[Platapus]

From a moral standpoint I can understand your concerns. It is a dicey question that every Kaluen needs to answer for themselves.

Sinking a fishing boat is probably no more or less immoral than sinking an unarmed cargo/passenger ship.

From a tactical standpoint, I can also understand your concerns. Fishing boats may have radio sets. Since we can't always tell, for the purpose of the argument we will assume that the fishing boat has a radio.

So a Kaluen is faced with a few options.

1. Avoid the fishing boat so they have no chance to use their radio
2. Attack the fishing boat submerged with a torpedo with some chance of blowing them up before they can send an SSS
3. Attack the fishing boat on the surface with deck gun and AAA with little chance of sinking them before they can send an SSS

Option two is not attractive. Using a torpedo to sink a fishing boat is not an efficient use of a torpedo. Torpedoing fishing boats, especially in rough weather, can be tricky as they have such a short length your aiming error angle is very tight. Fishing boats also have shallow drafts do your depth error angle is also very tight. Weather that would normally not cause your torpedo to miss a merchant, may cause you to miss a fishing boat. Additionally the displacement of the fishing boat is not very high and BdU wants tonnage. Even if you do hit the fishing boat, you can't guarantee that an SSS won't be sent.

This leaves options 1 and 3.

If we are concerned with naval and air patrols, keeping our presence concealed is the best way we can survive to sink our enemy (merchant shipping). Taking Option 3, removes this concealment and will, with high probability allow the enemy to know that your submarine is in the area. This in itself won't especially endanger your submarine. but it is information you are volunteering to the enemy. What are you getting in return for your gift? a 90 ton fishing boat. What about that juicy C3 just over the horizon that is now changing course? It is all about risk and payoff.

The chances of having your surface battle interrupted by air cover is also something to consider.

In many cases, fishing boats are a lot like aircraft. Are they worth fighting?

This leaves option 1 for consideration.

There is the emotional factor in making it difficult to "allow" a fishing boat to pass by unmolested. However, we in the KM and especially in the U-boats are military professionals. We do not attack through emotions, but according to military logic. Is it really in the best interest of my mission to take the time and risk (not only the risk to my boat but the risk of other merchant shipping being alerted) to sink a fishing boat?

Conversely, a bird in the hand is worth two in the bush. Normally we would not let a small merchant ship go past in hopes that a larger merchant ship will come along. But is there a limit to this train of thought?

So morally I don't think there is any difference between sinking a fishing boat and sinking an unarmed cargo/passenger ship.

Tactically, I will only sink a fishing boat on my return trip when I am already out of torpedoes. Even than, the fishing boat needs to be in the perfect position or most likely I will cruise by.

It is going to be a long war.