density of postflexion larvae of the dominant species, which were small yellow croaker（Larimichthys polyactis）, anchovy and gobies, was higher in frame net samples. These differences suggested that there was significant catch efficiency between the two nets. In conclusion, frame net is more efficient. However, when it comes to specifically collect fishes of elongated type（e.g. anchovy）, combination of multiple types of sampling gears should be in consideration. Conversely, an unique frame net is highly adequate for broad type fishes（e.g. croakers）.