Having been a fisherman myself, I can recall several occasions when I swore out loud when a big fish managed to exasperate me by either getting away or rejecting one of my flies at the last minute.
For example, I was a member of the Scottish international fly fishing team which took part in a competition in the spring of 1984 on Lough Conn against teams from Ireland (included some anglers from NI), Wales and England.
The match began at 10 in the morning and concluded at 6 in the evening. At around 4 in the afternoon a very large fish was attracted to and followed my orange tail fly for about 3 feet in choppy water off a skerry - causing a bow wave directly behind my artificial offering. Alas, the fish eventually lost interest and turned away, so I cursed it to the heavens in front of my boat partner, an Irishman, and our appointed boatman.
Therefore, I can empathise with swearing fishermen afloat, be they in trawlers/drifters at sea, or in rowing boats fitted with/without outboard/inboard motors on freshwater lochs.
Just for the record, the Irish team members each received gold medals as match winners, whereas my team was awarded the wooden spoon for finishing in last place.