Environment and development
in coastal regions and in small islands
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Coastal region and small island papers 3

Great Corn Island, Nicaragua

Joseph D. Ryan1,2,5, Leanne J. Miller3,5, Yamil Zapata2,4,5, Oliver Downs5, and Rodolfo Chan2,6

1 AEL/CEES, The University of Maryland System, Gunter Hall, Frostburg MD 21532 USA
2 Centro de Investigación y Documentación de la Costa Atlántica, Apartado 42, Bluefields, Nicaragua
3 Marine Research Institute, Florida Department of Environmental Protection, 100 8th Avenue SE, St. Petersburg FL 33701-5095 USA
4 Marine Biology Department, Universidad de las Regiones Autónomas de la Costa Caribe Nicaraguense, Bluefields, Nicaragua
5 Corn Islands Environmental Education Foundation, Office of the Mayor, Corn Island, Nicaragua
6 Marine Sciences Department, Bluefields Indian Caribbean University, Bluefields, Nicaragua

Mangrove wetlands, seagrass beds, and coral reefs are found throughout Nicaragua’s Caribbean coastal zone, but few scientific studies have been carried out to identify their distribution, productivity, or general physical condition. Preliminary data indicate that live coral coverage of several large coral reefs on the Nicaraguan shelf is declining. In view of the ecological importance of these habitats in sustaining east coast fisheries, Nicaragua joined the CARICOMP program in 1992. Great Corn Island was selected as the site for carrying out coral reef and seagrass monitoring. Although mangrove forests occur on the island, they are cut off from the sea except during short periods during the rainy season. To date, work at Great Corn Island has been limited to the coral reef site, which has been monitored for four years (1993-1997).

Introduction                                                                                             

Nicaragua’s humid Caribbean coastal zone (Fig. 1) traverses a broad range of environments that include brackish wetlands on the land, and nearshore and offshore benthic communities in the sea (Ryan, 1995). On the land, 90% of Nicaragua’s watersheds drain towards the east coast through eleven major rivers. The nutrient loads and freshwater pulsed by these rivers provide the basis for many of the environmental functions characteristic of the Miskito lowlands and the nearshore coastal boundary layer (Murray and Young, 1985; Ryan, 1992a). However, increased deforestation has led to high erosion along many of the watershed. This has resulted in higher sediment loads that are believed to have killed several of the large nearshore reef complexes on the central Nicaraguan shelf (Ryan, 1992a). The dynamics of marine environments on the continental shelf, the largest in the wider Caribbean, are primarily influenced by tropical storms, the westward flowing Caribbean Current, tradewinds (Roberts and Murray, 1983; Murray et al., 1982; Ryan 1992a, 1994), and ecological processes.

Fig. 1. Map of Nicaragua’s Caribbean coast and study area.

Mangroves, seagrasses, and coral reefs are found throughout this coastal zone. With the exception of their co-occurrence on the shallow Miskito Cays bank, they are not distributed according to the classical paradigm in which three habitats are tightly linked through physico-ecological processes over space and time (Ogden and Gladfelter 1983). This notwithstanding, the environmental functions of these three habitats provide resources and ecological services that contribute to the welfare of Nicaragua’s diverse coastal population (Ryan, 1995). For example, coral reefs and seagrasses produce scale-fish, lobster, and shrimp that account for more than 70% of the country’s seafood exports. This production provides jobs (artisanal and industrial fisheries) and subsistence incomes for local communities. Additionally, the habitats provide food and refuge for more than half of the remaining endangered green sea turtles (Chelonia mydas) that remain in the Atlantic Ocean (Carr, et al., 1978; Mortimer, 1983).

While most of the estimated 600 km2 of mangrove forests (Robinson, 1991) are largely confined to ten coastal lagoons and along river banks, they are also found in nearshore (Pearl Cays) and offshore (Miskito Cays, Corn Islands) marine environments. Seagrass meadows and coral reefs occur both nearshore and offshore across the Nicaraguan shelf. Although the areal coverage of these latter habitats is unknown, the seagrasses are among the most extensive in the world (Roberts and Murray, 1983). Most of the available data for coral reefs come from studies on the Corn Islands (Geister, 1983; Roberts and Suhayda, 1983; Ryan, 1994). The few coral data that have been collected for the important reef system in the Miskito Cays provide no scientific basis for quantifying coral abundance nor for evaluating their environmental condition, even though this progam has been operational for four years. Seagrass surveys have been carried out in the Miskito Cays by Ogden and Gladfelter (1983), Phillips et al. (1982), and Marshall (1994), and in the Pearl Cays and Corn Islands by Ryan (1992b, 1994).

Because of its remoteness from the rest of the country and a long history of civil war, the Caribbean coast of Nicaragua has remained relatively undeveloped. Less than 10% of the total population lives on that coast, and population densities are low. Approximately six inhabitants per km2 live in an area that comprises nearly half of the national territory (Robinson, 1991). The exception is Great Corn Island, one of only two inhabitated islands on the shelf, which has a population density of nearly 500 inhabitants per km2. Virtually all travel within the Caribbean coastal region is by water, due to the lack of roads. Human activities represent the greatest current threats to seagrasses and coral reefs. Habitat degradation (sedimentation and damaging fishing practices) and overfishing are common in nearshore environments, whereas high nutrient concentrations threaten the reefs offshore the Corn Islands. In nearshore areas, increased sediment loads are hypothesized to be responsible for an average living coral cover of 10% for corals found within 25 km2 of the mainland (Ryan, 1992a, b). The effects of overfishing of these habitats is unknown, but the decline in reef fish populations may be related to increased use of Jamaican-type fish traps and by-catch of reef fishes in shrimp trawlers (Ryan 1995). In offshore environments, many of the fringing reefs on Great Corn Island are believed to be declining as a result of sewage (Ryan, 1993; Broegaard, 1995).

Nicaragua joined the CARICOMP Program in 1992 and, after a careful search, decided to select Great Corn Island as its coral and seagrass monitoring location. The Corn Island site was selected for three reasons. First, it is outside the influence of terrestrial erosion that has led to river-borne sedimentation impacts on nearshore coral reefs and seagrasses. Second, both the island and its underwater environments are more accessible than at either of the other sites that were considered (Miskito and Pearl Cays). Finally, the lack of a close physico-ecological coupling between terrestrial mangroves and marine seagrasses and corals favored the selection of an offshore site.

The Setting at Great Corn Island                                                               

The two Corn Islands rise above the central Nicaraguan shelf to form the only inhabited islands along the Caribbean coast of Nicaragua (Fig. 1 and Fig. 2). Great Corn Island is located between latitudes 12°08'40"N and 12°11'83"N, and longitudes 83°04'24"W and 83°01'38"W. The island’s area is 10.3 km2 and it is inhabited by about 5,000 people. Lesser Corn Island, located 15 km to the northeast, is approximately half the size and has a population of about 500. A lobster and scale-fish fishery and two seafood processing plants, which produce over 40% of Nicaragua’s total seafood exports, provide the primary sources of income for the two islands.

Fig.2. Map of Great Corn Island showing major features and the location
of the CARICOMP monitoring sites.

General descriptions of physiographic features have been published (Conzenius, 1929; McBirney and Williams, 1965), as have descriptions of underwater environments (Geister, 1983; Roberts and Suhayda, 1983; Ryan, 1992b, 1994). The geological and hydrological forces in operation on the island have been summarized by Ruden (1993), who calculated that approximately one-half of the big island lies beneath the 2-m contour line, and that a sea level rise of only 0.5 m would innundate about one-third of the island. The island is made up of Tertiary (Miocene-Pliocene) basalts that protrude through late Tertiary carbonates. These basalt formations extend offshore, covering an area of approximately 10 km2 beneath the coral reefs. It has been hypothesized (Broegaard, 1995) that the island’s freshwater aquifer is geologically connected to the nearshore reefs (<1 km from the island) and that sewage may be seeping through fractures in the geological formations connecting the island and the nearshore reefs.

Climate and Oceanic Conditions                                                               

Tradewinds are the dominant force that controls processes such as erosion, groundwater movement, and wave energy around the Corn Islands. Winds blow steadily from the ENE at 7-10 m s-1, with a steadiness factor of 90% (Roberts and Suhayda, 1983). These and other physical forces (storms and currents) have sculptured the existing shape of the Corn Islands and the underwater reef formations.

There are no continuous air temperature or rainfall records for the Corn Islands, and logistical difficulties made it impossible to collect in situ data for more than six months. Hobo-temperature readings in 1994-1995 were consistent with long-term data for nearby San Andrés in 1959-1981. Given the absence of more recent long-term data, we present temperature recordings from the San Andrés dataset. Average air temperatures had an annual mean value of 27.4°C and a relative humidity of 81%. The coolest months are December-March, with an average of 27°C or lower. The warmest months are May-September (28°C). Extreme temperatures occurred in March 1973 (17°C) and July 1971 (34.4°C); these cannot be ruled out as factors in the observed coral degradation patterns. Average monthly rainfall is 50 mm; the rainy season is July-December (150-300 mm), with the highest rate in November.

Solar radiation is a critical parameter for coral and seagrass growth. López de la Fuente (1994) collected solar radiation data from Bluefields and found that the highest global radiation occurs in April, the lowest in December. The quantities of photosynthetically active radiation (PAR), which represents the amount of solar radiation available to perform photosynthesis, is proportional to solar radiation values.

Nicaragua’s Caribbean coast is characterized by one of the highest rainfall rates in the world, 3 m per year in the north and nearly 7 m per year near the Costa Rica border. An average of 3.5 m per year is assumed until more data can be collected. A relatively dry season occurs between January and April, and this period coincides with the strongest tradewinds. Peak rainfall is delivered between May and September. Annual evaporation has been estimated at 1.3 ´ 10-1 mm. The highest evaporation rates (154 mm) occur during the months of April and May, the lowest (86.9 mm) during November (Peralta-Williams, 1991).

Tropical storms impact the Corn Islands once a year on average, although three storms and a hurricane pounded the island in 1996. Hurricanes can be expected to strike the Nicaraguan coast once every 50 years, but three hurricanes have struck Corn Island in the past eight years. Joan (October 1988) was the most severe. Winds gusting up to 70 m s-1 struck Great Corn Island directly from the southeast, blasted across the island, and devastated 95% of all standing structures on the island. In the sea, the wave surge lasted for nearly one week, and wave heights >15 m were observed on the southeast side of the island during the peak of the storm. Bottom surges resuspended large sand banks and shifted them so that they spread over several extensive coral formations on the fringing reef in 1-7 m depth, and shallow (<7 m) Acropora formations were badly damaged. However, the most recent hurricane, Cesar, did very little damage to the reefs when it hit in 1996.

Few data exist on physical-chemical seawater conditions on the Nicaraguan shelf, and historical data for the Corn Islands are nonexistent. Tides are semi-diurnal and average less than 1 m in range. Average salinity during the initial phase of the monitoring program was 34.5‰; seawater temperatures averaged 28.5°C (Fig. 3). These measurements were made at the CARICOMP site.

Fig.3. Summary of water temperature data at the CARICOMP reef site,
Great Corn Island, Nicaragua.

Coastal Habitats on Corn Island                                                                

Approximately 15 percent of the island’s wetlands are composed of freshwater (Raphia taedigera) and mangrove (Rhizophora mangle) vegetation located in three different sections of the island (Fig. 2). Direct connection to the sea is blocked during most of the year by a beach dune system surrounding most of the island. However, during the wet season, heavy rains fill the wetlands until hydrostatic pressure builds up and breaks open the dune system. This allows for a temporary interaction, release of tannin-rich fresh waters, migration of crustaceans and scale-fish, between the wetlands and the offshore ecosystems. Many of the mangroves on Great Corn Island have been degraded by sewage discharge.

Marine environments surrounding Great Corn Island include numerous seagrass meadows. Dense mixtures of Syringodium filiforme, Thalassia testudinum, Halophila spp., and Halodule wrightii are commonly found in the nearshore backreef lagoon, whereas Thalassia testudinum and Syringodium filiforme dominate deeper offshore environments (6-8 m depth).

The largest coral reef system associated with Great Corn Island is the Cana Reef, a complex composed of fringing and patch reefs located on the northeast corner (windward side) of the island, which extends seaward 2 km and then curves towards the south. This system, which is 4 km long, consists of three separate reef formations (Fig. 2). Historically, the reef complex appears to have been best developed on the nearshore eastern and western borders (Geister, 1983), but the continuity essentially disappears in the center, where isolated patch reefs rise from the bottom. In general, there is little reef development on the leeward side of the island, presumably due to an island wave "shadow" effect that reduces current and wave activity behind Cana Reef (White, 1977).

On the offshore reefs, the coral is generally in good condition but many nearshore (<1 km from the island) corals are dead and covered by algae and encrusting sponges (Ryan, 1994). The most frequently observed encrusting sponges are Clathria spp., Cliona sp., and Agnanthosigmella varians, the latter covering most of the dead corals. Many of the nearshore corals appear to be declining due to subterranean discharges of sewage-contaminated groundwater originating from the island (Ruden, 1993; Ryan, 1994; Broegaard, 1995). Farther offshore, beyond the Cana Reef, patchy seagrass meadows and more robust corals abound. The outer coral reefs receive most of the incoming wave energy directed at the island; Acropora palmata and Montastraea annularis are the dominant frame-building corals throughout the fringing reef system.

The CARICOMP Reef Site                                                                      

In 1993, a set of five permanent transects was established on the CARICOMP monitoring site. The site is located at latitude 12°11'59.4"N and longitude 83° 03'28.5"W, within the offshore reef system in an area in which the reef is part of the fringing reef that is found at a depth of 12-15 m. The reef terminates at 20 m, where extensive sand pools extend seaward (Roberts and Suhayda, 1983; Geister, 1983; Ryan, 1994). The site is in the path of the predominant trade winds and wave energy (Fig. 2) and sand is frequently resuspended in the water column. The only human influence near the monitoring site is sporadic fishing by lobster divers, occasional fish trapping, and occasional hook-and-line fishing.

Based on chain transect surveys over the three year sampling period 1993-1995, mean substrate rugosity measured 1.91 (sd=0.132). Live cover at the site was dominated by algae (43.9%) and scleractinian corals (25.2%). Non-living substrate (e.g., gaps, holes, dead coral) made up 27% of the reef cover.

Data on the abundance of hard, soft, and hydrocorals were collected using a combination of chain transect and meter-square methods in order to make a more representative species characterization of the site. A total of 28 coral species was found using the two different methods (Table 1). Chain transects appeared to be more sensitive for finding smaller species (e.g., Scolymia, Mycetophyllia). Results indicated that a small group of coral taxa (Montastraea annularis, Agaricia spp., Porites spp., Millepora alcicornis, and Pseudopterogorgia spp.) represented over 90% of the corals sampled at the site (Table 1). Algae were dominated by the "fleshy" species, Dictyota spp. and Padina spp. Abiotic substrate made up nearly one-quarter (23%) of the site. Sponges and soft corals represented less than 5% of the total cover (Table 2). Video archives (8 mm video camera) were made by swimming 1 m above each of the 5 transects during each year from 1994 through 1996.

Table 1. Coral species composition comparing meter-square quadrats and chain transects, August 1994.
  Quadrat   Chain
Coral Species Composition Percent Rank Percent Rank
Montastraea annularis 48.5 1 4.93 1
Agaricia spp. 21.1 2 8.3 3.5
Porites astreoides 6.9 3 11.6 2
Agaricia agaracites 6.6 4 8.3 3.5
Pseudopterogorgia spp. 5.6 5 2.8 6.5
Millepora alcicornis 3.2 6 0.2 16
Porites porites furcata 1.6 7 1.5 9
Muriceopsis flavida 1.2 8 0 —–
Porties porites 1.0 9 6.2 5
Montastraea cavernosa 1.0 10 0 —–
Porites colenensis 0.9 11 0.9 12.5
Agaricia humilis 0.4 12 0 —–
Mycetophyllia spp. 0.4 13 0.6 15
Pseudopterogorgia bipinnata 0.3 14 2.8 6.5
Pseudopterogorgiaamericana 0.3 15 0 —–
Siderastrea siderea 0.1 16 0 —–
Porites porites divarcata 0.1 17 1.0 11
Gorgonia mariae 0.1 18 0 —–
Eunicea spp. 0.1 19 0 —–
Colpophyllia natans 0.1 20 0.7 14
Manicina areolata 0.1 21 0 —–
Dichoenia stokesii 0.1 22 0 —–
Agaricia aagarites danae 0 —– 1.7 6
Leptoseris cucullata 0 —– 1.1 10
Meandrine meandrites 0 —– 0.9 12.5
Mycetophylia aliciae 0 —– 0.5 15
Scolymia sp. 0 —– 0.1 17.5
Acropora cervicornis 0 —– 0.1 17.5
Species Number by Method   S = 22   S = 19
Total Species Number = 28        
 
Table 2. Mean percent cover and standard deviation (sd) of cover by different benthic categories, by transect and overall, 1993-1995.
  Transect Overall
Mean
Benthic Category 1 2 3 4 5
Algae mean 0.71 0.73 2.4 2.9 1.7 1.7
  sd 0.41 0.52 3.5 2.5 1.5 1.0
Stony Corals mean 18.9 30.7 20.2 24.4 32.0 25.2
  sd 4.0 5.4 3.9 1.1 5.6 6.0
Soft Corals mean 0.70 0.72 6.4 2.9 1.7 1.7
  sd 0.41 0.52 2.4 2.5 1.5 1.0
Sponges mean 1.8 1.3 3.1 0.81 1.9 1.8
  sd 0.65 1.3 1.3 0.91 2.3 0.86
Non-Living mean 34.4 22.2 23.8 32.1 25.2 27.5
  sd 11.0 4.6 18.4 7.2 7.4 5.4

The seagrass monitoring site is located in a wave-exposed environment 200 m seaward of two sites on the north coastline (Fig. 2). Each of the two sites consists of a meadow of mixed Thalassia and Syringodium located at <2 m depth. Seagrass sampling data are still unavailable due to a lack of equipment.

No mangroves were monitored at Corn Island due to lack of human resources during the first three years, but a site for future measurements has been selected in nearby Pearl Lagoon on the mainland.

Black sea urchins (Diadema antillarum) were not observed at the reef site. Densities in some of the shallow (<5 m) nearshore reefs around Great and Lesser Corn Islands were below 4 urchins m-2. Surveys of other urchins at the 5 transects (1 m on either side of each transect line) in October 1995 found a mean density of 29.2 (sd=25.2) Echinometra viridis. No correlation was found between urchin abundance and any of the six benthic categories nor substrate rugosity.

No bleaching was observed at the site during any of the three sampling periods. Sea fan surveys were carried out at both the CARICOMP site and at different locations around the Corn Islands. A total of 34 sea fans was examined. None of the sea fans examined on the reef (Gorgonia ventalina and Gorgonia flabellum) showed any evidence of disease, as reported for reefs in the eastern Caribbean by Nagelkerken et al. (1997). Along with the negative findings reported in nearby San Andrés Island and the Bay Islands, our results provide support for the finding that the disease is not widespread in the western Caribbean.

While most of the nearshore coral reefs at Great Corn Island (<1 km from the island) have undergone a major decline (<10% live coral cover) over the last decade, the percent live coral coverage (25.2%) at the deeper CARICOMP site is within the range of values reported for other Caribbean sites (CARICOMP, 1997). The annual variability of live coral cover was not significant over the three years, although standard deviations for the 5 transects at the CARICOMP reef site was high. This appears to be due to two problems. First, the failure to place permanent markers along the transects allowed for high variability in the placement of the chain each year. This problem has been overcome by hammering nails into dead coral along each of the transects. Second, the use of different divers to collect data each year was undoubtedly another source of variabililty. This problem has also been rectified by establishing a permanent sampling team of three divers from the region. Continuously recording temperature probes are now installed permanently at the reef, seagrass, and land sites.

Acknowledgments                                                                                    

Special appreciation is extended to the Norwegian Development Assistance Program, without whose funding and enthusiastic support this project could not have been possible. Invaluable support was provided by the Bluefields Marine Conservation Project of the United Kingdom, with assistance from the Royal Geographic Society, and by Gay Downs of the Corn Islands Mayor’s Office. Dr. Desirée Elizondo, Director of the General Directorate of the Environment, Nicaraguan Ministry of Natural Resources and the Environment, was instrumental in providing logistical support.

References                                                                                               

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