Fishing was modified fairly recently when baits were introduced. At that time, spear fishing was made (even) harder, and I would suspect tridents were reduced in efficiency as well.
The scenario where tridents have been used successfully in recent history is lantern fishing, i.e. fishing at night from a craft with a bright lantern or fire on the craft (the fire would be contained in a metal cage, often (always?) hanging off of the craft). I believe this kind of fishing is banned in many placed because it's too effective, but iron age Finland should have had no such laws...