Composition, abundance and biomass of a fish assemblage in a southern Brazilian coastal stream during polyhaline/euhaline condition

Abstract: The coastal streams of southernmost Brazil, Rio Grande do Sul state, are marked by a period of regular marine intrusion resultant from intense oceanic winds. In the present study we aimed to investigate the species composition, abundance and relative biomass of the ichthyofauna in the lower stretch of a coastal stream during summer, a period of regular marine intrusion. Estreito is a coastal hydrological complex composed by lakes, swamps and a perennial stream, located at the central-south portion of the coastal plain of Rio Grande do Sul state. During the summer of 2018, the ichthyofauna of lower Estreito stream was sampled by beach hauls applied in 17 random points distributed in a stretch of ca. 2km. Measurements at the sampled stretch revealed salinities between 19.3 to 31.3 ppt, characterizing the studied system as polyhaline/euhaline during summer. The sample of 4,533 specimens revealed the occurrence of 20 species, being the great majority marine-dwelling. The most abundant species were the anablepid Jenynsia lineata (70.3%), the cichlid Geophagus brasiliensis (19.3%) and the the mugilid Mugil curema (7.5%). The highest relative biomass was recorded for J. lineata, followed by M. curema and G. brasiliensis. The dominance of marine-dwelling species in the assemblage composition and the high abundance of limnic-estuarine J. lineata corroborate previous studies conducted in other washouts of Rio Grande do Sul.

摘要：巴西南部最南端的南里奥格兰德州（Rio Grande do Sul）沿岸溪流，因强烈的海洋风作用，会出现规律性的海水入侵时段。本研究旨在探究夏季——即该类沿岸溪流的常规海水入侵时段——目标河段的鱼类区系组成、丰度及相对生物量。
埃斯特雷托（Estreito）是一处由湖泊、沼泽与常年性溪流组成的沿岸水文复合体，坐落于南里奥格兰德州沿岸平原的中南部区域。2018年夏季，研究人员采用岸滩拖网法，在埃斯特雷托溪下游约2公里河段内布设的17个随机采样点开展鱼类采样。采样河段的盐度测定结果显示其范围为19.3~31.3 ppt，表明该研究区域在夏季属于高盐水（polyhaline）/真盐水（euhaline）生态系统。
本次采样累计获取4533尾鱼类标本，涵盖20个物种，其中绝大多数为海洋栖息物种。丰度位居前三的物种分别为四眼鱼科（Anablepidae）的线纹詹氏鱼（Jenynsia lineata，占比70.3%）、慈鲷科（Cichlidae）的巴西珠母丽鱼（Geophagus brasiliensis，占比19.3%）以及鲻科（Mugilidae）的墨西哥真鲻（Mugil curema，占比7.5%）。相对生物量最高的物种仍为线纹詹氏鱼，其次为墨西哥真鲻与巴西珠母丽鱼。
本研究中鱼类群落以海洋性物种为主，且淡水河口生境的线纹詹氏鱼丰度极高，这一结果与南里奥格兰德州其他沿岸入海通道的既往研究结果一致。