Fish breeder can't breed fish if the game is loaded after all fish is caught

Is this not an expected behavior? Fish breeder needs at least some fish on the field to breed additional fish.
Whether they complain or not is secondary. Are you sure that fish breeder breeds any fish before game save?

... but on the other side, I am not sure what is benefit of setting the start_amount to 0 in linked code...

If this is intended behavior, it should be the same whether you save/load the game or not. Also, it is not intuitive: in real life, a fish breeder would be able to bring some fish from one place to the other.
Also, fish breeder's animation shows him just releasing a fish. Why would he need some other fish to be there already to do it?

Yes, the behaviour should be the same, therefore I was asking if you are sure a fish breeder ever breed a fish on field where there were 0 fish before...

Yes, it might not be logical, but still should be consistent...

Yes, a fish breeder can breed fish, if there were some fish there initially (when the game started, or, due to this bug, when it was last loaded). If there never were any fish on a field, then a fish breeder cannot breed fish there. I think that this is the correct behavior. It's not good if you can make water permanently unusable for fishing, by catching all fish there (this applies to Atlanteans, as other tribes have different sources of renewable meat supply).

In the code, there are no checks that would prevent a fish breeder from breeding on a field where current amount is zero. This code [1] only checks that the current amount of a resource is less than the initial amount, and this code [2] does the same checks.

[1] https://bazaar.launchpad.net/~widelands-dev/widelands/trunk/view/head:/src/logic/findnode.cc#L113
[2] https://bazaar.launchpad.net/~widelands-dev/widelands/trunk/view/head:/src/logic/map_objects/tribes/worker.cc#L209

You seems to be right, I am surprised. Perhaps there is something else we are overlooking.

Are you interested to do actual coding and contributions to the game?

I agree that that code can go - I see no reason for it. We also need to fix the corresponding save function, and make sure that older savegames with depleted resources will still load properly.

Here is a savegame from bzr8593[trunk].
Started as network game, Last Bastion, no Opponentn, open gane.
After running with 65x for some minutes the fisher complained
"no fish". checking with the debug view I found "Resource fish 0/20"
at the coastline.

Sidenote Fishers can only fish at the coastline,
all the other fish will not be caught.

I played this as Atlatenr, but on load I found myself to be a barbarian?

Anyway: the debugview now shows "Resource none".

Next try with the bugfix branch.

Played the same with the new version, found that Fishbreeders can work again
and Debuggig shows recovery of fish resources.

Fixed in build20-rc1