Spatial variation in biomass, abundance and species composition of "key reef species" in American Samoa

Sabater, Marlowe; Tofaeono, Saolotoga

American Samoa is a small remote oceanic archipelago, increasing its importance of protection of its coral reefs and conservation of its marine resources. The fishery is important to the Samoan way of life and provides subsistence to its people. Problems associated with population growth, climate change, and chronic natural and anthropogenic disturbances are rapidly emerging, threatening coral reefs and its associated fisheries. Prior to management intervention, the status and underlying dynamics affecting the populations of reef fishes important to fisheries should be determined. Biologically and ecologically sound data assure fisheries manager that the intervention or strategy applied will be effective, efficient and provide maximum benefit for all resource users. A key reef species is defined as a fish species that is targeted for recreation and subsistence, including those secondarily targeted when primary species occur in small numbers. Determining abundance and biomass is useful to assess how much fish is available. Several surveys have been conducted in American Samoa by various off-island researchers but this is the only study that focused solely on fish known to be used as food. By focusing only on certain groups of species, the data generated will be more accurate. Reconnaissance surveys were conducted at 54 sites around Tutuila, Aunu’u, and associated reefs of Nafanua and Taema Banks. Replicated belt transect surveys were conducted in 24 out of 30 permanent monitoring sites around Tutuila, Aunu’u and Taema Banks. We used a hierarchical nested design to determine the effects of spatial scaling on species distributions, biomass, and abundance patterns. Our data shows that populations of key reef species in American Samoa are currently in a stable status. Seventeen percent of the sites surveyed are in very high biomass category (>40.1 mt/km2); 50% at high (20.1-40 mt/km2); 21% at moderate (10.1-20 mt/km2); and only 13% at low biomass category (5.1-10 mt/km2). The mean biomass value for Tutuila alone ranges from 5.99 mt/km2 in Aua to 106.59 mt/km2 in Pagaitua Point. Scaling analysis showed significant variations in abundance and biomass lies at higher spatial scales (from habitat level to exposure level), where significantly higher biomass occurs at point reef habitats and south shore sites. The high fish biomass in point reef areas is attributable both to species composition and effects of habitat quality on growth and survival. The high fish biomass on southern shores of Tutuila is due to the extent of true coral reef habitats which result in greater protection of key reef species and availability of food source. The species composition differ between exposure and between habitat such that species that have strong ties with true coral reefs are found mostly on the south shore and those that have wide habitat preference are found also in the north shore where a significant amount of the bottom substrate is colonized volcanic pavement. The results from this study could be utilized by the Marine Protected Area Program in the design and planning for the establishment of MPAs. Insights on scaling, species composition, spatial patterns in coral cover, and trends in fish biomass and abundance at various spatial scales are important considerations in the design of protected areas.