Selection of a spawning aggregation site by Chromis hypsilepis (Pisces: Pomacentridae): habitat structure, transport potential, and food availability

Spawning aggregations form when fishes migrate to a site from their normal feeding grounds and form temporary groupings for breeding. Spawning aggregation sites are spatially rare, and demonstrating differences between a spawning aggregation site and unselected sites nearby is the first step towards understanding the benefits provided by the aggregation site. Chromis hypsilepis (Pomacentridae) is a demersally spawning reef fish, one population of which reproduces in a large, single aggregation at a rocky reef off the central coast of New South Wales, Australia. This study compared the habitat structure (rugosity, reef slope, substratum particle size, and abundance of preferred spawning microhabitat) of the spawning aggregation site and several non-spawning sites, and tested the hypotheses that the spawning aggregation site provided greater off-reef larval transport and prey availability for brooding males. Substratum rugosity was significantly greater and the preferred spawning microhabitat was significantly more abundant at the spawning aggregation site. Reef relief and substratum particle size were not significantly different from non-spawning sites. Passively drifting surface current drogues released at the spawning aggregation site were more rapidly transported off the reef, but did not travel further or faster, than drogues released at a nonspawning site over a 12 h period. Biomass of the preferred prey (copepods of 0.441 to 1.49 mm equivalent spherical diameter) was not significantly greater, but was less variable, at the spawning aggregation site.