Fish habitat associations along a longitudinal gradient in a preserved coastal Atlantic stream, Brazil

ABSTRACT Habitat conditions at multiple scales are one of the major factors structuring ichthyofauna. Thus, we analyzed the fish habitat associations along the headwater-mouth gradient of a coastal Atlantic stream. We categorized the sampling sites into habitat units, so that in the middle reach these categories were statistically differentiated into riffles, runs and pools. Samplings were carried out quarterly from May 2009 to February 2010 using electrofishing. Principal component analysis (PCA) indicated an environmental gradient from higher water velocity and rocky bottom to deeper and sandy areas in the headwater-mouth direction. A total of 1,495 individuals belonging to 27 species were captured, being 13, 18 and 22 from headwater, middle and mouth reaches, respectively. Shannon diversity was slightly higher in the middle reach, while beta diversity showed higher rates of addition than turnover in species along the longitudinal gradient. Fish structure, evaluated by DCA (detrended correspondence analysis) scores, showed significant differences between upper reaches and mouth reach, but the middle riffles did not differ from headwater habitats. In the middle reach, mesohabitat analysis distinguished riffles, with higher abundance of fast-water crenuchids, from pools, with a higher abundance of lentic-water characids. These results suggest that environmental differences along the stream determine the wider structural patterns. However, the middle reach amassed species from upper areas and lowlands in structured fish mesohabitat associations, possibly implying distinct local ecological interactions. These findings contribute to the assessment of stream conservation status and to recognize eventual direct impacts on fish structures along longitudinal gradients.摘要 多尺度栖息地条件是塑造鱼类区系（ichthyofauna）结构的核心驱动因素之一。为此，本研究针对大西洋沿岸一条溪流的上游-河口梯度，开展了鱼类栖息地关联特征分析。本研究将采样点位划分为栖息地单元，其中中游河段的点位经统计学区分后，可归类为急流浅滩（riffles）、平流区（runs）与深潭（pools）三类。本研究于2009年5月至2010年2月间按季度开展采样，采用电渔法（electrofishing）进行数据采集。主成分分析（PCA）结果显示，沿上游至河口的方向存在一条环境梯度：从高流速、石质底质的区域，逐步过渡至水深更深、底质为沙质的区域。本次采样共捕获1495尾个体，隶属于27个物种；其中上游河段、中游河段与河口河段的物种数分别为13种、18种与22种。香农多样性指数（Shannon diversity）在中游河段略高；而沿纵向梯度来看，β多样性（beta diversity）的物种新增率高于物种更替率。通过去趋势对应分析（detrended correspondence analysis, DCA）得分评估的鱼类群落结构显示，上游河段与河口河段的群落结构存在显著差异，但中游的急流浅滩生境与上游栖息地并无显著区别。在中游河段，中型生境（mesohabitat）分析可区分出两类生境：急流浅滩中栖息着丰度更高的急流型溪鳉类（crenuchids），而深潭生境则以静水环境脂鲤类（characids）占比更高。上述结果表明，溪流沿线的环境差异决定了鱼类群落结构的整体分布格局。不过，中游河段汇聚了上游与低地区域的物种，形成了具有明确结构的鱼类中型生境关联群落，这或许暗示当地存在独特的物种间生态互作关系。本研究结果可为溪流保护现状评估提供参考依据，同时有助于识别沿纵向梯度对鱼类群落结构造成的潜在直接影响。