Require different construction levels
Use a fishing potion or a fish pie to temporarily increase your fishing level by 3, or an admiral pie to increase it by 5. This makes catching fish much quicker and saves time. That is if of course, you have the inventory space to do so.
When harpooning fish you may catch a larger version of the fish. The larger version can be stuffed by the Taxidermist in Requiem gold. The stuffed fish can be mounted on the wall in your Player Owned House skill hall. Different fish require different construction levels to mount them.