This depends on a few factors, such other animal life and similar environmental hazards, piracy, competition (not all of it fair or even ethical), and very thin profit margins for all but the larger fishing concerns.

For example, fishing in and around the Myr is mostly hazardous because it's easy to get stuck or lost if one doesn't know the land. Fishing around Arbas Gaelith means staying in unpopular areas or hiring mercenaries to protect your ship from larger and better armed groups. Fishing around Sahaladir means being very good because no one will give one a leg up over those with more skill.