Reef Fish Depend on Corals and Natural Outcrops: Are Predation and Migration Linked to the Fractal Characteristics of Habitats?
John F. Caddy
Abstract.—This paper focuses on interactions between reefs, fisheries, and their management and discusses the relationship between reef fish and their habitat. Hard bottom is much less extensive than fine sediments on most continental shelves but supports epifauna/flora as cover for reef fish and serves as habitat for their prey species. Corals, epifauna, and macroflora on natural reefs function as feeding, shelter, and aggregation sites and often have fractal characteristics. Size frequencies of catches from unfished banks may reflect the aggregate of crevice size spectra present, and this influences growth and natural mortality rates at size. It is suggested that migration is initiated when juvenile fish outgrow cover in nursery areas, which only offered predator protection at smaller sizes. Juveniles are then forced to move further offshore where cover for protecting larger reef fishes is often more available. Large cavities are rare on most natural outcrops or reefs, but these attract large fish and artificial reefs are often designed to have large cavities needed by mature reef fishes. Once artificial reefs are located by fishermen, large, resident fishes are readily captured. Unless some offshore reefs or refugia are protected, spawning potential could be affected.
Although natural mortality (Mt ) declines with age for many free-living species (see Caddy 1991), information on reef-associated species suggests that natural mortality rises when potential bottlenecks occur; thus, M at size or age can increase for older individuals due to a shortage of suitable cover. Bottlenecks in recruitment may be due to a limited size-specific holding capacity of the present habitat or excessive distances of migration to habitats needed to survive to the successive life stage. Anthropogenic impacts on epifauna could also reduce holding capacity of habitats by eutrophication, hypoxia, or impacts from excessive use of bottom gear. A conceptual model suggests how increments in habitat complexity and proximity of habitats between sequential life history stages may reduce population bottlenecks and improve productivity of reef resources. It also suggests that the trophic development of reef ecosystems was linked to their fractal surface characteristics.