黄海中部小黄鱼的食物组成和摄食习性的季节变化

根据2001年3月至2002年1月在黄海中部海域进行的4个季节的定点底拖网调查,应用聚类分析、单因素方差分析和列联表检验等方法,对小黄鱼(Pseudosciaena polyactis Bleeker)的食物组成和摄食的季节变化进行研究.结果表明,小黄鱼摄食的饵料种类有30余种,甲壳类(磷虾类和虾类)和鱼类是其主要的饵料类群,二者在食物中所占的重量百分比之和为97.45%.优势饵料种类有太平洋磷虾(Euphausia pacifica)、脊腹褐虾(Crangon affinis)、细螯虾(Leptochela gracilis)和赤鼻棱鳀(Thryssa kammalensis).小黄鱼的摄食强度有显著的季节变化,秋季最高,春季和冬季较低.食物组成也随季节的不同而有所变化,夏季主要以虾类为食,其它季节则主要以磷虾类为食.聚类分析的结果表明,小黄鱼春、夏季的食物组成与秋、冬季相比,存在较大差异.通过与历史资料进行比较发现,黄海小黄鱼的食物组成发生了较大的变化,其中鳀鱼(Engraulis japonicus)在食物中所占的比例有明显的下降.

Diet composition and seasonal variation in feeding habits of small yellow croaker Pseudosciaena polyactis Bleeker in the central Yellow Sea

Small yellow croaker, Pseudosciaena polyactis (Bleeker), is an important bottom fish distributing from the northern East China Sea to the Bohai Sea. The central Yellow Sea is its important feeding and overwintering ground. Small yellow croaker is highly exploited in the Yellow Sea and plays an important role in the marine ecosystems. In spite of its high abundance and importance to fisheries, only limited information is available on its feeding ecology. In the present papers, the diet composition and seasonal variations in the feeding of small yellow croaker in the central part of the Yellow Sea were examined to provide more quantitative information on the feeding ecology. A total of 1 688 specimens of small yellow croaker at standard length (SL) of 80-178 mm were sampled from seasonal bottom trawl surveys by the R/V 'Bei Dou' in the central part of the Yellow Sea from March 2001 to January 2002.Based on stomach contents analysis, the small yellow croaker was a generalist and more than 30 prey species were ingested. Crustaceans were the most important prey groups, occupying 92.76% of the stomachs with food and accounting for 77.44% of the total food by weight. Among the crustaceans, euphausiids made an important contribution to the diet, followed by decapods. At the species level, Euphausia pacifica was the most frequent prey, followed by Themisto gracilipes, Leptochela gracilis, Calanus sinicus and Crangon affinis. Fishes were the second most important prey group. The most important fish prey was Thryssa kammalensis. The relative importance of cephalopods, sagitta and polychaetes was comparatively low.The feeding intensity, as indicated by the percentage of empty stomachs (PES) and mean stomach fullness index (MSFI), varied significantly among seasons. The feeding activity was the highest in autumn and the lowest in spring and winter. A chi-squared test revealed that there were significant seasonal differences between the ingestion of euphausiids, amphipods, copepods and "others". No significant differences were detected between fish and decapods. Decapods were more important in summer, whereas euphausiids were more important during other seasons. Cluster analysis employing Bray-Curtis similarity index revealed two major groups, linking at 48.46% similarity: one group consisted of specimens sampled in spring and summer (average similarity 56.12%), while the other group included autumn and winter samples (average similarity 60.82%). At species level, E. pacifica, L. gracilis, C. affinis and Apogon angustatus dominated the diet in spring and summer, whereas T. kammalensis, C. sinicus and T. gracilipes were the dominant preys in autumn and winter. The observed selection for some prey items coincides with an increased abundance of these preys in the environment.Compared with historical data in the mid-1980s, the diet composition of small yellow croaker in the Yellow Sea varied significantly. The proportion of a small pelagic fish, Engraulis japonicus, in the diet markedly decreases and the reduced importance of E. japonicus seems to be offset by other abundant preys in the habitat, such as E. pacifica, L. gracilis and C. affinis. Direct surveys to estimate E. japonicus abundance in the Yellow Sea indicates that its biomass has declined sharply since 1999 due to the high fishing pressure. The decreased proportion of E. japonicus in the diet of small yellow croaker is consistent with the decline of its biomass in the Yellow Sea.