From the Super Mario Wiki, the Mario encyclopedia
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The Fishing Hut is a room inside of the Secret Mine in Luigi's Mansion: Dark Moon. The Fishing Hut is accessed by taking the right door inside of the Ice Lake. The Fishing Hut is a small room that is presumably used to store fishing equipment. Several fishing poles are kept here with their lures still intact and there is a bucket with some fish in it. A large fish is also hanging on the left wall. The door on the south leads back into Ice Lake and the floor below leads into Under the Ice.
During Cold Case, Luigi must go to the Fishing Hut to free the Toad assistant that is trapped inside of the portrait. When Luigi first enters, a Boo laughs at him and then disappears into the floor. The Toad portrait is not visible, but if Luigi sucks up the snow on the ground, the reflection of the portrait can be seen. Luigi must shine the Dark-Light Device on the north wall to reveal the portrait and then use the Dark-Light again to free the trapped assistant. Pulling all the lures hanging from the fishing lines causes the bucket of fish nearby to spit out coins, bills, and Gold Bars.
Starting on Cold Case and most missions after, once Luigi enters the Fishing Hut, if he looks below the floorboards on the left side, there is a sapphire trapped in snow. Vacuuming up the snow releases the gemstone for Luigi to collect.