Many scriptures forbid the consumption of animal flesh. Some examples of such scriptures are given below:
We ought to destroy those who eat cooked as well as uncooked meat and eggs. (Atharva Veda 8.6.23)
Its definitely a great sin to kill innocents. Do not kill our cows, horses and the people. (Atharva Veda 10.1.29)
He who desires to augment his own flesh by eating the flesh of other creatures, lives in misery in whatever species he may take his [next] birth. (Mahabharata, Anu.115.47)
Those who are ignorant of real dharma and, though wicked and haughty, account themselves virtuous, kill animals without any feeling of remorse or fear of punishment. Further, in their next lives, such sinful persons will be eaten by the same creatures they have killed in this world. (Srimad Bhagavatam 11.5.14)
If there were nobody who ate flesh, then there would be nobody to slay living creatures. The man who slays living creatures kills them for the sake of the person who eats flesh. If flesh were not considered as food, there would then be no destruction of living creatures. It is for the sake of the eater that the destruction of living entities is carried on in the world. Since, O you of great splendor, the period of life is shortened by persons who kill living creatures or cause them to be killed, it is clear that the person who seeks his own good should give up meat altogether. Those dreadful persons who are engaged in the destruction of living beings never find protectors when they are in need. Such persons should always be molested and punished even as beast of prey. (Mahabharata, Anu.115.29-32)
That man who seeks to multiply his own flesh by (eating) the flesh of others has to live in this world in great anxiety, and after death has to take birth in indifferent races and families. High Rishis given to the observance of vows and self-control have said that abstention from meat is worthy of praise, productive of fame and Heaven, and a great satisfaction itself. This I heard formerly, O son of Kunti, from Markandeya when that Rishi discoursed on the sins of eating flesh. (Mahabharata, Anu.115.34-36)
Bhishma said: That man who wishes to increase his own flesh by the meat of another living creature is such that there is none meaner and more cruel than he. In this world there is nothing that is dearer to a creature than his life. Hence, one should show mercy to the lives of others as he does to his own life. Forsooth, O son, flesh has its origin in the vital seed. There is great sin attached to its eating, as, indeed, there is merit in abstaining from it. (Mahabharata, Anu.116.11-13)
There are many more verses than these present in the scriptures. I am not going to quote them since there are too many.
Now coming to my question, people in various parts of India see fish as Sattvic. For example, in my state (i.e. Bengal), many Brahmins say fish is Sattvic. Some even go to the extent of calling them "sea-vegetables". However, this does not only happen in Bengal. Many in Odisha, Bihar and the Goud Saraswat Brahmin community in Coastal South-Western Andhra Pradesh; see fish as Sattvik.
In addition to this regional/cultural issue, we have seen prominent individuals such as Ramakrishna Paramahamsa and Lahiri Mahasay to be pescetarians when their disciples Swami Vivekananda and Yukteshwar Giri advocating vegetarian diet. (I discuss the vegetarianism issue of Swami Vivekananda and Ramakrishna Paramahamsa in this question) So it can be said, Swami Vivekananda and Yukteshwar Giri took fish as vegetarian.
Are there any scriptural justification of their views? Do any scriptures call fishes as sea-vegetables and not as "meat"? Do any scriptures classify fishes as Sattvic for eating?