Prop Bailey Nicholas crashed over for the Eels' first try before impressive centre Ereatara Smallman crossed out wide after a smart pass by Luke Cairns Nelson put him into space.
Smallman was in the next play from the restart, running the ball up 45 metres before a loose pass was intercepted by fleet-footed Kyle Maggs, who just beat the cover defence to score for the Tigers.
Second rower Kereama Kiwi scored a superb long-range try to put the match beyond the Tigers but it was dynamic centre Smallman who scored the try of the match with his second four-pointer after he cut the Tigers defence to shreds.
Fullback Shevez Williams impressed with his accurate goalkicking in the tricky cross breeze, missing just one kick from seven attempts.
Under-15 Eels' Smallman, Kiwi and Nicholas will join clubmates Tarquin Court, Ryan Gordon, Wilz Rutene, Josh Weedon (seniors) and Jacob Tana (under-17) in the Waicoa Bay reps to contest the rugby league nationals next week in Auckland.
Otumoetai Eels 40 (Ereatara Smallman 2, Kereama Kiwi 2, Bailey Nicholas, Amanaki Pasi, Hans Amoroa tries; Shevez Williams 6 goals).
Southport Tigers 28 (Beau Davidson, Kyle Maggs, Caleb Bonning, Lachlan Douglas, Mitchell Giles, Louis Barnett tries; Barnett 2 goals).