01 / Itoyo
Sticklebacks that never go to seabuild nests on the bottom of a pond.
The itoyo (イトヨ, three-spined stickleback) grows up in the sea and swims upriver to spawn, but the fish of Hongan Shōzu (本願清水) never go down to the sea. From around May into autumn, the males build nests on the pond bottom and guard the young. They need spring water that stays below 20°C even in summer, and the city calls this the southern limit of the landlocked form. The pond is said to have been dug by Kanamori Nagachika (金森長近) as the castle town's water source.
- National Natural Monument, 1934
- Southern limit of the landlocked form
- Water below 20°C
