Weak Effects From Human Impact on Composition and Trophic Structure of Fish Communities in a Tropical Lowland River System

ABSTRACT Myanmar is a biodiversity hotspot and holds at least 570 different freshwater fish species with a high amount of endemism. Nevertheless, freshwater systems in Myanmar are highly threatened by anthropogenic perturbation such as pollution. We examined impacts on streams from nutrient enrichment and catchment degradation on the fish community composition and trophic structure. We studied 12 stream sites, and treated these as two groups (six impacted vs. six less impacted), as well as covering a gradient in degradation. We did quantitative electrofishing, collected potential food resources, invertebrates and fish for stable isotope analysis of δ 13 C and δ 15 N, and measured a number of environmental site variables. To determine fish diversity, we extracted DNA from tissue samples of 193 specimens, thereby identifying a total of 44 different species from 20 families. The most impacted sites had higher nitrogen concentrations and lower minimum O 2 saturation. Fish biomass m −2 increased with degradation as it correlated positively with total nitrogen concentration and negatively with minimum O 2 saturation. Fish communities did not differ significantly between the two groups nor across the environmental gradient, except that pairwise similarities were negatively related to differences in catchment forest cover. Likewise, trophic niches of the fish did not differ significantly between the two groups nor across the environmental gradient, but fish at the more impacted sites had significantly lower minimum δ 15 N signal and higher δ 15 N range compared to the least impacted. The tropical fish assemblages studied were diverse and showed overall weak response to human impact. Likewise, there was little indication of differences in tolerance to environmental stress among taxa. Thus, in contrast to previous studies on macroinvertebrates in the same region, our results suggest that fish communities are less suitable as bio‐indicators of environmental degradation and biodiversity loss in these tropical lowland streams.