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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?
Thanks.
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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.  
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That makes a lot of sense, thanks, MM.