CRAMP Rapid Assessment. Results Summary
Coral Reef Community Structure (see: Coral for detailed results)
Hawaiian reef communities can be characterized as “Porites reefs”, structured mainly by wave energy and anthropogenic factors. Coral cover in the MHI is approximately 22%, with corals of the genus Porites comprising half of the coverage. Three species of Montipora and Pocillopora meandrina comprise the overwhelming majority of the remainder.
No single factor accounts for the variation observed in coral cover and diversity, but several parameters are highly correlated with coral assemblages. Sites with high coral cover and diversity are characterized by low wave regimes and low levels of silt and human population. Topographical complexity and distance from streams are equally important in explaining coral reef variability.
Corals are stratified by depth and degree of hydraulic stress, with higher coral cover found at deeper sites with lower wave regimes. High wave energy and circulation showed inverse correlation with levels of silt. Silt is winnowed out of sediments by high wave energy.
Reef Fish Community Structure (See Fish for detailed results)
The extremely high spatial variability that exists among fish populations can be attributed in part to their schooling behavior and acute mobility. A large sample size can help reduce the effects of this variability.
Fish community structure in the MHI can be characterized as follows. The mean value is nearly 10,000 fishes per hectare, weighing 2,640 kg with most fish ranging in size between 5 and 15 cm. Smaller fishes are found on the windward sides of the islands. An average of 17 species were recorded per 25 m transect, with the Saddle Wrasse, Thalassoma duperrey (hīnālea), observed most frequently. The vast majority of species recorded are either indigenous or endemic, with relatively few introduced fish species. The density of non-native species is higher in shallower waters, contrary to overall fish densities that increase with depth. Only a few alien fish species occur in Hawai’i, yet the introduced Bluestripe Snapper, Lutjanus kasmira (ta’ape) has been very successful and shows the highest biomass of any reef species in the state.
Some of the most popular fish species in the aquarium trade, the Yellow Tang, Zebrasoma flavescens (lauīpala), the Orangespine Unicornfish, Naso lituratus (umaumalei) and the Gold-ring Surgeonfish, Ctenochaetus strigosus (kole), are among those with the highest densities statewide.
A strong anthropogenic influence was shown by the negative correlation of fish assemblage characteristics with parameters linked with human impact. Along with sediment organics from land-based sources, the impacts of over-fishing and human population pressure can be seen in declining fish assemblage characteristics. The size structure of fish communities is a strong indicator of over-fishing. The low number of individuals in the largest size class is clearly evident. Fishing pressure can also be detected by changes in abundance of populations of popular food fish as well as overall fish populations. The effect of fishing is also reflected by higher fish abundances in areas with stronger management protection. Sites within marine reserves are among those with the highest fish densities in the state. In sharp contrast, sites with heavy anthropogenic impacts exhibit consistently low fish populations. Over-fishing is visible in the absence of fishes of recreational and commercial value from the upper hierarchy of dominant species. The trophic structure also relates to over-fishing. Extremely few piscivorous fishes are found in the MHI relative to the Northwestern Hawaiian Islands (NWHI), where fishing pressure is minimal.
Other evidence that declines are associated with anthropogenic influence is the low rank of O’ahu compared to the other MHI in fish assemblage characteristics. This strong link to human population density is evident regardless of the high number of marine protected sites surveyed on this island.
Fish populations are strongly influenced by biological and physical factors. As in the case of coral communities, fish populations are stratified by depth and heavily influenced by topographical relief. Coral cover and richness and coralline and turf algae explain a portion of the variability in fish assemblage characteristics.
Sediments (See Sediments for detailed results)
Heavy terrigenous input is strongly associated with high levels of organic material and fine grains in the bulk sediment samples. Sites with limited water circulation are most heavily impacted. Silt and clay that overwhelm the system can become the dominant forcing function on community structure, strongly influencing both coral and fish populations. Coral settlement can be blocked at sites containing large amounts of sand that can be mobilized by waves and currents. Sites that contain sediments with high levels of basalt and low levels of carbonates appear to be less impacted by sedimentation. These sites tend to be shallow, high wave energy habitats on exposed coastlines that are primarily dominated by successionist coral species such as the rose or cauliflower coral, Pocillopora meandrina. Both a vertical and horizontal stratification of sediment composition and grain-size is apparent for some parameters. Organics and large grain-sizes decrease, while CaCO3 increases with depth. A horizontal carbonate gradient is characterized by increases in carbonate fraction of bulk sediment with increasing latitude, reflecting the greater coral reef development of the older islands.
Other signs of anthropogenic stress may result from artificial fish feeding or eutrophication. These can be characterized by high levels of organic compounds accompanied by low proportions of silt and clay.
Last Update: 04/21/2008
By: Lea Hollingsworth
Hawai‘i Coral Reef Assessment & Monitoring Program
Hawai‘i Institute of Marine Biology
P.O. Box 1346
Kāne‘ohe, HI 96744