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copyright statement: the images
on this site are to be used only for non- commercial
purposes, such as personal use or not-for-profit education and outreach
activities. Please email me at burdickdr at hotmail.com to request use of these images for more than personal (i.e., home) use.
- Importance of Guam's reefs
- Health of Guam's reefs
- - Health intro
- - Reef health tour
- - - - Northern reefs
- - - - East-central and southeastern reefs
- - - - Apra Harbor reefs
- - - -Southwestern reefs
- - - - Western reefs
- Threats to Guam's reefs
- - Algae overgrowth
- - Anchor damage
- - Coral bleaching
- - Crown of thorns outbreaks
- - Debris
- - Disease
- - Divers/snorkelers/ swimmers
- - Illegal fishing
- - Kayaks and other human-powered watercraft
- - Ocean acidification
- - Overharvesting
- - Sedimentation
- - Sewage discharge
- - Stormwater runoff
- - Terpios sponge
- - Vessel groundings
- - Windsurf boards and kiteboards
- Current management
- Recommendations
- Get involved |
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How healthy are Guam's reefs? |
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The health of Guam’s reefs is highly variable, with many reefs showing signs of degradation, while others support diverse, relatively healthy reef communities. But because long term monitoring efforts have only recently begun (with the exception of the Department of Agriculture's creel monitoring), it is difficult to objectively assess the health of Guam’s reefs and how the condition of the reefs has changed over time. With this in mind, reefs described as “healthy” should be considered so only relative to other, more degraded reefs on Guam, and relative to reefs of the past few decades only as described by relatively limited data sets. The historical data that do exist, however, suggest a signficant decline in reef health, such as the decline in fore reef coral cover from 50% to 25% between the 1960s and the 1990s, a precipitous drop in the catch per unit effort for reef fish in the 1980s, and an estimated 86% decline in reef fish catches since the 1950s.
While we don't have as much as data as we would like, even a conservative assessment of the overall health of Guam's reefs based on the available data - especially in comparision to reefs in the past - would suggest that many of Guam's reefs are not doing very well. Unfortunately, Guam's reefs are not unique in this regard - the decline in the health of Guam's reefs over at least the last several decades parallels the decline in the health of coral reefs across the Indo-Pacific. As a result of this decline, the ability of Guam's coral reefs to provide important services to Guam’s current and future residents is compromised.
In the past, Guam’s reefs have recovered after drastic declines, but continued degradation of water quality, chronic crown of thorns seastar outbreaks, low numbers of important herbivorous (algae-eating) fishes and other threats make Guam’s reefs less resilient – meaning they are less able to recover from disturbances such as major storms or severe coral bleaching events. A particularly distressing sign of declining reef resilience is the large decrease in rates of coral recruitment (i.e., the settlement of young corals onto the reef) in the last few decades. In areas without successful coral recruitment, recovery will likely be a long process – if it happens at all.
The news isn't all bad
Some much needed good news is that the number of fish in Guam’s marine preserves has increased significantly only 3 years after enforcement began in 2001. In fact, the amount of fish in the Piti Bomb Holes and Achang Reef Flat preserves increased by more than 100%. Preliminary evidence suggests that the increasingly abundant large fish in the preserves produce large numbers of eggs that drift with currents to nearby reefs, while others swim into adjacent non-protected areas, where they can be caught by fishermen.
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Extensive coral growth near Gabgab Beach, Apra Harbor. Photo by Dave Burdick. |
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An Acropora-dominated reef community on a shallow fore-reef terrace off the southeast coast . Photo by Dave Burdick. |
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A reef community near Anae Island on the southwest coast that is heavily impacted by regular sedimentation events |
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An extensive macroalgal bloom
(Padina sp.) near Apaca Pt. along the southwestern coast |
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| How are Guam's reefs doing? A brief guide to the reefs of Guam* |
| *The information provided below was taken from a hand-out released by the Bureau of Statistics and Plans. The hand-out is available as a PDF here. |
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Northern reefs |
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slideshow |
| The northern reefs are generally considered to be in better condition than reefs in the south, and although they may be exposed to elevated nutrient levels through groundwater discharge, northern reefs are not affected by the intense levels of sedimentation experienced by many southern reefs. In general, the highest coral cover and diversity on Guam is found in an area beginning roughly at Falcona Beach on the northwest coast, continuing clockwise around the northern coast, and extending down to Pagat Point on the eastern side of the island. The abundance of medium-to-large fish is slightly higher on northern reefs compared to reefs in other parts of the island, possibly due to the relatively better habitat quality and restricted fishing access. Crown of thorns sea star outbreaks may have significantly altered the coral communities in the northwestern part of the island in the last few years, however, including at least some of the reef extending north from Falcona Beach to Ritidian Point. |
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slideshow
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East-central and southeastern reefs |
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The health of reefs along the central and southern portions of the east coast is highly variable; some reefs adjacent to large river mouths have been degraded by sedimentation, while other reefs appear relatively healthy. Some of the reefs reported as relatively healthy in 2005, such as the forereef slope off the Achang Reef Flat Marine Preserve and the east side of Cocos Lagoon, have since experienced crown of thorns seastar outbreaks. Other areas previously known to have relatively high coral cover and diversity, such as the northern part of Pago Bay and at sites south of Agfayan Bay and Talofofo Bay, have also been heavily impacted by crown of thorns predation.
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Apra Harbor reefs |
slideshow |
| Although Apra Harbor is home to the busiest port in Micronesia, a large U.S. Navy base, and numerous recreational facilities, it contains both patch and fringing reefs with some of the highest coral percent cover (>80%) on the island. While the harbor reefs appear to be doing relatively well, the impacts of the increased turbidity, pollution, overharvesting, and invasive species associated with the area’s use as a port and naval base have not been fully assessed. Several acres of reef were removed from the entrance of the Inner Harbor by the Navy in 2006 and 2007 and additional areas, including a large area near Polaris Point, are expected to be lost or degraded due to other planned construction and dredging activities in the harbor. |
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slideshow
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Most of the fringing reefs and patch reefs along the southwestern shore remain in poor to fair condition, depending on their proximity to river mouths. Data from NOAA surveys conducted in 2005 suggest that these reefs had the lowest average coral cover on the island. This is supported by University of Guam Marine Lab data from this region. A 10 km stretch of reef in this area was reportedly heavily impacted by sedimentation from a poorly planned coastal road project in the early 1990s. The reefs in this area continue to experience high levels of sedimentation from erosion caused by wildland arson, off road vehicle use and other activities. |
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West-central reefs |
slideshow
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| Several large bays, including Piti, Asan, Agana, and Tumon, are located along the central western coastline. Both Piti and Tumon Bays host a wide diversity of habitats and possess areas with vibrant reef communities, but there are also large expanses of severely degraded reef in these bays. Chronic crown of thorns seastar outbreaks and poor water quality appear to be the main impacts to these reef communities. Asan Bay is heavily impacted by fishing, with fish stocks decreasing in this area since monitoring began in 2001. The reef communities in Asan Bay are also heavily impacted by sediment- and nutrient-laden river and stormwater discharges. The health of coral communities in West and East Agana Bays varies; coral cover is relatively high, especially along the shallow reef front and forereef slope, but fish abundance is low. |
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