Read this Animal Crossing: New Horizons Switch (ACNH) Guide on Giant trevally.
Yeah, I'm pretty well-trevalled."
Head to the end of the Pier and fish off of it. It's also worth noting that the shadow of the Giant Trevally is on the larger side, meaning you'll be catching a lot of Sea Bass in the process. If you’re looking to catch a Giant Trevally, you’ll want to make sure your fishing during the right months in the correct location. The Sea (Pier) location is the wooden dock stretching off your island into the sea. "I caught a giant trevally!
—New Horizons. Find out the spawn conditions, sell price, what time of the day and year it spawns, and more! The giant trevally is a rare fish introduced in New Leaf. The shadow size for Giant Trevally in Animal Crossing New Horizons is large. This fish is available from May through October at any time during the day. Here you can find Giant Trevally's Shadow Size, Sell Price, and its Locations and Times available for both Southern and Northern Hemisphere. This page will teach you how to catch a Giant Trevally in Animal Crossing: New Horizons (ACNH) for the Nintendo Switch. It has a large shadow, comparable to the Sea Bass, and can only be found by fishing off a pier. The location to catch the rare Giant Trevally in Animal Crossing New Horizons is the Sea (Pier). Giant Trevally are one of the rarer fishes in Animal Crossing: New Horizons. If they don’t spawn you can scare away the other fishes and use bait to force other fish types to spawn in. In New Leaf, it is only found in the waters around the island; in New Horizons, it now spawns by the pier.
Look for Extra Large shadows.