Not to be too precise, but that is not a "Sewer", that is a storm drain. And if close enough to a retention pond it could hold fish. But, if it is holding enough water then its just an example of poor drainage in that neighborhood and could mean it is subject to flooding.
Only thing you's catch out of a sewer is rats and cockroaches. Trust me, you won't be finding any fish, alligators, or teenage mutant turtles in a real sewer.
Yep. Its also commonly called "storm sewer". Im a civil engineer, and I design storm drains almost on a daily basis. We commonly call these "storm drains" and "storm sewer" interchangeably. That manhole cover likely has "City of ___ Storm Sewer" stamped on it.
There could easily be water holding in there due to a number of reasons.
The likely scenario is there has been some sedimentation (trash & dirt) built up in the storm drain system. When the downstream pond rises during a large storm event, water can back up into those storm drains (often designed this way on purpose), then fish could swim up into the pipes. Once the water recedes, the fish are stuck behind walls of sedimentation (acting like a dam in the pipes).
Its not that unlikely.