In Animal Crossing: New Horizons, Dorado is one of the most valuable fish, priced at 15,000 bells. You can Buy Animal Crossing Bells on the safe site. For islands in the northern hemisphere (December to March for the southern hemisphere), fishing is only done from June to September from 4 am to 9 pm.
Because Dorado is a river fish, it is relatively easy to spot when it swims around. It is one of the only big fish that appears in the river, so be sure to observe its time from any big shadow you see. You don't need to be at a high place, so please feel free to check every part of the river on the island.
If you are very keen to quickly capture Dorado, we also advise that you make lots of bait. If you are fishing for the first time, you can Buy Animal Crossing Bells to help you. Build a good supply and you could kill time to throw the bait and catch any big fish that appears. If you spawn smaller fish, you may catch them or scare them by sprinting nearby.