LS
You haven't been reading the rest of the thread have you?
Yes in the original it says casting the 'lead' in verse 2 line 2 but in line 4 it says 'shoot their net'
Verse 3 'Methinks I see them shoot their trawl'
Verse 4 'Methinks I see the net on board'

I would guess they are casting the lead to find the shallower Dogger Bank where the fish often congregate. The net may well have had some lead weights on it, but casting the lead usually refers to the 'lead line' to test the depth.