Angler's Tranquility is an Aspiration Reward Trait. Sims will this unique reward trait when they succesfully complete "Angling Ace" from the aspiration Nature. There is no limit on the number of aspirations that can be completed, and thus no limit on the number of aspiration reward traits that can be earned. To complete an aspiration, sims are expected to complete milestones.
This trait can make all negative moodlets while fishing disappear. This is very handy if your sim likes fishing. This way sims will only get the biggest and freshest fish in lakes and ponds, without having to worry about getting sad or angry and scaring the fish away.
It is also recommended to get this trait if your sims like dishes that include fish. This way your sims will only catch the juiciest and most delicious fish.