It is widely believed that attached diatoms are the best food source for freshwater ayu Plecoglossus altivelis (Temminck et Schlegel) in Japan. However, sensory testing by a panel consisting of 40 untrained tasters and analyses of gut content showed that there was no significant correlation between preference scores for fish taste and the dominance of diatoms in the algal assemblages fed upon by ayu in the Yahagi River, Aichi Prefecture, Central Japan. Ayu that fed on the filamentous blue-green

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... algae Homeothrix varians was evaluated higher for aroma and fat content than those that fed on diatoms. Ayu feeding on diatoms were collected in the lower reaches of the Yahagi River which receive domestic sewage. In the absence of more detailed analyses, exposure to sewage is considered to confer a characteristic aroma on Ayu.