Mark 1:16

Now as he walked by the sea of Galilee
The same with the sea of Tiberias, ( John 6:1 ) ,

he saw Simon:
whose surname was Peter, the son of Jonas:

and Andrew his brother;
the brother of Simon,

casting a net into the sea;
of Galilee, in order to catch fish:

for they were fishers:
by occupation, this was their trade and business, by which they got their livelihood; (See Gill on Matthew 4:18).