ASALAMU ALAIKUM,

1) The shark possesses all the characteristics of a fish. Hence it is classified as a fish and can be eaten.

2) Whale, dolphin is classified as a mammal and is therefore not permissible to eat.

- Their (sahabah) eating was exeptional, because ALLAH TAALA gives RUKHSAT (PERMIT) for those who is in desperate necesity .

#ALLAH TALA says:

He has forbidden you only Al-Maytatah (meat of a dead animal), blood, the flesh of swine, and any animal which is slaughtered as a sacrifice for others than All?h (or has been slaughtered for idols etc. or on which All?h's Name has not been mentioned while slaughtering). But if one is forced by necessity, without wilful disobedience, and not transgressing, then, All?h is Oft-Forgiving, Most Merciful.

9: Surah Number: 16, Ayah Number: 115