Polymorphic spawning-nest types in a local population of the fluvial sculpin, Cottus nozawae (Teleostei: Cottidae)

The nesting behavior of a fluvial sculpin Cottus nozawae population was investigated in a stream–reservoir system on the Sainai River, northern Honshu Island, Japan in 2003, 2004 and 2006. Except for the usual rock nest-spawning type (RSN), two new types of spawning nest were found: hole nest spawning (HNS) and crevice nest spawning (CNS). The HNS type was characterized by males digging one or more nest holes in the wall of the banks of the stream and reservoir, or on the sandy silt bottom of the reservoir. The CNS type involved the use of a crevice between large boulders or crevice of crag for their nests. The HNS type was dominant in the reservoir section, whereas the RNS type was dominant in the rapid riffles of stream sections. The HNS and CNS types appeared to be rarely or first observed in the Cottus species. These new nest-spawning types might have occurred in relation to the artificial environment changes in the river by dam construction.