Larval fish are likely to be vulnerable to rapid flow and predation, and need favorable nursery site for their safe growth. Larval fish of most species require zooplankton as primary diet, however, few zooplankton (Daphnia, Rotifer, etc) are in running water. So, larval fish in a river have to depend on phytoplankton (attached algae). Attached algae grow anywhere on the substrate in running water, though their species composition is different among locations. Therefore, I tried to evaluate the quality of nursery site as feeding site of larval fish by comparing the digestive tract content and the algae resource.

Investigation was conducted in Ado River in Shiga prefecture, Japan, from July to September, 2010. Four sites (in river) selected were artificial construct (slow riffle with groundsills as spur dike), pool (shallow and stagnant water), sandbar (shallow and stagnant water, muddy bottom), reed zone (slow run water, surrounded by reed belt).

Attached algae were most abundant on groundsill of artificial construct, and larval fish in this site depended on attached algae on groundsill. In contrast, larval fish in reed zone took the least amount of food (algae). Artificial constructs are usually said to damage the natural environmental, but, it is plausible that artificial structure service good feeding site for larval fish.