This skill seems to be implemented since at least version 1.11 of the server ; probably earlier.
This skill is only available through a skill tool named “fishing pole” .
The archetype name of the fishing pole is fishing_pole .

There is no treasure list, that contains fishing_pole, as there are for lockpicks and flint_and_steel,
making it unavailable to the average player as random find in dungeons and purchase-able in shops.
A Dungeon Master can get himself such a fishing pole by the DM command create :
create 1 0 fishing_pole .

The only archetype that contains some harvest-able fish is “shallow_sea” , which has a light-blue face,
and is usually found at ocean's coasts on world maps.
All other water archetypes do not contain fish as treasures: lakes, rivers, ponds.

Only useful on shallow_sea tiles, that do not block movement.
The shallow_sea archetype by default has move_block all set ( old no_pass 1 );
so only a DM is able to apply the use_skill command successfully.

Syntax : use_skill fishing

The shallow sea tiles provide fish as random treasure:
Some shallow sea tiles do not contain fish at all, saying
“You can't %s anything here.”, skill→slaying ;
as it would say for all the many other ground tiles, as for grass, trees, cobblestones.

When a shallow_sea tile contains fish as treasure, than the message for success is
“You %s some %s”, skill→slaying, item→name
and for failure
“You fail to %s anything.”, skill→slaying .

Sets the skill into the shoottype range foreground.

This works also for a normal player, to fire the skill onto a direction of a bordering shallow_sea map tile.

Syntax : ready_skill fishing

The skill is internally called SK_HARVESTING :

• #define SK_HARVESTING 38 server/include/skills.h
• Function do_skill() in server/server/skill-util.c calls
  • do_harvest() in server/server/c_misc.c

This skill is unavailable to a normal player on the current Metalforge server v1.12 svn r17660 .

An entry from the CHANGES file of the arch package :

See also Crossfire Traffic March 2009

Server Version 1.50.0 seems to have some bug in the code preventing any fish as treasure in shallow_sea tiles.
At least I get only “You can't fish anything here.” from shallow_sea tiles.

Server Version 1.60.0: Works.

Server Version 1.70.0: Works.