Background: At high densities, terrestrial and marine species often employ alternate reproductive tactics (ARTs) to maximize reproductive beneits. We describe ARTs in a high-density and unished spawning aggregation of the squaretail grouper (Plectropomus areolatus) in Lakshadweep, India. Results: As previously reported for this species, territorial males engage in pair-courtship, which is associated with a pair-spawning tactic. Here, we document a previously unreported school-courtship tactic; where territorial males court multiple females in mid-water schools, which appears to culminate in a unique 'school-spawning' tactic. Courtship tactics were conditional on body size, local mate density and habitat, likely associated with changing trade-ofs between potential mating opportunities and intra-sexual competition. Counter-intuitively, the aggregation showed a habitat-speciic inverse size-assortment: large males courted small females on the reef slope while small males courted equal-sized or larger females on the shelf. These patterns remained stable across two years of observation at high, unished densities. Conclusions: These unique density-dependent behaviours may disappear from this aggregation as overall densities decline due to increasing commercial ishing pressure, with potentially large consequences for demographics and itness. [ABSTRACT FROM AUTHOR]