Q:

Does "hatcheries" mean "a farm where eggs are hatched under artificial conditions, especially those of fish or poultry"?

Ok, so 

A hatchery is a facility where eggs are hatched under artificial
  conditions, especially those of fish or poultry - Source (Wikipedia)

But I often see people use terms like "Fish hatcheries" or "Poultry hatcheries".
I am wondering why people use the plural "hatcheries" in this case.
So, does "hatcheries" mean "a hatchering farm where eggs are hatched under artificial conditions, especially those of fish or poultry"?
And of course, it is "a hatchering farm" because "it has many facilities where eggs are hatched under artificial conditions, especially those of fish or poultry"
The online dictionary only has the definition of the singular "hatchery", no definition of the plural "hatcheries".
Also, what is the word fitting to the meaning of "a hatchering farm where eggs are hatched under artificial conditions, especially those of fish or poultry" ?

A:

Hatcheries is plural and designates multiple facilities; if you're speaking of a single facility you use the singular hatchery.
What is probably confusing you here is that singular hatchery may be used to designate either a single structure (or pool or henhouse or whatever) where hatching is carried out, or a collection of such structures at a single place.
This ambiguity is very common. An institution you refer to as a school, for instance, may include several schools—a law school, a medical school, a business school, a graduate school, an engineering school, and so forth. A single grain elevator may be expanded to comprise several elevators, and you may then refer to the entire campus either as "Bunge's elevators at Hannibal" or as "Bunge's elevator at Hannibal".  
At one time, in fact, it was not unusual to see an integrated collection of multiple facilities of the type Z referred to as a Zs—singular article with plural noun. This is rare now, but you may still encounter something like this:

He'd seen the remarkable transformation from a peacetime docks to a docks at war. 

And at least one such use is still 'standard'—we speak of manufacturing facilities as a steel works or an auto works.

