I'm lost again on countable vs uncountable.

There are a lot of fish in the river.
Cattle are a source of meat.
Fish is/are a source of protein. (I feel 'is' is better but it doesn't follow the cattle pattern )

What do you think?
Fish and cattle are both common plurals for the animals.
Fish is also the common uncountable form for the foodstuff. That's why 'is' seems more normal.  
That makes a lot of sense, thanks, MM.