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It has long been assumed that structural shelter of the appropriate size is a primary limiting resource for coral-reef fishes. We tested two corollaries of this limited shelter hypothesis for fishes occupying 12 concrete-block reefs in a seagrass bed off St. Thomas, U.S. Virgin Islands.
Each of three experiments involved six reefs (two replicates of each of three treatments) and lasted up to 27 months. The first corollary was that, holding reef size and hole size constant, reefs with more holes should support more fishes of that size than reefs with fewer holes. On reefs
with 0, 12, and 24 large holes, the number of large resident fishes (squirrelfishes, groupers, and moray eels) increased directly with the abundance of holes. The availability of small shelters (gaps between blocks) was approximately constant on the reefs, so small resident fishes did not
show the same pattern. When we subsequently reduced the small shelters on the reefs, the abundance of small fishes declined, especially on reefs where predatory fishes were most numerous, while the abundance of large fishes was unaffected. The second corollary was that, holding reef size and
hole number constant, reefs with small holes (4 × 6 cm), should support more small fishes than reefs with large holes (12 × 14 cm), which should support more large fishes. This prediction was also verified. Beyond the effects of shelter variables, there was a statistically significant
negative relationship between the number of resident piscivores and the maximum number of small fishes on any reef. This pattern suggested that piscivores set the upper limit to the number of small fishes on a reef. Overall, it appears that an indirect interaction exists between large shelters
and small reef fishes: an increase in the abundance of large shelters causes an increase in the abundance of large piscivorous fishes, which in turn causes a decrease in the local abundance of small prey fishes. This phenomenon suggests that artificial reefs designed for persistent fisheries
should include both small holes for small fishes (as refuges from predation) as well as large holes for predatory “target” species (as home sites).
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