Competition for shelter space causes density-dependent predation mortality in damselfishes

The long-standing interest in density dependence in demographic rates of organisms stems from its influence in bounding population fluctuations and in shaping spatial patterns of abundance. Despite growing evidence that early mortality of marine reef fishes can be density dependent and can involve predation, the underlying biological mech- anisms have not as yet been fully explored in any system. Here we examine the causes of density-dependent juvenile mortality for two tropical damselfishes, Dascyllus flavicaudus and D. trimaculatus. These species shelter in branching corals or anemones, and they feed on plankton above their microhabitats during the day. Field experiments confirmed that density-dependent juvenile mortality of both Dascyllus species arose from predation and that most of the density-dependent loss could be attributed to small-bodied, resident piscivores (e.g., sandperch, squirrelfish) rather than larger, tran- sient species (e.g., jacks). Over the diel cycle, mortality was strongly density dependent during the dark when damselfish were sheltering but not during daylight when fish were actively foraging. Infrared video recordings revealed the species of predators responsible for most losses and indicated that most predatory events occurred from late twilight to early night, when damselfishes were in shelters and not feeding. Individuals were most at risk when located near or just outside the perimeter of a shelter. The proportion of a cohort in the riskiest areas of a microhabitat increased with density. The cause of the increased fraction of individuals at risk with increasing density was intraspecific interactions among sheltering fish jostling for space in the safest regions; this resulted in the displacement of less aggressive individuals to riskier locations. Thus, density-dependent mortality in both damselfishes arose from interference competition for refuge space from crepuscular and nocturnal predators.

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