you would rather say "fish were plentiful in the sea" than "..was..".

I think you could say both, wsieb.

"fish was plentiful in the area" treats "fish" as a general commodity.

Conversely - I don't know why - but "sheep was plentiful in the area" doesn't work at all.

Is it significant that you can say "I eat fish" but you wouldn't generally say "I eat sheep" ?

- As "lamb was plentiful in the area" sounds OK.