Getting it just right

I cast my net into the sea
Red fish, blue fish both may be
Do all the fish I wish to get
End up in the bottom of my net?

There may be some
that I missed
there in the depths...
I can only guess.

There may be others
I'm sad to say
In my net on this fine day.
Ones I did not wish to catch,
which I'll return to sea
at end of watch.

The perfect catch
contains only the ones I wish;
those special red and blue fish.

More than those
are just a bother;
taking time to sort
one way or other.

(In the information retrieval world, there are two terms: 'recall' and 'precision. Recall refers to the ratio of relevant documents retrieved to the number relevant documents in the universe. Precision refers to the ratio of relevant documents retrieved to the total number of documents retrieved.)