Archived Science Features: Distribution and Abundance of Humpback Whales in the Hawaiian Islands: Results of 1993-1998 Aerial Surveys Beach Watch: A Shoreline Survey Program

Science Feature Fish and Coral Population Changes in Fagatele Bay National Marine Sanctuary from 1985-1998 By Nancy Daschbach, Manager NOAA's Fagatele Bay National Marine Sanctuary Introduction Fagatele Bay National Marine Sanctuary is located 14° south of the equator just east of the International Dateline in American Samoa. The bay sits in an eroded volcanic crater that is just one of many volcanic extrusions that form the high island of Tutuila. The inner part of Fagatele Bay holds a well-developed coral reef system featuring a reef flat and reef slopes characterized by a terraced structure down to about 65 m. Its outer flanks have less well-developed reef structure due to the vertical cliffs and rougher water conditions. The steep sided cliffs that surround Fagatele Bay provide protection from anthropogenic threats. The isolation of the bay from population centers has provided de facto protection from most anthropogenic impacts with the exception of illegal practices. At a recent workshop held to develop a five year coral reef monitoring and research plan overfishing was identified as a critical environmental issue for American Samoa (Craig et al., in press). Although spearfishing is prohibited, nocturnal commercial spearfishers (utilizing scuba gear) routinely fish in the bay as well as any other accessible nearshore reef areas. In addition, rare incidents of dynamite fishing that wreak serious damage to the benthic habitat have been reported. In 1997, one such incident fractured the side of a huge Porites lobata bommie that has been estimated to be 300 years old (Birkeland, pers. comm.). Although human impacts have been relatively slight, Fagatele Bay has been struck by several major perturbations in the past twenty years. In the 1970s, the Bay was considered one of the most spectacular reef areas on the island. Its southwest-facing entry to the sea afforded it protection from most storms tracking from the northwest, the prevailing direction for hurricanes. The long western arm of Matautuloa ridge ending in Steps Point (the southern most point of land in American Samoa, and by extension, the U.S.) protects the inner bay from the southeast trade winds and swells. (top) The crown-of-thorns seastar (Acanthaster planci) is a voracious coral predator. In 1978-79, the entire island of Tutuila was damaged by a population explosion of Acanthaster planci, the crown-of-thorns starfish. This corallivorous echinoderm destroyed over 90 percent of the coral in Fagatele Bay, and in the following years, the nature of the reef populations changed. Six years later, in 1985, a long-term monitoring program was established to track the changes and recovery of the reef. In 1990 and 1991 hurricanes struck Tutuila; the 1991 Hurricane Val was particularly devastating to Fagatele Bay. In 1994, a warm water event in the Pacific likely was the cause of massive coral bleaching resulting in significant coral mortality. Most recently, in the late (austral) summer of 1998, several months of extremely low tides during a drought left reef flats exposed for hours to the tropical summer sun. This resulted in a die-off of all exposed reef biota. These destructive incidents are a natural part of tropical reef ecosystems that likely are adapted to this "boom and bust" dynamic. Biological Survey: Methods A team of coral reef biologists assembled and headed by Dr. Charles Birkeland of the University of Guam Marine Laboratory have surveyed Fagatele Bay four times since 1985 (Birkeland et al., 1987, 1994, 1996). Results of three of the surveys have been compared in a paper in Pacific Science, which will be released in October (Green and Birkeland, 1999). Figure 1. Map of Fagatele Bay National Marine Sanctuary showing location of permanent lines at each site; and map of Tutuila Island, American Samoa, showing location of sanctuary (from Green and Birkeland, 1999) Click on the map to see a larger map. In 1985, 1995, and 1998, coral, fish, algae and invertebrate specialists conducted surveys along lines (fig. 1) that were established in 1985 and earlier (six survey lines lie in Fagatele Bay and there are an additional 12 lines at sites around Tutuila and the island of Aunu`u). Each line was surveyed along multiple isobaths at 3, 6, 9, and 12 m for all lines, and for those lines with a reef platform, an additional three transects were added. Not all lines or transects were visited in all surveys due to weather and sea conditions. Only coral and fish were censused in the 1988 survey.   Corals were surveyed with the point-quarter method; points were selected by tossing a hammer at 16 locations along each transect line. With each fall of the hammer, four corals were selected: the closest in each imaginary quadrant to the center. Each coral was identified, measured (length and width), and the distance from its center to the quadrat center recorded. From these data, coral density, cover, species richness, and diversity (Shannon-Weiner Diversity Index) were calculated. Fish were identified to species and counted within 1 m on either side of the transect and 2 m above it. Multiple swims were made over the transect to record different types of fish: the initial swim recorded those fish most readily disturbed by a diver entering the water; on the returning swim was a search for the more sedentary and cryptic or hidden species. Fish density, species richness, and diversity were calculated. Biological Survey: Data synopsis Figure 2. Mean coral abundance at various transects located in Fagatele Bay National Marine Sanctuary. Alison Green and Birkeland compared fish and coral data from 1985, 1988, and 1995 to determine if any trends were noticeable. In general, the corals showed increases in mean abundance at almost all depths surveyed (Fig. 2). Species richness showed modest gains at most depths, but it more than doubled at the 9 m reef slope transects. Diversity, too, increased at that isobath although in general diversity stayed below 1.0 H' on the Shannon-Weiner Diversity Index. Percent coral cover at most depths was seen to be increasing in the 1988 survey, but decreased, most dramatically at the 9 m depth, in 1995. Similar decreases were seen in colony size at three of the five depths surveyed, but, paradoxically, increases occurred at the 12 m and 3 m depths. (top) Figure 3. Mean fish abundance at various depths in Fagatele Bay National Marine Sanctuary. Fish abundance dropped dramatically between 1985 and 1988 but showed a modest rise in 1995 (Fig. 3). In general, diversity increased over the 10 year period. Species richness was slightly higher in 1995 than in 1985, but dropped in the 1988 survey. The latest survey completed in 1998 shows that species richness experienced a slight decline on the reef slope and reef flat (perhaps due to habitat loss following the reef flat die-off earlier that year) but was consistent with the 1995 data at other depths (Green, 1998). Green reports that the obvious disparity between the 1998 data from other data may be due to an observer artifact: the methodology was slightly different (i.e. all divers were working in the water at the same time the fish swim was being conducted, which may have contributed to a lower count). Biological Survey: Conclusions It is clear from the data that although the coral communities appear, in general, to be recovering (within the constraints of repeated perturbations), the fish communities are not keeping pace. Several factors may contribute to this observation. Evidence supports the allegation that illegal fishing occurs and adversely impacts the reef fish communities. Few large reef fish such as the maori wrasse (Cheilinus undulatus), sharks, and the larger serranidae and scaridae, appear in the counts. These species can be present and abundant at other sites in American Samoa, particularly those areas that are not heavily fished. The coral reefs of Fagatele Bay National Marine Sanctuary have experienced natural cycles of disturbance and recovery. The data for the 1998 coral survey have yet to be reviewed, however, it is apparent that, at least qualitatively, the corals are healthy and rapidly recovering. Coral cover has increased and coral size in some species appears to be growing at an almost logarithmic rate. One factor seems to contribute to the recovery of Samoa's corals, the coralline algae. Porolithon onkodes is an ubiquitous benthic organism in Fagatele Bay with cover approaching 75 percent in some areas. Coralline algae "glue" loose rubble together, ultimately providing a stable platform for settling coral spats (Birkeland, pers. comm.). In conclusion, Fagatele Bay provides a small model for tropical coral reefs in the Indo-Pacific region. Recent history of multiple perturbations underscore the lack of long-term stability and variability in these systems, but suggest that adaptive strategies by the reef communities allow them to rebound from these enormously destructive events. Literature Cited Birkeland, C.E., R.H. Randall, and S. Amesbury. 1994. Coral and reef-fish assessment of the Fagatele Bay National Marine Sanctuary. Report to the National Oceanic and Atmospheric Administration, U.S. Dept. of Comm. 126 pp. Birkeland, C.E., R.H. Randall, R.C. Wass, B. Smith, and S. Wilkins. 1987. Biological resource assessment of the Fagatele Bay National Marine Sanctuary. NOAA Tech. Mem. Ser. NOS/MEMD 3. 232 pp. Birkeland, C.E., R.H. Randall, A.L. Green, B.D. Smith and S. Wilkins. 1996 Changes in the coral reef community of Fagatele Bay National Marine Sanctuary and Tutuila Island (American Samoa) over the last two decades. Report to the National Oceanic and Atmospheric Administration, U.S. Dept. of Comm. 233 pp. Craig, P., N. Daschbach, S. Wiegman, F. Curren, and J. Aicher. 1999. Workshop Report and Development of a 5-year plan for coral reef management in American Samoa (2000-2001). American Samoa Government. 28 pp. Green, A. 1998. Fish Communities. Report to the National Oceanic and Atmospheric Administration. U.S. Dept. of Comm. 36 pp. Green, A. and C. Birkeland. 1999. Twenty Years of Disturbance and Change in Fagatele Bay National Marine Sanctuary, American Samoa. Pacific Science 53: 376-400.  (top)