Looking at the rules as written it seems slightly abusive to me that along a river length of twelve miles people could establish and maintain a stable enough population of fish to feed approximately 250 people every month, year after year. The more implausible factor is that this can be done every twelve miles along the same body of water.
Throwing in the ideas of waste disposal and resource consumption this seems a bit much.
I am toying with the idea of making a Fishery style building for use with cities (limit 1 per city) that reduces consumption by 1 plus an additional 1 per adjacent hex with a water resource.
Am I over thinking this? Is there some justification to my concerns or are they overblown considering what could be achieved with basic magic in a Golarion-tech-level society?
I would just make a ruling and move on, but I have one player pitching and moaning about how I'm 'cheating them out of Consumption reduction' if I think of changing this rule. (Or the rule that you can build a sawmill in every forest hex and a quarry/mine in every mountain hex.)
The rules are pretty abstract, and I imagine they assume that people aren't just eating fish. That they have some goats and chickens, a small garden plot, etc...
And anyway, 250 isn't actually that many people. You could fit that many people into the bleachers at a small school gymnasium and have a lot of elbow room. So a twelve-mile stretch of a decent sized river doesn't seem that unreasonable to me.