CHAPTER 11

PRECIOUS CORALS

Background

Precious corals have been highly valued as raw material for making jewelry and various

art objects since antiquity. They consist of a diverse assemblage of coelenterates belonging

primarily to three orders of the class Anthozoa: Gorgonacea, Zoanthidae, and Antipatharia. The

most valuable of the precious corals are species of the genus Corallium in the order Gorgonacea.

The historically famous red coral of commerce from the Mediterranean Sea, Corallium rubrum,

belongs to this genus. Other highly valued species of Corallium include C. japonicum, C.

elatius, and C. konojoi from the far western Pacific between latitudes of 19oN and 35oN and C.

secumdum and C. sp. nov. from the Hawaiian Archipelago and Emperor Seamount complex.

Other gorgonians that are considered valuable include several gold corals in the family

Primnoidae (Primnoa resedaeformis and P. willeyi) from Alaska and the bamboo corals in the

families Isisiidae (Acanella spp.) and Lepidisisidae (Lepidisis olapa) in the western Pacific.

Another gold coral found in Hawaiian waters, Gerardia (= Parazoanthus) sp., belongs to the

order Zoanthidae. The last major group of precious corals are the black corals in the order

Antipatharia. Of the 200 species of black corals known in the world’s oceans, about ten (mostly

in the genus Antipathes) are used for the commercial production of jewelry.

Semi-precious corals include an even more diverse grouping of coelenterates. They

include mainly the stylasterine corals and Allopora in the class Hydrozoa, the blue corals

(Heliopora) in the Anthozoan order Coenothecalia, the organ pipe corals (Tubipora) in the

Antohzoan order Stolonifera, and several gorgonians in the family Melitodiidae, order

Gorgonacea. Like precious corals, semi-precious species are also used primarily for jewelry.

However, because of their abundance and high porosity, they are not as highly valued. Stony

corals (scleractinians) are even more porous than semi-precious varieties and are almost never

used for jewelry. Stony corals are, however, sold as curios or as decorations in many part of the

world. The value of stony corals imported to the United States averaged $1.0 million annually

from 1975 to 1980 (Wells, 1981a). In contrast, the value of the precious coral industry in

Taiwan and Japan (major production centers) in 1981 was about $50 million (Grigg, 1982a).

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Ecology of Precious Corals

Depth is perhaps the most convenient parameter with which to distinguish the ecological

requirements of various groups of precious corals. None of the precious corals is a reef building

species per se. All are ahermatypic species that lack zooxanthellae. The shallowest species are

commercial varieties of black coral that almost all occur within SCUBA diving depths. Except

for the Mediterranean Sea population of Corallium rubrum, which has an extremely broad depth

range (10-250 meters), all other precious corals occur at various depths below the euphotic zone.

Patterns of distribution and depth zonation of all significant species of precious corals are

summarized in Table 11.1. Excluding Alaska, two rich depth zones exist in the Pacific Ocean,

one between 100 and 400 meters and the other between 1.0 and 1.5 km. The former zone

includes the most valuable species of Corallium, the Hawaiian gold coral (Geradia) and the

bamboo corals. The 1.0-1.5 km zone is confined to the Emperor Seamonts in the north Pacific,

where the majority of the world’s production of Corallium (sp. nov.) is now harvested (Grigg,

1982b). In Alaska the primnoid gold corals have the broadest depth range of all, generally

between 50 and 800 meters (Cimberg, et al., 1981).

It is evident from Table 1 that for at least Corallium spp. (the most valuable group of

precious coral) all known commercial concentrations are found north of 19oN latitude.

Corallium is known to exist in the southern hemisphere but not in commercial quantities. A

survey by CCOP/SOPAC (Cooperative Committee for Offshore Prospecting in the South

Pacific-United Nations Development Program) determined the presence of Corallium in the

Solomon Islands, Vanuatu, Fiji, Tonga, Samoa, and the Cook Islands (Grigg and Eade, 1981).

Of these areas, the Solomon Islands hold the most promise. Corallium is also known to occur in

Indonesia, particularly the northeastern islands (Talaud and Sangi) but to date only small

colonies have been recovered (Bayer, 1950).

General ecological requirements of all precious corals include the following: the

presence of a firm substratum, relatively strong bottom currents, and the absence of significant

sources of sediment. There is an interaction between all three of these variables. Strong bottom

currents tend to prevent sediments from accumulating, thereby exposing rocky substrata.

Because of the longevity of precious corals, which is on the order of 75 years, the stability of the

habitat is as important as its suitability (see Grigg, 1975). For example, the only areas where

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Table 11.1. Distribution of major species of precious coralsSpecies Common

nameWhere found Depth

range (m)Reference

Corallium rubrum Red coral of commerce

Mediterranean Sea, primarily coasts of Sardinia, Corsica, southern Italy, Sicily and northern Tunisia

10-250 Belloc (1950), Marchetti (1965a), Lacaze-Duthiers (1864)

Corallium secundum

Angel skin or pelle d’ange

Hawaiian archipelago from Hawaii (20oN) to the Milwaukee Banks (36oN)

350-475 Grigg (1974)

Corallium sp. nov. Midway deep-sea coral

Midway Island to Emperor Seamounts, 28o-36oN

1,000-1,500

Grigg (1982a)

Corallium japonicum

Aka-sango Japan, Okinawa & Bonin Islands, 26o-36oN

100-300 Grigg (1982a) and Kitahara (1902)

Corallium konojoi Shiro-sango Japan to northern Philippines, 19o-36oN

50-150 Grigg (1982a) and Kitahara (1902)

Corallium elatius Momoiro-sango

Northern Philippines to Japan, 19o-36oN

150-330 Grigg (1982a) and Kitahara (1902)

Primnoa resedaeformis, Primnoa willeyi

Alaskan gold coral

Southeastern Alaska (Dixon Entrance) to Amchitaka, Aleutian Islands

10-800 Cimberg, et al. (1981)

Gerardia sp. (= Parazoanthus)

Hawaiian gold coral

Hawaiian archipelago & Emperor seamounts

300-400 Grigg (1974)

Antipathes dichotoma

black coral Main Hawaiian Islands, Indo-West Pacific region

30-100 Grigg (1976) and observations from the Starr II submersible, which supercede depth ranges given in Grigg (1974)

Antipathes grandis black coral, pine or umimatsu

Main Hawaiian Islands (Hawaii to Niihau)

45-100 Grigg (1976) and observations from the Starr II submersible, which supercede depth ranges given in Grigg (1974)

Antipathes spp. black coral Caribbean Sea Taxonomy, patterns of distribution & abundance yet to be adequately described.

Antipathes spp., Cirrhipathes sp.

Philippine Sea Taxonomy, patterns of distribution & abundance yet to be described.

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large beds of C. secundum have been located in Hawaii are in environments where sediments

virtually never accumulate. Between 1971 and 1975 a large-scale survey using the submersible

Star II was used to investigate all potential sites for precious coral in the major Hawaiian Islands.

Thirty-one dives to 400 meters were completed. These surveys showed conclusively that most

shelf areas near 400 meters are periodically covered by shallow lenses of sand and silt. Only in

habitats always free of sediment were large and abundant stands of Corallium found.

In habitats well removed from terrestrial sources of sediment such as seamounts,

sedimentation rates would be expected to be much lower. In such cases the strength and

consistency of bottom currents may be of less importance. The Emperor Seamounts may

provide one such example (Grigg, 1974). Even so, the large scale correlation between the

position of the Kuroshio Current in the western Pacific and the location of rich beds of Corallium

emphasizes the overall importance of bottom currents.

Bottom currents are also an important ecological factor in terms of transporting food and

carrying away metabolic wastes. Many species of both precious and non-precious deep sea

corals exhibit various adaptations in skeletal morphology, branching and orientation apparently

to maximize the exposure of feeding surfaces to water-borne food particles (Grigg, 1965, 1972,

1976; Wainwright and Dillon, 1969; Warner, 1981). Many species of pink, black, and gold coral

form fan-shaped colonies that orient at right angles to the prevailing current. The bamboo coral,

Lepidisis olapa, is unbranched but forms long coils that trail in the current, a shape that

presumably increases the feeding efficiency of polyps. The feeding habits of most precious

corals are unknown, although it has been shown that several species of black coral are largely

planktivores (Grigg, 1965; Lewis, 1978; Warner, 1981).

Light may be the most important factor in terms of setting the upper depth limit for a

number of precious corals. Grigg (1965) has presented strong evidence that the larvae of the

black corals Antipathes grandis and A. dichotoma are negatively phototaxic. This behavior has

the advantage of concentrating settlement at depths below the wave base where wave induced

abrasion is minimal or absent. The pattern of distribution of Corallium rubrum in the

Mediterranean Sea, where in water less than 30 meters deep colonies are found only in dimly lit

caves, suggests a similar larval behavior (Marchetti, 1965b). Furthermore, the relative shallow

occurrence of species of Corallium in the region of the Kuroshio Current may be due to the high

productivity and turbidity of the water. Kuroshio translates into English as black current.

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In general temperature does not appear to play a strong role in the ecology of most

species of precious coral except possibly in setting lower depth limits. Species of Corallium in

the Pacific Ocean are found in waters that range between 8o and 20oC. Similarly, C. rubrum in

the Mediterranean Sea occurs at depths between 5 and 300 meters, which may involve as much

as a 10oC difference in temperature (Barletta, et al., 1968). Evidence that temperature may set

the lower depth limit for two species of black coral is the correspondence between their lower

depth ranges and the top of the thermocline in the major Hawaiian Islands (100 m, see Sechel,

1962).

The growth rate of all precious corals is relatively slow. For species for which data exist

there appears to be an approximate relationship between maximum size and growth rate. Species

that form large colonies tend to grow faster (Table 11.2). If this is true generally, one

consequence would be that many species would be characterized by roughly the same longevity.

However, to be cautious the data must be said to be too meager to generalize at this point. In fact

they only represent commercial grade pink and black corals and may only reflect differences

between these two groups.

Table 11.2. Growth rates of precious corals

Species

Maximum

height (cm)

Growth rate

(cm y-1) Location Reference

Corallium secundum 75 1.0 Hawaii Grigg (1976)

Corallium rubrum 45 0.5-2.0 Mediterranean Bauer (1909)

Antipathes dichotoma 250 6.4 Hawaii Grigg (1976)

Antipathes grandis 300 6.1 Hawaii Grigg (1976)

Antipathes salix 250 4.5 Caribbean Olsen and Wood (1980)

The growth rates of black corals in Table 11.2 are based on direct measurements in the

field. For C. secundum the estimate is based on an assumption that concentric growth rings that

are visible in all sections (Brown, 1976) are annual. The rate of C. rubrum is inferred from rates

of recovery of harvested grounds off Algeria in the Mediterranean Sea (Bauer, 1909).

All species of precious coral except one develop distinct non-coalescing colonies.

Branches grow together only within colonies (Grigg, 1965 for Corallium and Antipathes spp.;

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Kishinouye, 1904 for Corallium, Grigg, personal communication for Lepidsis and Acanella).

The exception to this rule is Gerardia sp., which in Hawaii is always found in association with

Acanella as a parasitic overgrowth. This behavior for Hawaiian specimens has previously been

noted by Brown (1976). Gerardia is therefore only found in areas where colonies of Acanella

previously exist.

What is known of the reproductive biology of precious corals suggests that within most

species sexes are separate. This behavior applies to C. rubrum (Vighi, 1970), C. secundum and

A. dicohtoma (Grigg, 1976). For these species the reproductive cycles are annual, and spawning

occurs during summer months. For C. rubrum gametogenesis is affected by temperature, and

shallow colonies mature earlier in the year (Vighi, 1970), as does Muricea californica off

California (Grigg, 1977a). Age at reproductive maturity in C. secundum and A. dichotoma is

similar (~12 years old), and the two species have similar life spans (~75 years). Fertilization in

C. rubrum is internal, and larvae are released as fully mature planulae (Lacaze-Duthiers, 1864).

As mentioned above, patterns of settlement suggest that C. rubrum larvae are negatively

phototaxic. The same appears true for A. dichotoma and A. grandis.

Asexual reproduction by fragmentation is common in many species of coral (Highsmith,

1982), including some precious corals. However, in contrast to reef building species whose

fragments frequently regenerate, reattachment of fragments is uncommon for species of both

Corallium (Kishinouye, 1904) and Antipathes (Grigg, pers. comm.). On the other hand, colonies

of Antipathes have been successfully transplanted if firmly secured by an artificial base (Grigg,

1965).

Recruitment and mortality rates appear to be quite low for most precious corals. This is

an obvious corollary to their longevity. A notable exception is Corallium rubrum, for which

heavy settlement has been frequently observed in shallow caves in the Mediterranean Sea.

Interestingly, C. rubrum is the smallest and probably the shortest lived of all the commercial

species of Coralllium. Mortality rates have been measured for two species of black coral and

one species of Corallium in the field (Table 11.3). The best estimates vary between 4% and 7%

per year. This means that turnover of a population occurs about once every 15-25 years.

In favorable environments for C. secundum and A. dichotoma in Hawaii, the age

frequency distribution for both species has been found to be relatively stable (Grigg, 1976),

suggesting that recruitment and mortality are approximately in steady state. In contrast, precious

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coral populations surveyed near unsuitable habitats in Hawaii all exhibit truncated or highly

uneven age-frequency distributions for both species (Grigg, 1984). Hence the shape of the age-

frequency distribution (smooth versus uneven) of precious corals might serve as an index of

habitat stability.

Table 11.3. Mortality rates of precious corals.

Species Locality Mortality (% per year) Reference

A. dichotoma Hawaii 7 ± 2 (Grigg, 1976)

A. salix Caribbean 4 (Olsen and Wood, 1980)

C. secundum Hawaii 6.6 (Grigg, 1976)

The most common cause of mortality in marginal habitats for Corallium in Hawaii is

smothering by movement of sand along the bottom. Steep slopes in close proximity to shallow

water are often unsuitable because of downslope transport of sand and other debris. Sand

deposits have also been observed to completely obliterate exposed limestone terraces that would

otherwise serve as ideal substrata for black corals in Hawaii. Differences in the retention of sand

on the insular shelves of the Hawaiian Islands is probably the main factor that accounts for

variability in the abundance of black coral around the Hawaiian Islands.

In environments essentially free of heavy sedimentation, the most common source of

mortality for both Corallium and Antipathes in Hawaii is toppling caused by organisms that bore

into and weaken the site of basal attachment. Encrustation is a secondary cause of mortality to

the black corals, particularly at shallow depths.

Nature of Skeleton and Criteria for Evaluation of Raw Material

The skeletons of precious corals provide the raw material for the industry. Red and pink

corals consist of a very hard, high magnesium calcium carbonate. Black and gold corals of the

genus Gerardia have entirely protein skeletons, while the bamboo corals consist of skeletons that

have alternating sections of calcium carbonate and protein. Finally, the gold corals in the genus

Primnoa have skeletons of protein that are abundantly infused with calcite (CaCO3) spicules.

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The value of the raw material of any species of precious coral depends primarily on its

size, color, abundance, and condition (whether collected live or dead). Precious corals less than

1 cm in diameter directly above the base rarely have any commercial value. Rich beds generally

consist of much larger colonies (3-10 cm in basal diameter). Color varies according to species

and locality where collected. The color of at least some species of Corallium is due to the

presence of organic matter in the skeleton (Kishinouye, 1904). The organic substance is actually

a matrix of spicule sacs that are cemented together to form the skeleton. As for the effect of

color on value, fashion trends change. During the 1980s the red varieties of Corallium were

considered the most valuable. During the 1970s angel-skin varieties were preferred (Grigg,

1984).

Abundance is also an important factor that affects value. As a general rule, the greater

the abundance, the lower the price.

Condition refers to the state of the precious coral when it was collected. For species of

Corallium, Japanese fishermen have four terms to distinguish condition: ikiki = alive, tachigareki

= dead but still attached, ochii = dead but fallen, and mushikui = dead, fallen, and “worm” eaten.

For black, gold, and bamboo corals, the same general criteria apply, except that for black coral

an additional factor is the degree to which dead portions of the skeleton are encrusted.

History of the Fishery and Methods of Harvest

Precious corals have been treasured by humans since Paleolithic times. Evidence of this

are perforated beads of red coral uncovered in Germany with human remains and other artifacts

that date from the Aurignacian Period about 25,000 B.C. (Tescione, 1965). The history of

precious coral and its cultural importance over the millennia from early Greece up through

modern times has been exhaustively researched by Tescione, who has presented his findings in

two magnificent volumes (Tescione, 1965, 1968). Other recent historical accounts include a

paper by Hickson (1924), in which emphasis is placed on early trade routes between Europe and

the Middle East. Marco Polo is said to have carried precious corals east across the silk road.

Two other unpublished accounts of recent historical development exist, one on the Italian fishery

over the last 100 years by (Apa, 1971) and the other on the recent history of the industry in Japan

8

and Okinawa by Morita (1970). A condensed popular version of the history and mythology of

precious corals has been published by Grigg (1977b).

It was undoubtedly the presence of C. rubrum in the Mediterranean Sea that secured a

place for precious coral in the history and culture of early humans. Perhaps because of its luster

and color (blood red), hardness and tree-like shape, red coral was considered a symbol of

immortality and a panacea for many ills and more. Actually much more. No history would be

complete without some mention of the magic attributed to red coral by the ancients. The

Romans took ground Corallium powder as an antidote to poison (Wells, 1981b). It was also

believed to be a cure for all manner of stings, to be a comfort for fainting spirits, to counteract

fascinations, to protect man against sorcery, to purify the blood, to cure imbecelicity of the soul,

melancholy, mania, and other maladies. It was also believed to be a protector against the evil

eye and the shade of Satan. Similar powers were also attributed to black coral in some countries.

In Indonesia, for example, folklore maintains that a black coral bracelet worn on the right arm

increases virility, while one worn on the left arm cures rheumatism (Wells, 1981b). Many of

these and other attributes arose from the myth of Perseus and the Gorgon Medusa. Precious

coral was thought to have arisen from soft algae petrified by the stare of the severed Gorgon

head and stained red by its blood. Magical properties of the coral were said to have been

conferred by Minerva pleased by the exploits of her brother Perseus, who had killed the Gorgon

monster and cast its head into the sea.

The history of precious corals was confined to the Mediterranean Sea until the early

1800’s, when Corallium was discovered in the Pacific in the sea off Japan. For nearly 5,000

years a fishery had existed in the Mediterranean waxing and waning depending on supply (new

discovery of beds) and demand and various political struggles for hegemony over the sea. Coral

fishing started in Japan in the time of Bunka (1804-1818) but did not flourish until after the Meiji

Reform in 1868. Prior to that year, any corals brought up were confiscated by daimyo or

Shoguns. The Pacific fishery was centered on grounds off Japan, Okinawa, the Bonin Islands,

and Taiwan until about 1965, when a huge strike was made north of Midway Island in

international waters on the Milwaukee Bank. For about 20 years most of the world’s harvest

came from the Milwaukee Bank area and surrounding seamounts in the Emperor Seamount

chain. Several hundred tonnes of precious coral were harvested from this area annually by

Japanese and Taiwanese fishermen (Grigg, 1984). Current production in the far western Pacific

9

is about 10% of this level (20 tonnes). For comparison, the annual harvest in the Mediterranean

Sea is only about 5 tonnes per year (Boulhel, 1981; Hunnan, 1980), about 2.5% of the production

in the Emperor Seamounts.

In both the Mediterranean and the Pacific, the primary method of collection of Corallium

spp. has been by dredging. The dredge used in the Mediterranean (the engegno) is actually a

heavy cross with attached nets that are dragged across the bottom. Japanese and Taiwanese coral

fishermen dredge with coral mops (nets weighted down with natural stones) that are also pulled

across the bottom. In Hawaii a small submersible was used for a time (1971-1978) to harvest all

species of precious coral. However, depressed coral prices in the late 1970’s in combination

with increasing operational costs halted this operation.

In addition to Corallium, other species of precious coral presently harvested include two

species of black coral (A. dichotoma and A. grandis) and one species of gold coral (Gerardia) in

Hawaii, two primnoid gold corals (Primnoa spp.) in Alaska, the black coral A. dichotoma in Fiji,

Tonga, and Palau, and several undescribed species of black coral in the Philippine Sea and the

Caribbean Sea (Table 1). Harvest levels vary between these areas depending on supply and

demand. However, in the aggregate global annual production of black and gold coral is about 10

tonnes and 1.0 tonne, respectively. In all areas black corals are collected by SCUBA divers,

whereas both Garardia and Primnoa species are harvested with dredging devices.

Economics

The precious coral industry has always been characterized by periods of boom and bust.

Several boom periods are in fact famous in areas where they have occurred since they resulted in

a literal coral rush. In the Mediterranean Sea, a discovery of large beds between Sicily and Tunis

in the 1880’s led to an unprecedented rush of almost 2,000 vessels, which rapidly depleted the

grounds (Tescione, 1968). The same thing occurred near Okinawa and the Miyako grounds in

1963 but on a smaller scale (Morita, 1970). In 1965 another discovery on the Milwaukee Banks

in the Emperor Seamounts led to a temporary coral glut in the late 1960’s. This was followed by

yet another period of over-supply in 1980-81 due to the discovery of Midway deep-sea coral at

depths of 1.0 to 1.5 km, also in the Emperor Seamounts (Grigg, 1982a). In years when supply

has been excessive, the price of raw material may actually fall below the break-even level. This

10

occurred in 1982 and caused fishing effort in the Emperor Seamounts to fall by a factor of three

(Grigg, 1984).

In 1982 the value of the pink coral industry in Taiwan and Japan was about $50 million.

To this must be added sales of imported coral products in other countries such as Italy and the

United States. The lack of economic stability that has traditionally plagued the industry in many

parts of the world is a consequence of overharvest and rapid depletion. With the exception of

Hawaii, this pattern has resulted from the lack of any successful efforts to manage the fisheries.

In Hawaii, a management plan for black corals was developed by Grigg (1984). At the present

time commercial landings of black corals in Hawaii are averaging only about 1.0 tonne y-1, which

is well below the estimated maximum sustainable yield (see below). Virtually all black coral

products sold in Hawaii today are produced from corals harvested in the Philippines and

converted to finished products in Taiwan.

Conservation and Management

Precious corals are vulnerable to over-exploitation for the following reasons:

1. They are characterized by generally slow growth rates and population turnover.

2. Being sessile, they are readily exposed to repeated fishing pressure.

3. Many beds of precious corals exist in international waters where no management

authority exists.

4. In areas where jurisdiction is clear, few or no management guidelines exist.

5. Where laws have been passed, enforcement has been difficult.

Several of these problems were first appreciated by the Arabs, who attempted to rotate

fishing effort on the grounds off Tunis in the tenth century, but to no avail (Bauer, 1909).

Attempts were also made to limit entry into the Okinawa fishery in 1963 during the “Miyako

coral rush”. Unfortunately lack of enforcement in both of these cases undermined the success of

these attempts, and the result was resource depletion.

Aside from problems of enforcement and jurisdiction, another serious difficulty in the

management of precious corals historically has been the lack of knowledge of their population

11

biology. Growing appreciation of this problem led to an increasing number of studies on the

biology of precious corals. Given even a minimal understanding of the natural history of a

species of precious coral, there are a number of approaches that can be taken to management of

the resource. These include the following:

1. A total ban or moratorium on the taking of coral. This approach was taken in the Virgin

Islands for black coral and in the Philippines by an executive order by Ferdinand Marcos

for reef building corals in general.

2. Reserves. Reserves for the protection of C. rubrum in the Mediterranean Sea have been

established by the government of Spain (Hunnan, 1980) and in the Hawaiian Islands for

pink, gold, and bamboo corals (Anonymous, 1980).

3. Limited entry. Also practiced in Spain for C. rubrum in the Mediterranean Sea (Hunnan,

1980).

4. Benign neglect. Practiced by most countries of the world. Fortunately past history has

shown that to date precious corals become economically extinct before they are close to

biological extinction. An example is C. rubrum in the Mediterranean Sea, which has

withstood harvesting on and off for over 2,000 years without suffering extinction. This

example, however, may be atypical, since C. rubrum is perhaps the most fecund of the

precious corals (Grigg, 1984).

5. Placement on Appendix II of CITES (Convention on International Trade in Endangered

Species of Wild Flora and Fauna, United Nations). Export is allowed only if an export

permit has been issued by the country of origin. Both black (Antipathes spp.) and pink

(Corallium spp.) corals have been added to Appendix II. Unfortunately, enforcement of

CITES Appendix II has been inconsistent.

6. Specific size or weight quotas. Currently practiced in Hawaii as provided by state and

federal law (Anonymous, 1980).

Before guidelines for size and/or weight limits can be developed for any species of

precious coral, it is necessary to have estimates of basic demographic rates such as growth,

recruitment, and mortality. With the use of such data, Grigg (1984) developed a management

plan for black coral in Hawaii based on the fishery model of Beverton and Holt (1957).

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Description of the Model

The model is best described as a yield per recruit model. Data requirements for the

model include at least the following: measures of distribution and abundance, growth data in

terms of weight increase per unit time, rate of instantaneous mortality and rate of instantaneous

recruitment. While not absolutely required, information on age of sexual maturity is also useful

in the application of the model. Often growth data are expressed in terms of linear increases per

unit time. In order to convert linear increases to weight increases, an expression relating length

to weight must be derived. The most important limitation of the model is the assumption of

steady state. However, for many species of corals (precious and non-precious) this is probably

not a serious constraint. Conditions are most stable in the most favorable environments, where

balanced rates of recruitment and mortality would be most likely to occur. Such appears to be

the case for populations of C. secundum and A. dichotoma in Hawaii (Grigg, 1976). Many

species of reef coral have similar life histories, i.e., they grow slowly, have a long lifespan, and

have low rates of recruitment and mortality. Because of this fact, the Beverton and Holt model

may have widespread application to management of coral resources.

The model works in the following way. Given knowledge of rates of growth, mortality,

and recruitment for a species, a cohort of 100 (for example) organisms is allowed to gain weight

until a point is reached where growth gains are overtaken by mortality losses. A cohort is a

group of recruits all beginning life at the same time and place. Maximum production of the

cohort occurs at the point where losses due to mortality overtake gains from growth (Fig. 11.1).

As the cohort ages and reaches a point of maximum longevity, production declines to zero.

In this case, the assumption of steady state means that the yield of a single cohort over its

lifespan is equal to the yield of all cohorts or age classes present in a single year. The precision

and accuracy of the Beverton and Holt model depends primarily on the accuracy of the data

collected and the degree to which the assumption of steady state is violated. Even in the case of

precious corals and other species that have relatively stable population dynamics, it is important

to recognize that nature is never without some variability. For example, variations in annual

recruitment are almost certain to occur. Thus it may be advisable to introduce year-specific

estimates of certain population parameters into the model. While this modification is possible, it

13

does not make the analysis more complex. As a first approximation it may be useful to initially

apply the model using invariant estimates of growth, recruitment, and mortality.

Figure 11.1. Average biomass of an A. dichotoma colony recruit versus time. The average

biomass increases up to age 28 because growth of surviving colonies more than offsets

population losses due to natural mortality. Beyond age 28, losses of biomass due to natural

mortality are greater than gains associated with the growth of the surviving colonies.

The results of the model as it has been applied to the black coral fishery (primarily

Antipathes dichotoma) in Hawaii are as follows:

1. Standing crop of Maui black coral bed – 84,000 colonies age two years and older1

2. Growth rate – 6.4 cm per year

1 Colonies younger than 2 years of age were assumed to be sufficiently small that they would be missed in a survey.

14

3. Weight increase is described by the equation: wt = 0.19 · (H)2.05, where H is the height

of the colony in centimeters, and wt is the weight of the colony in grams.

4. Instantaneous rates of natural mortality and recruitment were determined by an analysis

of the age-frequency distribution of a portion of the unfished population (Fig. 11.2). The

age-frequency distribution of A. dichotoma was obtained by measuring 152 colonies

selected randomly. Height data were converted to age data by dividing by the growth

rate (6.4m y-1). The instantaneous rate of natural mortality, M, was calculated from the

regression of the natural logarithm of the percentage of the colonies in each age class

against time (Fig. 11.2). The value of M determined from the least squares regression

line was 7% per year.

5. In steady state, coral mortality must be balanced by recruitment. If there are 84,000

colonies age two or older and if M = 7% per year, then natural mortality must be (7%)

(84,000) = 5,880 colonies per year. Hence recruitment at age two years must be 5,880

colonies per year. Yield per recruit is obtained by plotting the biomass of a cohort of

5,880 recruits divided by 5,880 versus time (Fig. 11.1).

Figure 11.3 summarizes the various components of the model. The height of a colony is

assumed to be directly proportional to the age of the colony (Fig. 11.3A). Because the colonies

are basically two-dimensional, the weight of a colony is almost directly proportional to the

square of the height or equivalently to the square of the age (Fig. 11.3B). Percentage survival

declines exponentially with time, the rate of decline being 7% per year (Fig. 11.3C). The yield

per recruit is the product of the weight of an individual colony and percentage survival (Fig.

11.3D). Weight increases with time, survival decreases with time, and the product peaks (in this

case) at an age of 28 years. The peak in the yield per recruit is 1.28 kg. Since there are 5,880

recruits, if all colonies are harvested at an age of 28 years, the total yield would be (5,880)(1.28

kg) = 7.5 tonnes per year.

An important caveat to this conclusion is the requirement that recruitment not be affected

by the proposed harvesting plan. Since A. dichotoma becomes sexually mature at an age of

about 12 years, the proposed strategy would allow give the coral 16 reproductive years.

15

Figure 11.2. Percentage of Maui Antipathes dichotoma colonies versus age from data reported

by (Grigg, 1976). Straight line is a linear regression fit to the log-transformed data. The slope of

the line is -0.068 per year.

Figure 11.4 shows the actual record of black coral harvests in Hawaii since the

implementation of the fisheries management plan. The actual landings have been averaging just

about 1.0 tonne per year with an ex-vessel value of about $42,000 per year. These are not very

impressive figures. Grigg (2001) resurveyed the Maui black coral beds in 1998, and the results

of his survey are shown in Fig. 11.5. He found no colonies older than 27 years, which seems

consistent with the management plan. The mortality rate estimated from the age-frequency

distribution was about 10% per year, 3% higher than the mortality rate estimated from his 1975

survey. What is the explanation for the increase in mortality?

16

Figure 11.3. (A) Length of A. dichotoma colony, (B) weight of individual colony, (C) %

survival of colonies, and (D) Yield per recruit as a function of colony age.

By eliminating older colonies through harvesting, the management plan has

effectively reduced the number of colonies in the stock. Hence the same rate of mortality for the

younger colonies translates into a higher overall percentage loss. In other words, if the

percentage of mortality is calculated from the ratio (number of colonies dying)/(total number of

colonies), the fact that there were fewer total colonies in 1998 would logically imply a higher

rate of mortality, even though the numerator in the ratio was unchanged. However, in this case

the increase in mortality is higher than expected if the fishery took only twenty-eight year old

colonies. The management plan that was actually implemented allowed harvesting of colonies as

young as 19 years, i.e., colonies 122 cm in height. Colonies smaller than this height accounted

for about 74% of the black coral colonies in the 1975 survey. Eliminating all colonies 19 years

17

and older through harvesting would thus reduce the number of colonies by a factor of 1/0.74 =

1.35, which is comparable to the observed increase in mortality, i.e., 10%/7% = 1.43.

Figure 11.4. Commercial landings of black coral in Hawaii and ex-vessel value of the corals.

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Figure 11.5. Percentage of Maui Antipathes dichotoma colonies versus age from data reported

by Grigg (2001). Straight line is a linear regression fit to the log-transformed data. The slope of

the line is -0.098 per year.

19

References

Anonymous, 1980. Fishery management plan and proposed regulations for the precious coral fishery of the Western Pacific region. U. S. Fedeeral Register 45 (180), 60957-61002.

Apa, M. P., 1971, Status of the precious coral industry in Italy, Torre del Greco, 1971. unpublished manuscript.

Barletta, G., Marchetti, R., Vighi, M., 1968. Ricerche sul corallo rosso. Instituto Lombardo (Rend. Sc.) B 102, 119-144.

Bauer, M., 1909. Precious Stones, 2nd ed. Dover, New York.

Bayer, F. M., 1950. A new precious coral from North Bornea. Washington Academy of Science 40 (2), 59-61.

Belloc, G., 1950. Inventory of the fishery resources of Greek waters, Food and Agriculture Organization of the United Nations, Rome.

Beverton, R. J. H., Holt, S. J., 1957. On the dynamics of exploited fish populations. H.M. Stationery office, London.

Boulhel, M., 1981. Amenagement des resources vivantes de la zone littorale tunisienne. Management of living resources in the Mediterranean coastal area. Studies and reviews / General Fisheries Council for the Mediterranean (Vol. 58, pp. 95-130), FAO, Rome.

Brown, G., 1976. Two new precious corals from Hawaii. Australian Gemmologist 11, 371-377.

Cimberg, R. L., Gerrodette, T., Muzik, K., 1981. Habitat requirements and expected distribution of Alaska coral (Final report to NOAA for contract 27-80), NOAA, Boulder, Colorado.

Grigg, R., 1965. Ecological studies of black coral in Hawaii. Pacific Science 19, 244-260.

Grigg, R., 1972. Orientation and growth form of sea fans. Limnology and Oceanography 17, 185-192.

Grigg, R., 1974. Distribution and abundance of precious corals in Hawaii. In: Cameron, A. M., Campbell, B. M., Cribb, A. R., Endean, R., Jell, J. S., Jones, O. A., Mather, P. and Talbot, F. H. (Eds.), Proceedings of the 2nd International Symposium on Coral Reefs (Vol. 2, pp. 235-240), Great Barrier Reef Committee, Brisbane.

Grigg, R., 1975. Age structure of a longevous coral: A relative index of habitat suitability and stability. American Naturalist 109, 647-657.

20

Grigg, R., 1976. Fishery management of precious and stony corals in Hawaii (Sea Grant Technical Report 77-03), University of Hawaii, Honolulu.

Grigg, R., 1977a. Population dynamics of two gorgonian corals. Ecology 58, 278-290.

Grigg, R., 1977b. Hawaii's precious corals. Island Heritage Publishing, Honolulu, Hawaii.

Grigg, R., 1982a. Economics and future development of the precious coral fishery in the Pacific. Infofish 2, 8-11.

Grigg, R., 1982b. Status of the precious coral industry in 1982 (Technical Report), Food and Agriculture Organization of the United Nations, Rome.

Grigg, R., 1984. Resource management of precious corals: A review and application to shallow water reef building corals. Marine Ecology 5 (1), 57-74.

Grigg, R., 2001. Black coral: History of a sustainable fishery in Hawaii. Pacific Science 55 (3), 291-299.

Grigg, R., Eade, J. V., 1981. Precious corals. In: Exon, N., Tiffon, D. and Gauss, G. (Eds.), Report on the inshore and nearshore resources training workshop, Suva, Fiji, CCOP-SOPAC, Suva, Fiji.

Hickson, S., 1924. An Introduction to the Study of Recent Corals. Manchester at the University Press, London, U. K.

Highsmith, R. C., 1982. Reproduction by fragmentation in corals. Marine Ecology Progress Series 7, 207-226.

Hunnan, P. J., 1980. Mediterranean specis in possible need of protection (Report to IUCN), Aquatic Biology Consultancy Services, Ltd., Chelusford, Essex, U. K.

Kishinouye, K., 1904. Notes on the natural history of corals. Journal of the Imperial Fisheries Bureau 14 (1), 1-32.

Kitahara, T., 1902. On the coral rishery of Japan. Journal of the Fisheries Bureau 13 (3), 1-13.

Lacaze-Duthiers, H., 1864. Histoire naturelle du Corail. J. B. Bailliere, Paris.

Lewis, J. B., 1978. Feeding mechanisms in black corals (Antipatharia). Journal of Zoology 186, 393-396.

Marchetti, R., 1965a. Ricerche sul corallo rosso della costa Ligure e Toscana, I. Distribution geografica. Instituto Lombardo (Rend. Sc.) B 99, 255-278.

21

Marchetti, R., 1965b. Ricerche sul corallo rosso della costa Ligure e Toscana, II. Il promontorio di Portofino. Instituto Lombardo (Rend. Sc.) B 99, 279-316.

Morita, M., 1970, Story of sango, to commemorate the tenth anniversary of coral industry in Okinawa. unpublished manuscript.

Olsen, D. A., Wood, R. S., 1980. Investigations on black coral in Salt River Submarine Canyon Saint Croix, U.S. Virgin Islands (Final Scientific Report 80-12), National Undersea Laboratory System, St. Croix, Virgin Islands.

Sechel, G. R., 1962. Atlas of the oceanographic climate of the Hawaiian Islands region. Fishery Bulletin 61, 371-427.

Tescione, G., 1965. Il corallo nella storia e nell'arte. Montanino editore, Napoli, Italy.

Tescione, G., 1968. The Italians and their coral fishing. Fausto Fiorentino, Napoli, Italy.

Vighi, M., 1970. Recerche sul ciclo reproduttivo del corallo rosso (C. rubrum [L.]) del promontorio di Portofino. Lincei-Mem. Sci. fisiche, ecc. Ser. III 10, 1-26.

Wainwright, S. A., Dillon, J. R., 1969. On the orientation of sea fans (genus Gorgonia). Biological Bulletin, Marine Biological Laboratory, Woods Hole 136, 130-139.

Warner, G. F., 1981. Species descriptions and ecological observations of black corals (Antipatharia) from Trinidad. Bulletin of Marine Science 31, 147-163.

Wells, S., 1981a. International Trade in Corals, IUCN Conservation Monitoring Centre, Cambridge, U. K.

Wells, S., 1981b. IUCN Invertebrate Red Data Book. IUCN, Cambridge, U. K.

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