I assume that you are talking about the line where Mr. Weazly asks who put the dynamite in the salmon pool. This comes early in the story when Mr. Franklyn is being indignant because other of the men are suggesting that he might possibly use all the money meant for the outing to buy alcohol for himself. He offers to resign his post as treasurer of the outing.
At that point, Mr. Weazly happens by and speaks the line about the dynamite. Basically, what he is asking is who has caused the argument -- why there is a commotion going on. It's a bit of an overstatement since the commotion isn't all that big.
It is possible to "fish" by using explosives. An explosion in the water kills the fish and they float to the surface. The water is, of course, very disturbed. So Mr. Weazly is using this as a metaphor for the argument that is going on. He wants to know why the "waters" in the shop are disturbed.