Why does the Torah refer to non-Kosher fish as "Sheketz" (abominable)?
Rashi: To render Asur a mixture containing them if they add taste.
Seforno (Pasuk 2): The Torah refers to the species that contaminate the soul but not the body
Targum Yonasan: To include the juice and the brine of non-Kasher fish in the Isur.
If they constitute more than one in sixty. See also Sifsei Chachamim.
Incorporating non-Kasher species of fish (Pasuk 11 & 12), birds (Pasuk 13 & 20), locusts (Pasuk 23) and rodents (Pasuk 41).
Since all of these are not subject to Tum'ah (Seforno).
Why does the Torah write specifically "mi'Besaram"?
Rashi: To preclude the fins and the bones from the prohibition.
And what is the extra word "Teshakeitzu" coming to teach us?
Rashi and Ramban: It teaches us that Yavchushin (gnats
[Rashi]) that have been sifted
are included in the prohibition.
Targum Yonasan: It comes to place an Isur Hana'ah on Sheratzim.
The Ramban maintains that 'Yavchushin' are not gnats