From Bulbapedia, the community-driven Pokémon encyclopedia.
Seaking Catching Day is a small annual competition is at a lake in Johto. It appeared in the episode Hook, Line and Stinker. It is a fishing competition to see who can capture the largest Seaking. Contestants may use any rod, reel, or lure they like and may capture as many Seaking as they like—but only one may be entered in the competition and it must be captured using a Lake Ball. Competitors can also have one of their own Pokémon registered to assist them in their captures. The first prize winner gets a trophy, as well as a year's supply of chocolate candy.
Andreas won in a previous competition. Misty won in the competition featured in Hook, Line and Stinker.