One of our specialty areas includes the design, installation, and repair of dry fire hydrants. We often get called in to assess why a dry fire hydrant does not perform at its desired flow rate...or in some cases does not flow at all. In the vast majority of cases, a dry fire hydrant that once flowed but no longer flows has some type of obstruction... commonly sediment. Occasionally, we find fish living in the dry fire hydrant pipe which is a problem not easily corrected.
There are a couple of ways that fish get into a dry fire hydrant pipe. The first suspect is the strainer. When large fish are found in the piping system then the strainer is probably missing or damaged in some manner to allow fish to enter the pipe. The second suspect is strainer design where small fish enter the pipe to live in the protected area and grow too big to swim back out of the strainer. If the strainer does not have a back flush opening (flap) then those fish are going to get sucked up against the suction head screen and cause a flow obstruction.
Unfortunately, when fish are found as the obstruction problem in a dry fire hydrant the strainer probably needs changed out...which means lowering the water level in the water source or completing the strainer change-out underwater. Neither of those solutions is easy; both must begin with a total back-flush of the system once the old strainer is removed and before the new one is installed. All of our new, dry fire hydrant installations use suction strainers that have a built-in back-flush feature.