Abstract
Algal vulnerability to the grazing loach, Niwaella delicata, was investigated in the Ogawa stream, a tributary of the Kiso River flowing through Nagano Prefecture, Japan. To evaluate the vulnerability, Chesson's electivity index was calculated using the relative abundance of ten dominant algal taxa in the gut contents of the loach and benthic algal assemblage. The index values differed significantly among the algal taxa and the index values of diatoms increased with their size. The results showed that vulnerability to the loach differed among algal taxa. Prostrate filamentous cyanobacteria and the large diatom, Ulnaria ulna, were susceptible to loach grazing, while upright filamentous cyanobacteria and small diatoms such as Achnanthidium convergens, Fragilaria perminuta, Gomphonema parvulum, Cymbella turgidula var. nipponica and Encyonema sp. were less susceptible to loach grazing.
