Specialization within a shifting habitat mosaic underpins the seasonal abundance of a tropical fish

Habitat availability can be a key driver for the distribution and abundance of animals occupying heterogeneous landscapes. How species respond to regular changes in patch habitat structure, however, remains poorly understood, especially within seasonal seascapes. We explored the importance of patch habitat quality for the seasonal population dynamics of a widespread reef fish, Leptoscarus vaigiensis, within a shifting mosaic of seaweed patch habitats. Following dietary and behavioral assessments of L. vaigiensis microhabitat preferences, we used best subsets model selection to explore the relative importance of seaweed canopy attributes (cover, height, density) and the abundance of putative competitors and predators for explaining regional variations in L. vaigiensis density across summer and winter. Dietary analysis (n = 53 individuals) and behavioral observations (n = 1014) indicated that L. vaigiensis are habitat specialists that primarily consume brown seaweeds and prefer areas within patches that have tall (>30 cm height) canopy‐forming seaweeds (Sargassum, Sargassopsis) at low to intermediate densities (1–5 holdfasts per m2). Accordingly, we found seaweed percent cover and canopy height were two of the most powerful predictors for L. vaigiensis abundance, with major seasonal shifts in L. vaigiensis abundance from summer to winter strongly related to patch‐level changes in canopy cover and height. We conclude that patch habitat quality, based upon microhabitat preferences, can be a powerful predictor for the response of species to a seasonally shifting habitat mosaic, and that high‐quality patches that provide seasonal refuges should be a focus for spatial conservation and management.