Fish scale
From Inkipedia, the Splatoon wiki
Fish Scales are a new item in Splatoon 3. They are obtained from damaging the King Salmonid in Salmon Run Next Wave.
Earning
Fish scales can only be obtained from reaching an Xtrawave in Salmon Run Next Wave by damaging the King Salmonid. There are three types of fish scales, bronze scales, silver scales and gold scales, with the rarer varieties more likely to drop after higher hazard levels.[1]
Use
Fish scales can be exchanged for Splashtag banner designs or items like various Boss Salmonid statues and stickers for the lockers. They can also be exchanged for different colored Grizzco approved alternate work outfits.[1]
