Mating successfully at high densities often requires species to employ unusual reproductive tactics. We reportunique courtship behaviours in an un shed, high-densityspawning aggregation of squaretail groupers (Plectropomus areolatus) that are potentially associated with alternative reproductive tactics (ARTs). Aggregating males aretypically known to court females in small territories (paircourtship), which is often associated with a pair-spawning tactic. However, we also observed the largest malessimultaneously courting several females in mid-watershoals – a unique, high-cost-high-benefit courtship tactic which appears to result in a novel school-spawning tactic. Counter-intuitively we observed an inverse size-assortment in individuals–large males courted smallerfemales and vice-a-versa, likely linked to different pay-offs with competitive ability and local mate density. Theseunique, high-density behaviours are threatened to belost, with increasing commercial fishing pressures on theP. areolatus aggregation.