Trophic structure of the fish community throughout the reservoirs and tributaries of the Middle and Lower Tietê River (São Paulo, Brazil)

Abstract Aim: We investigated the use of food resources throughout the reservoirs and tributaries of the Middle and Lower Tietê River. Methods The samplings were made in July 2000 (dry period) and January 2001 (rainy period). The fish samples were collected using two batteries of gill nets consisting of 8 nets with 10 meters in length and 1.5 meters wide with different mesh sizes, dip net and dragging nets. The stomach contents were dissected, the portion studied was the stomach and the anterior third of the intestines. Food remains found in the stomach were analyzed under a stereoscopic microscope and its identification was performed to the lowest taxonomic category. Results The analysis of stomach contents returned 30 types of food sources, being 21 autochthonous and 8 allochthonous. Most of the diet of the species in the tributaries consists of terrestrial insects, followed by vegetal material, seeds, crustaceans and mud, whereas in the reservoirs the main diet consists of aquatic insects, vegetal material, seeds and scales. The species of fish analyzed were grouped into 13 trophic categories. In both reservoirs and tributaries, most of the diet was carnivorous, whereas the omnivorous diet occurred only in the reservoirs. Species from rivers are more specialized and in reservoirs they are more generalist, making it a better survival strategy. Conclusions Fishes use the coastal areas of the reservoirs and even of the tributaries to feed on superior plants and insects that have terrestrial and aquatic stages. The trophic characteristics exhibited by the fish in the reservoirs depend on the structuring of the community during the colonization process, the characteristics of each reservoir, the introduction of exotic species of fish and alternative food sources.

研究目的：我们针对蒂特河中下游库区及其支流的食物资源利用情况展开了调查。
研究方法：采样工作分别于2000年7月（旱季）与2001年1月（雨季）完成。鱼类样本采集采用两套刺网组合，每套包含8张网具，单网长10米、宽1.5米，网目尺寸各不相同，同时辅以抄网与拖网。对鱼类胃含物进行解剖分析，取样部位为胃及肠道前1/3区段。胃内检出的食物残渣经体视显微镜观测，并被鉴定至最低分类学等级。
研究结果：胃含物分析共识别出30类食物来源，其中21种为内源(autochthonous)食物，8种为外源(allochthonous)食物。支流中鱼类的主食构成以陆生昆虫为主，其次为植物材料、种子、甲壳类与泥沙；而库区鱼类的主要食物则为水生昆虫、植物材料、种子及鱼鳞。本次分析涉及的鱼类物种可划分为13个营养类群。无论库区还是支流，多数鱼类的食性为肉食性，而杂食性仅在库区鱼类类群中出现。支流鱼类的食性更具特化性，库区鱼类则食性更泛化，这属于更为适宜的生存策略。
研究结论：鱼类会利用库区乃至支流的沿岸区域，取食兼具陆生与水生生活史阶段的高等植物与昆虫。库区鱼类所展现的营养生态特征，取决于群落拓殖过程中的群落结构、各库区自身的环境特征、外来鱼类物种的引入以及替代食物资源的供给情况。