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).
1
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
2
Table 11.1. Distribution of major species of precious corals
Species
Common
Where found
Depth
name
range (m)
Corallium rubrum Red coral of
Mediterranean Sea,
10-250
commerce
primarily coasts of
Sardinia, Corsica,
southern Italy,
Sicily and northern
Tunisia
Corallium
Angel skin or Hawaiian
350-475
secundum
pelle d’ange
archipelago from
Hawaii (20oN) to
the Milwaukee
Banks (36oN)
Corallium sp. nov. Midway
Midway Island to
1,000deep-sea
Emperor
1,500
coral
Seamounts, 28o36oN
Corallium
Aka-sango
Japan, Okinawa &
100-300
japonicum
Bonin Islands, 26o36oN
Corallium konojoi Shiro-sango
Japan to northern
50-150
Philippines, 19o36oN
Corallium elatius
MomoiroNorthern
150-330
sango
Philippines to
Japan, 19o-36oN
Primnoa
Alaskan gold Southeastern
10-800
resedaeformis,
coral
Alaska (Dixon
Primnoa willeyi
Entrance) to
Amchitaka,
Aleutian Islands
Gerardia sp. (=
Hawaiian
Hawaiian
300-400
Parazoanthus)
gold coral
archipelago &
Emperor seamounts
Antipathes
black coral
Main Hawaiian
30-100
dichotoma
Islands, Indo-West
Pacific region
Antipathes grandis
black coral,
pine or
umimatsu
Main Hawaiian
Islands (Hawaii to
Niihau)
Antipathes spp.
black coral
Caribbean Sea
Antipathes spp.,
Cirrhipathes sp.
Philippine Sea
3
45-100
Reference
Belloc (1950), Marchetti
(1965a), Lacaze-Duthiers
(1864)
Grigg (1974)
Grigg (1982a)
Grigg (1982a) and Kitahara
(1902)
Grigg (1982a) and Kitahara
(1902)
Grigg (1982a) and Kitahara
(1902)
Cimberg, et al. (1981)
Grigg (1974)
Grigg (1976) and observations
from the Starr II submersible,
which supercede depth ranges
given in Grigg (1974)
Grigg (1976) and observations
from the Starr II submersible,
which supercede depth ranges
given in Grigg (1974)
Taxonomy, patterns of
distribution & abundance yet
to be adequately described.
Taxonomy, patterns of
distribution & abundance yet
to be described.
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.
4
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
Maximum
Growth rate
height (cm)
(cm y-1)
Location
Corallium secundum
75
1.0
Hawaii
Corallium rubrum
45
0.5-2.0
Antipathes dichotoma
250
6.4
Hawaii
Grigg (1976)
Antipathes grandis
300
6.1
Hawaii
Grigg (1976)
Antipathes salix
250
4.5
Caribbean
Species
Reference
Grigg (1976)
Mediterranean Bauer (1909)
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.;
5
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
6
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 agefrequency 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)
A. dichotoma
Hawaii
7±2
A. salix
Caribbean
C. secundum
Hawaii
4
6.6
Reference
(Grigg, 1976)
(Olsen and Wood, 1980)
(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.
7
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).
12
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.
18
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
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