Water-level fluctuations lead to changes in the diet of an omnivorous fish in a floodplain

Abstract The hydrological regime leads to fluctuations in the availability of food resources for fish, which may reflect in their diet variation. Our main goal was to analyze the relationship between water-level fluctuations and the diet and the body condition of Trachelyopterus galeatus in the Upper Paraná River floodplain. The fish used in the study were sampled quarterly along nine years, at nine sampling stations. Our results showed that diet of T. galeatus varied in response to oscillations in the hydrometric level, with the consumption of terrestrial invertebrates being positively related to hydrometric level, while the consumption of aquatic invertebrates was negatively related. The trophic niche breadth and feeding activity were not affected by hydrometric level, but the fish body condition was positively affected. The high food plasticity allows T. galeatus to consume the most available resources (e.g., terrestrial invertebrates in high water levels), which probably contributed to the increase in its body condition in higher hydrometric levels. In summary, the hydrological dynamic is fundamental for the diet variation of omnivorous fish, which can take advantage of the resource availability according to the hydrometric level, increasing its body condition when allochthonous resources as terrestrial invertebrates are supposed to be more available.

摘要
水文情势（hydrological regime）会引发鱼类食物资源可获得性的波动，而这类波动可通过鱼类的食性变化体现出来。本研究以巴拉那河上游泛滥平原（Upper Paraná River floodplain）中的粗盔鲶（Trachelyopterus galeatus）为研究对象，核心目标为分析水位波动与其食性及身体状况之间的关联。本研究的鱼类样本于9年内每季度在9个采样站点采集得到。
研究结果表明，粗盔鲶的食性会随水文水位（hydrometric level）的波动发生改变：陆生无脊椎动物的摄食占比与水文水位呈正相关，而水生无脊椎动物的摄食占比则呈负相关。营养生态位宽度（trophic niche breadth）与摄食活动并未受到水文水位的影响，但鱼类的身体状况却得到了正向提升。较强的食物可塑性（food plasticity）使得粗盔鲶能够优先取食最为易得的食物资源（例如高水位时期的陆生无脊椎动物），这或许是高水文水位条件下其身体状况得以改善的原因。
综上，水文动态对于杂食性鱼类的食性变化至关重要；这类鱼类可根据水文水位状况灵活利用可获取的食物资源，当陆生无脊椎动物这类外源资源（allochthonous resources）更为易得时，其身体状况也会随之提升。