Mark 1:16

1:16 As he went along the Sea of Galilee, he saw Simon and Andrew, Simon’s brother, casting a net into the sea (for they were fishermen). ¹ 

Mark 1:20

1:20 Immediately he called them, and they left their father Zebedee in the boat with the hired men and followed him.

[1:16]  ¹ sn This is a parenthetical comment by the author.