Theoretical considerations suggest that a shortage of suitable cover for reef-dependent motile organisms may lead to high predation mortality and/or migration, constrain adult biomass and spawning potential, and result in a bottleneck in ‚sheries production. For free-living ‚sh and motile invertebrates, the natural mortality rate often descends steeply with size and age toward maturity, but from simple simulations, the fractal properties of structurally complex marine habitats suggest that mortality rates may rise again for older, crevice-dependent ‚shes. That natural cover for commercial-sized organisms is limited can be inferred from the rapid colonization of arti‚cial reefs. Estimating hole-size availability in the local environment could be important in optimizing crevice sizes prior to installing arti‚cial reefs.