Why fish is halal in Islam besides not being slaughtered ?

In Islam foods and meat are defined by The Almighty either halal or haram.Islamic form of slaughtering animals , poultry is involves killing the animal through the jugular vein or wind pipe.Saying the Takbeer at the time of sacrifice is paramount to ensure a successful Qurbani.

fish underwater

Fish use a mechanism called “ counter current oxygen exchange ” to maximize the amount of oxygen in their blood.Fresh oxygenated water taken into the mouth is uninterruptedly pumped out through the gills in one direction, while blood in the lamellae flow in opposite creating the countercurrent flow of the blood and water on which a fish survival depends .It is scientifically proven, when a fish comes out of water and dies, its blood redirects and move to the parts of it epiglottis ,leaving the pure meat to eat.

Prophet Muhammad (P.B.U.H) said concerning the sea:

"its water is a mean of purification and its dead meat is permissible."( Narrated and classed as saheeh by al-Tirmidhi (69). Also said : "there are two dead animals that are lawful for us (to eat): they are fish and the locusts." 

It has no blood clotting, microbes, excretion or harmful blood left after its death .
Ibn Al-Qayyim said: the dead animals are forbidden because fluids, excretion, harmful blood remain in it, hence slaughtering is a reason to make it lawful to eat because it removes these excretions and blood . As regards death, itself it does not lead to prohibition since it take place by slaughtering in the same way it take place by other reasons, so if there is no blood and excretion in an aminal, which are removed by slaughtering according to Islamic rites, it does not become forbidden by it death and it is not a condition to slaughter it in order to make it lawful to eat, as is the case of the locusts."
some of the verses are :
The only things which are made unlawful for you are the flesh of dead animals, blood, pork and that which is not consecrated with the Name of God. But in an emergency, without the intention of transgression and rebellion, (it is not an offense for one to consume such things). God is certainly All-forgiving and All-merciful.^[8]

He has only forbidden to you dead animals, blood, the flesh of swine, and that which has been dedicated to other than Allah . But whoever is forced [by necessity], neither desiring [it] nor transgressing [its limit], there is no sin upon him. Indeed, Allah is Forgiving and Merciful. -- Quran, Al-Baqara 2:173 ^[7

 Pork is haram in islam:  

Pork meat is forbidden in islam due to some reasons.They eat their own droppings without this they would be unable to obtain any nutritional value .Hamsters, guinea pigs, chinchillas, hedgehogs eat their own poop which are thought to be a source of vitamin B and K , produce by gut bacteria. Most of the diseases are caused by eating Pork. Pork is a real garbage gut. It will eat anything including urine, excrement, dirt, decaying animal flesh, maggots, or decaying vegetables. They will even eat the cancerous growths off other pigs or animals. The meat and fat of a pig absorbs toxins like a sponge. Their meat can be 30 times more toxic than beef or venison.When eating beef or venison, it takes 8 to 9 hours to digest the meat so what little toxins are in the meat are slowly put into our system and can be filtered by the liver. But when pork is eaten, it takes only 4 hours to digest the meat. We thus get a much higher level of toxins within a shorter time. 
Quran clearifies the prohibtion as :
The only things which are made unlawful for you are the flesh of dead animals, blood, pork and that which is not consecrated with the Name of God. But in an emergency, without the intention of transgression and rebellion, (it is not an offense for one to consume such things). God is certainly All-forgiving and All-merciful.^[8]

     

Prohibition of pork in Torah :

passages of Leviticus enlists the animals that are emitted to eat in Jewish community .Animals having cloven hooves chew their own cud and are edible like cows, sheep, deer but pigs dont not chew their own cud .

         And the pig, because it has a cloven hoof that is completely split, but will not regurgitate its cud; it is unclean for you. You shall not eat of their flesh, and you shall not touch their carcasses; they are unclean for you.

— Leviticus 11:7–8

And the pig, because it has a split hoof, but does not chew the cud; it is unclean for you. You shall neither eat of their flesh nor touch their carcass.

— Deuteronomy 14:8