ABOUT HAWAIIAN REEFS

Hawaiian Geology
Turtle Reef is named for the coral-rich reefs extending along the geographical area of the Kona coast, on the west side of the big island––Hawaii.

ISLANDSThe Hawaiian archipelago is the peak of a chain of volcanoes (known as the Hawaiian-Emperor seamount chain) formed over the past 70 million years on the sea floor of the Pacific Ocean as the Pacific tectonic plate passes over a hot spot in the mantle below the Earth’s crust. While the hot spot has remained more or less stationary, the Pacific plate moved northwards at first for 30 million years and then north westwards––a change in course, perhaps caused by the collision of India with Asia. For the past 40 million years the plate has continued to move in this direction at a steady clip of 3.4 in/year (9 cm/yr). In the course of geological history the Hawaiian Hot Spot has budded off some 107 volcanoes on the ocean floor to form a chain of 132 islands, reefs and shoals stretching across more than 1500 miles of the Pacific Ocean.

As the volcanoes move further from their point of origin the plate beneath them cools, becomes denser, and they begin to slide back into the ocean. Corals that have become established around the perimeter of the volcano build a limestone ring around the island called a fringing reef. As fast as the volcanic superstructure sinks the corals keep pace building a limestone coral reef perimeter underwater.

Erosion inexorably reduces the superstructure of the volcanoes and they gradually become smaller and eventually disappear beneath the waves to form flat-topped seamounts called guyots. The oldest of the volcanoes are the guyots in the Northwest Pacific. The youngest island, Hawaii (approximately 500,000 years old) lies in the Southeast. Today Mauna Loa and Kilauea are still adding lavas to the mass of Hawaii.

Finally, the newest volcano in the Hawaiian-Emperor Chain is called the Lo'ihi is in the process of forming on the seafloor some 25 mi (40 km) southeast of Hawaii Island and will, in due course, break the surface of the Pacific.

The Hawaiian Marine Ecosystem
Centered in mid-Pacific the Hawaiian Islands range from 19 degrees to 29 degrees north of the equator. Reef-building corals depend upon abundant sunlight for energy so the closer they are to the equator the more productive they can be. As a rule coral reefs exist mainly between the latitudes of 30 degrees north and south of the equator. Optimal temperatures for their growth is around 26-27 degrees centigrade and corals can rarely build reefs where temperatures drop below 18 degrees centigrade. Consequently, the Hawaiian Islands exist in a region towards the limit of coral reef growth in the Northern Hemisphere.

CORAL REEFThe greatest diversity of coral reef species lies in the ancient seas centered in the area of the Philippines, Indonesia and Papua New Guinea called the “Coral Triangle” where there are some 2000 species of marine fishes. The Coral Triangle lies at the center of the two great basins of the Indian and Pacific Oceans. Away from this ancient sea area biodiversity decreases.

The distribution of species in the Pacific Ocean depends upon each species dispersing from one reef or island where they are established to the next island down-current. Large animals may be able to swim between distant island archipelagoes, but most small animals, including corals, depend upon their larvae colonizing distant shores carried on oceanic currents. For those animals with long larval phases, such as surgeonfish and moray eels, colonization of distant reefs is much easier than for those whose larvae spend too little time floating in the currents as plankton.

The Hawaiian Islands lie in the center of the North Pacific Ocean where an ocean current, called a gyre, revolves around it in a clockwise direction. Warm waters from the tropical western Pacific move north, northeast and finally eastwards before joining with cool waters from Alaska to form the California Current in the eastern Pacific that in turn becomes the North Equatorial Current as it heads westward back toward the Coral Triangle. As they circulate around the gyre, ocean currents from the tropics bud off huge vortices of warm water that retain their flora and fauna and may have been responsible for transporting much of the marine life to the Hawaiian archipelago. The shore life that has colonized Hawaii island-hopped across the Pacific on these currents from the west or from the south to colonize the Hawaiian archipelago. Due to the relative isolation of the Hawaiian Islands and their northerly location they are much less diverse than centers of biodiversity nearer the equator such as the Coral Triangle . For example, the number of inshore fishes shallower than 600 feet is only around 680 species––a third of the Coral Triangle’s diversity.
 
Due to their remoteness some 25% of the Hawaiian fishes are endemic (unique to these islands). Since the Hawaiian marine fauna is so isolated, a species arriving at the archipelago may become established  in the absence of competition. Subsequently, intense intraspecific competition (competition between members of the same species) would likely culminate in evolutionary change as this species adjusts perfectly to its new environment. Evolution of new species into endemic forms was possible as a direct result of their population’s isolation.

Isolated marine ecosystems such as those around the Hawaiian Islands are less resistant to change than more biodiverse regions such as the Coral Triangle where competition between species is intense. As with other isolated fringing coral reefs around the world, the Hawaiian coral reef ecosystem has changed in recent memory because they are less resistant to the changes we impose.

For example, certain species, such as bluestripe snapper, (Lutjanus kasmira) were introduced into the Hawaiian marine ecosystem by Fish and Game in the 1950’s. Subsequent studies indicate that this species successfully established itself and preys on native species such as small fish that are newly settled as well as many shrimp, crabs and other crustaceans. Bluestripe snappers also have impacted the marine ecosystem by edging out other native nocturnal species, such as the yellowfin goatfish (Mulloidichthys vanicolensis), from prime refuge sites used during the day.

Another species, the Peacock Grouper (Cephalopholis argus), was also introduced (1956) as a fishery’s candidate and has since become a common member on Hawaiian reefs. It has been a failure as a fishery species because it accumulates ciguatera toxin and is banned from commercial sale. Since the 1980’s the Peacock Grouper’s population has increased fifteen times. One study on Hawaii Island’s Kona coast showed that this species was consuming on average 30 tonnes of fish per square mile annually.

While Turtle Reef has healthy populations of corals and reef fishes the largest and most desirable species have been decimated by overfishing. Sharks, jacks, large snappers, giant groupers and other large predatory fishes are rare or absent. Relentless overfishing and the introduction of exotic species has compromised the inherent efficiencies of a healthy reef ecosystem. The consequences of human impacts are hard to measure but, as with other ecosystems, it often results in loss of diversity and productivity and ends with the simplification of the ecosystem.

 

Turtle Reef's Community Dynamics
Coral Reef communities are as complex as our own. They are populated by creatures as alien and different from us as creatures from another planet. We associate them with sport; a living; a food item… or occasionally entertainment… even a pet.

On Turtle Reef in the Hawaiian Islands we discover the similarities of our own communities with these creatures living in this alien world. Who would imagine that these animals live in a society akin to cities in the sea? Every member of the reef plays a vital role in the ecosystem upon which others are dependent in a similar way that individual human workers foster society’s needs.

Daily life follows discernable patterns that are regulated by the conductors of this orchestra––the cycles of sun and moon. Individuals, species and associations of different organisms interact in direct and indirect ways. Associations and social gatherings play important roles have critical consequences. Lifelong partnerships are common.
 
Reef Health
Coral reef species are engaged in intense competition to survive because of the scarcity of, thus competition for, resources such as food and shelter. Species have adapted to these challenges with multiple strategies. Most reef species have home ranges or are territorial and thereby establish an area large enough to support them and which they may defend. They avoid direct competition by partitioning their habitat, specializing in diet and foraging strategies, and by exploiting different times of the day to feed. Approximately 60% of species are diurnal (active during the day), another 30% nocturnal, (active only at night) and 10% are crepuscular (active during twilight hours). Some species are generalists and may be active both day and night. Most coral reef species have a broad range of food sources to which they can switch when their preferred diet is unavailable.

Often viewed through the lens of a constant struggle between predators and prey, coral reef ecosystems function in exquisitely refined and complex dynamic balance. While in competition for space and resources, organisms are also in mutually beneficial associations. Each of them fulfills important functions in the efficient transfer of energy from the primary producers to apex predators and ultimately back to bacteria, the terminal consumers. All parts of this complex system are necessary for the most efficient transfer of energy and the maximal diversity and productivity of coral reefs. People’s impact on coral reef ecosystems far outstrips any other single species and, as a result, we cause changes to occur in coral reef systems that often impacts the natural efficiency of the ecosystem. Loss of coral reefs globally (reportedly more than 33% over the past 50 years) is directly related to the proximity of human populations to them. Human populations inadvertently exert stress on marine creatures by affecting water quality through pollution, by over fishing and physical damage.

Poor water quality appears to have the greatest adverse impacts since this is the medium in which reef life is immersed. For example, coral reefs close to human populations may suffer from bacterial levels that are more than 1000 times greater compared to remote locations (for example the Kiribati Islands). High levels of nitrates and phosphates encourage the growth of phytoplankton and benthic algae that directly compete with corals for light.

Over fishing runs a close second by disproportionately removing certain target species, thereby disrupting important relationships that have consequences across the entire ecosystem. Coral reef ecosystems are diverse enough to have redundancy in the routes that energy flows through the system. Over time chronic over fishing effectively reduces these target species to functional extinction. One way of assessing this impact is by measuring the mean trophic levels on the reef system over time, or in relation to comparable systems not impacted by fisheries. Since predators are preferentially targeted, in heavily fished reefs the trophic level shifts downwards toward the primary producers indicating a loss of functionality. In Hawaii, many of the reef-associated predators, such as jacks that were once common are locally depleted.

Additionally, since Hawaii is remote it was never colonized by some of the usual and common species associated with coral reefs. Government managers introduced species with unforeseen consequences to the indigenous species and their role in the ecosystem (please see about section on blue striped snapper and peacock grouper - see above).

Bottom line, the improvement of water quality reduction in fishing and protection from introduced and exotic species improves coral reef health.


1 The Coral Triangle is the most diverse marine region on the planet, matched only by the Amazon and Congo basin rainforest ecosystems. Defined by marine areas containing more than 500 species of reef-building coral, it covers around 6 million square kilometres of ocean across six countries in the Indo-Pacific – Indonesia, Malaysia, Papua New Guinea, Philippines, Solomon Islands, and Timor-Leste.