Below is from the reference Jackdale provided in his earlier post. Interesting that the commentator says that the master of the vessel must display the lights and shapes in order to claim "fishing vessel" status. For dayshape, two cones with apexes together. I've heard this before but have yet to see it printed from any definitive source.
[COLREGs / Inland] The term "vessel engaged in fishing" means any vessel fishing with nets, lines, trawls, or other fishing apparatus which restrict maneuverability, but does not include a vessel fishing with trolling lines or other fishing apparatus which do not restict maneuverability;
As a general rule, this definition includes most commercial fishing vessels (while fishing) and excludes most recreational or sport fishing vessels. The term "lines" in the phrase "fishing with nets, lines, trawls" refers to lines such as long-lines which may be miles long and to which are attached at regular intervals many leaders and hooks. The term "trawls" refers to large open-mouthed nets that are towed through the water by one or two specially equipped fishing vessels (trawlers). Not included in the definition are vessels fishing with trolling lines (for example, a sport fisherman's rod and reel with the line towed astern), which do not restrict maneuverability.
The use of nets, lines, or trawls is presumed to restrict maneuverability while the use of trolling lines is presumed not to restrict maneuverability. The master determines whether the fishing apparatus restricts maneuverability; if a collision occurs, the court may subsequently make the determination. In any case, a master electing to take on vessel-engaged-in-fishing status is required to display the day shapes and lights prescribed by Rule 26.