What are the Sunnahs related to fishing, and is it obligatory to say "Allahu Akbar" (God is Greatest) before taking the fish out of the water or cutting it, like other slaughtered animals? What is the ruling on catching fish with a spear, stone, or explosives while they are still in the water?

First: Fishing is not an established Sunnah (definition), nor is it among the slaughtered animals for which the بسم الله (mentioning God's name) is required. If one says بسم الله before fishing, it is good, not because it is a specific Sunnah, but because it is among matters of importance.

Second: It is permissible to fish with a spear if no other alternative, such as nets, is available, especially for large fish, based on the Almighty's saying: "O you who have believed, Allah will surely test you with something of game that your hands and spears attain," (Quran 5:94). The fisherman must observe excellence in hunting and choose the gentlest means, according to the Prophet's (PBUH) saying: "Indeed, Allah has prescribed excellence in everything." And avoid harming oneself or others, according to the Prophet's (PBUH) saying: "There should be no harm nor reciprocating harm."