It depends alot on the elevation of the lake. Where I fish (Paulina Lake at around the 6,000' level) we go as soon as the ice is off which is usually around May. Once the season opens the lower lakes are probably your best bet. Some lakes the kokes don't want to come out and play until the water warms up a bit. Keep in mind that early in the season your target depth will be up high, and as the water warms up throughout the summer they go deeper. When their up high you probably won't see as many on your fish finder because of the cone angle of the sonar and the boat/motor spooking them to the side of the boat.