The sparkle you are asking about is a certain interaction that a mermaid can do that causes the option to catch more different types of fish at that spot. It is called Aquatic Lure.
This is what it says over at the Carl Sims site on how to do this.
Island Living's Mermaids can use an ability called, "Aquatic Lure" on fishing spots. This costs 30 hydration, so top off before you use it. When this spell is cast, the Sim pulls out a big conch and blows. The horn sounds and a glow appears on the fishing spot. Now fishermen at this spot fish as though they're 2 levels higher, and fish catch rates are improved (as opposed to treasure/seaweed). This effect lasts at a minimum of several hours.
https://www.carls-sims-4-guide.com/skills/fishing/
Here is the information for the patch notes that were added back in June about fishing, it talks about the sparkles you are seeing.
Fishing Additions:
Brace yourself, we’re about to cast you into a deep dive…
- We added several new interactions to allow players to fish in different ways and interact with other Sims around fishing activities.
- Sims can perform a few new Fishing-based socials to gain useful info (via UI TNS/Notebook) and push NPCs to Fish.
- Fishing is now joinable.
- Improved fishing interaction tuning and autonomy to make the interaction more efficient and fun.
- Upper skill levels now provide more meaningful rewards, with new Interactions added to Fishing skill levels
- New high skill cast interaction “Angle for Big Catch” increases chance of getting rarer fish.
- New high-skill VFX visuals on Rare fishing spots.
- Ability to now “Mentor Sims in Fishing.”
- Fishing UI has improved information, including Notebook info with Bait information.
- Tuning for fish that can be caught is unique per world & more fishing spots added to some of our previously shipped worlds.
- New bait preference system applied to most existing fish that modify catch chances.
https://forums.thesims.com/en_US/discussion/960976/june-18th-2019-patch-notes#latest
