This might be a dumb question but I want to know for a shinylocke. I'm asking this to figure out how it works so I can know what makes chain fishing chain fishing, to make sure I can do this in sun.
Shiny-boosting chain fishing is only in X and Y, while Sword and Shield's chain fishing mechanics provide a higher chance of encountering Brilliant Pokemon. Unfortunately, both types are unhelpful in your situation. SOS chaining is probably your best alternative.
Fishing (Bulbapedia)
