This is the flip side of the previous mitzvah (to check for the signs of kosher fish) and it has the same inter-relationship with Mitzvah #155 that Mitzvos #153 and #154 have regarding animals. Accordingly, the Talmud in Chullin (66b) tells us that eating a non-kosher fish violates both a positive mitzvah (#155) and a negative mitzvah (this one).
As we’ve stated, the reason for eating kosher is to keep away from things that God knows to be spiritually harmful. Seriously, look at some of these bottom-feeders and scavengers. They’re absolutely repulsive. Would we really want to emulate them and internalize their characteristics?
This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in the tractate of Chulin (66a-67a) and is codified in the Shulchan Aruch in Yoreh Deah 83. It is #173 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #95 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.