Dietary variation and food selection by an algivorous loach (Pseudogastromyzon myersi: Balitoridae) in Hong Kong streams

Abstract

The algivorous balitorid loach Pseudogastromyzon myersi is abundant in Hong Kong streams where it may exert top-down control on benthic algal assemblages as reported for grazing fish in temperate and neotropical streams. Dietary selectivity by P. myersi was investigated in two shaded and two unshaded streams during the wet and dry seasons, thereby allowing for variation in the potential bottom-up influences of light and flow on algae. Fish stomach contents were compared with the benthic algal assemblages to assess selectivity and to test whether algal growth form influenced susceptibility to grazing. Diatoms and filamentous cyanobacteria dominated the diets of the fish, regardless of stream or season; most of the remainder of the stomach contents comprised fine particulate organic matter (FPOM). Stalked diatoms (Gomphonema) and filamentous cyanobacteria (Homeothrix) were the most important dietary items, with the former selected during the dry season and the latter in the wet season. Adherent diatoms (Achnanthes) were underrepresented in the diet, and filamentous chlorophytes were rarely eaten. Seasonal changes in diet were minor. Interstream variations reflected differences in the proportions of Gomphonema, Homeothrix and FPOM ingested, and were unrelated to shading. Grazing by P. myersi may influence algal composition and productivity by removing loosely attached diatoms and facilitating growth of filamentous cyanobacteria.