Hi:

Actually counting fish is very easy. Just as long as a few facts are
known.
1) How do the fish get into the boat?
2) How big are the fish? Are you talking 0.25 Kg squid or a 200 Kg
tuna?

But even more important..... WHY do you want to know how to estimate
catch size on other people's boats?

Also who is going to use, and for what purpose, is the data being
collected? Are you a NGO, host nation, commercial fishing association,
or associated with a do-good save he the whales group?

By your inability to describe the situation I imagine you have no
commercial fishing experience and also do not understand basic
statistical sampling methods.

So help me out here. Why do you want to know how to estimate catch size
of someone else's boat. Do you have the owner/operator's
permission to do that?

Bob