Ibn Ezra on Leviticus 11:10

שרץ המים. הם הקטנים שנבראו מן המים ובמים נפש חיה מזכר ונקבה. ואחר שאמד שקץ הם לכם אמר ושקץ יהיו לכם לפרש מה שהוא שקץ והוא מבשרם לא תאכלו והנה הדג יקרא בשר רק מה שאמרו חז״ל בנדרים הוא על מנהג דברי אלה הדורות והוסיף כל אשר אין לו סנפיר וקשקשת להיות כלל לכל מים כי לא הזכיר בתחלה רק הימים והנחלים: OF ALL THAT SWARM IN THE WATERS. They are the small creatures that were created from the water.¹⁵See Gen. 1:20. The waters contain living creatures that are both male and female.¹⁶I.E.’s interpretation of and of all living creatures that are in the waters. After stating they are a detestable thing unto you Scripture goes on to say, and they shall be a detestable thing unto you (v. 11) in order to explain in what way they shall be detestable namely, ye shall not eat of their flesh (Ibid.). Note, fish are called flesh.¹⁷For our verse states, ye shall not eat of their flesh. However, what the sages of blessed memory said with regard to vows¹⁸The rabbis say that fish are not considered flesh. See Hullin 104a, “One who takes a vow not to eat flesh is permitted to eat fish and locusts.” refers to the custom of their generations.¹⁹Post-Biblical generations. In Rabbinic and contemporary times fish are not considered flesh. Scripture adds, Whatsoever hath no fins nor scales to include all waters, for Scripture earlier mentioned only the seas and the rivers.