Data from: Alternative reproductive tactics and inverse size-assortment in a high-density fish spawning aggregation

At high densities, terrestrial and marine species often employ alternate reproductive tactics (ARTs) to maximize reproductive benefits. We describe ARTs in a high-density and unfished spawning aggregation of the squaretail grouper (Plectropomus areolatus) in Lakshadweep, India. 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-offs between potential mating opportunities and intra-sexual competition. Counter-intuitively, the aggregation showed a habitat-specific 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, unfished densities. These unique density-dependent behaviours may disappear from this aggregation as overall densities decline due to increasing commercial fishing pressure, with potentially large consequences for demographics and fitness.

在高密度种群中，陆生与海洋物种通常会采用替代性繁殖策略（alternate reproductive tactics, ARTs）以最大化繁殖收益。本研究针对印度拉克沙群岛海域一处高密度、未受捕捞的方形尾石斑鱼（Plectropomus areolatus）产卵集群，对其ARTs展开描述。此前针对该物种的已有研究显示，领域雄鱼会开展配对求偶行为，对应配对产卵策略。本研究首次记录到一种此前未被报道的集群求偶策略：领域雄鱼在中层水集群中对多只雌鱼进行求偶，最终形成独特的“集群产卵”模式。求偶策略受体型大小、局部配偶密度以及栖息生境的调控，这大概率与潜在交配机会与性内竞争间的动态权衡相关。出人意料的是，该产卵集群呈现出生境特异性的逆体型选配现象：在礁坡生境中，大体型雄鱼会求偶小型雌鱼；而在陆架生境中，小体型雄鱼会求偶体型相等或更大的雌鱼。在连续两年针对该高密度未受捕捞种群的观测中，上述行为模式始终保持稳定。随着商业捕捞压力加剧导致整体种群密度下降，该集群所特有的这类密度依赖型行为或会消失，进而对种群结构与进化适合度产生重大影响。