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Article Excerpt
Introduction
Around the year 1500, discoveries by Spanish explorers of sources of pearls, gold, and spices in the New World were a powerful stimulus for Spain to expand into the Americas. Samples of these resources, which Christopher Columbus and later crews brought back to Spain, so aroused public enthusiasm in Spain that navigators, explorers, and adventurers began to organize expeditions to seek the treasures of lands beyond the "Western Ocean." Columbus first saw the pearls in the Gulf of Paria, Venezuela, on his third voyage, where local Indians bad brought them from the Caribbean coast of Venezuela located to the northwest (Mosk, 1934, quoted by Galtsoff, 1950b; Morison, 1942; Hanson, 1967; Wagner, 1992).
The Spanish subsequently organized harvesting programs for pearl oysters in Venezuela and Colombia and began to ship huge quantities of pearls to Spain and other European countries for ladies adornment. The first Spanish town in the New World was established in 1528 on the Venezuelan island of Cubagua to serve as a center for harvesting pearl oysters and collecting pearls. The pearls from Venezuela, whose northeast shores became known as the "Pearl Coast," were relatively small, weighing 2-5 carats, but they were harvested in the largest quantities of any location in the New World. Within a decade or two following the discovery of the Venezuelan pearls, the Spanish found pearls and developed programs to harvest them on beds around islands off the Pacific Coast of Panama (Galtsoff, 1950a; MacKenzie, 1999) and in the Gulf of California, Mexico (Townsend, 1892; Kunz and Stevenson, 1908). They also searched for pearls in what is now the United States, but found none in its marine environments (Kunz and Stevenson, 1908).
By the late 1500's, the pearl oysters in Venezuela and Colombia had become much scarcer as a result of intensive fishing by hundreds of divers (Landman et al., 2001). Documentations of the pearl production may be the first records of resource declines in any of the world's marine fisheries that were brought about by intensive harvesting stimulated by strong market demand. In this case, large beds of natural pearl oysters that had been scarcely harvested beforehand were harvested intensely, albeit by primitive hand methods, and the beds were slowly depleted.
In 1948, Paul S. Galtsoff of the U.S. Bureau of Commercial Fisheries (now the National Marine Fisheries Service, NOAA) spent 2 months on Margarita Island, Venezuela, at the request of the Venezuelan Government which wanted him to recommend measures for managing its pearl oyster industry. This followed his similar stay in Panama for the same purpose (Galtsoff, 1950a). Galtsoff (1950b) reviewed the history of the Venezuelan pearl oyster industry, made observations and recommended research and management strategies, and later described them in that paper. Nothing has appeared in the international literature regarding this fishery since his paper was published. The senior author visited Margarita Island from 20 to 31 January 2002 to determine the history of this fishery industry between 1948 and 2002, collect additional earlier historical material, and photograph pertinent scenes. L. Troccoli and L. B. Leon S. have had long associations with fisheries in the Margarita Island-Cubagua Island-Coche Island region and contributed to this paper printed and verbal information about the fishery, the biology and ecology of the pearl oysters, and additional photographs.
The Pearl Oyster in the Caribbean Sea
The species of pearl oyster in the Caribbean Sea is the Atlantic pearl-oyster, Pinctada imbricata Roding 1798. It ranges beyond the Caribbean Sea to as far north as North Carolina (Ruppert and Fox, 1998) and south to Brazil (Abbott, 1974). The main fisheries for it have been off the coasts of Venezuela and northeastern Colombia. In Venezuela, the harvesting has been centered on beds near the clustered islands of Margarita Island, Cubagua Island, and Coche Island, 12-18 km off its northern coast (Fig. 1). Colombia's pearl oyster fishery was 1,000 km to the west on beds off the Guajira Peninsula close to the Venezuelan border. Bohlander (1992) reported that the explorer Alonso de Ojeda, in about 1500, observed people fishing for pearl oysters in what is now Lake Maracaibo, Colombia, but pearl oyster harvests there have not been described elsewhere to our knowledge.
[FIGURE 1 OMITTED]
The shells (valves) of pearl oysters are somewhat similar to some other oyster species. Its left valve is more concave than the right, and it has a byssal opening, a structure not universal in oysters (Fig. 2). The valves, which rarely exceed 7 cm in length, have three sections: a periostracum, a prismatic layer, and a nacre layer (Leon et al., 1987). The color of the outer surface of the valves varies from white to bronze and occasionally to black (Cervigon, 1998). Owing to its relatively small size and because its valves are thin, Atlantic pearl-oysters have not been used in the mother-of-pearl trade, which deals in ornaments, knife handles, and buttons from the larger pearl oyster shells harvested off the west coast of North America and in Asia. By 1900, thousands of tons of the pearl oyster shells lay in heaps along the Venezuelan coasts and in smaller quantities on the Colombian coast, where they had been left for centuries by oyster shuckers (Kunz and Stevenson, 1908).
[FIGURE 2 OMITTED]
Pearl Oyster Biology
At the time of Galtsoff's (1950b) survey in 1948, knowledge of the spawning, setting, and growth of the pearl oyster was scarce, but it was known that its larvae will set on hard objects with a clean surface, and growth of young pearl oysters is rapid. Some biological information relating to reproduction has since been gathered.
The Atlantic pearl-oyster is a protantric hermaphrodite. The small mature pearl oysters are males; the large pearl oysters are females. Their gonad surrounds their digestive diverticulum. Sperm and eggs are spawned into the water where fertilization takes place. The unfertilized eggs are 47-50[micro] in diameter; the sperm are 60[micro] long. In a laboratory study, the larvae resulting after fertilization took 20-25 days to grow to settlement size: 215[micro] (Ruffini, 1984) (Fig. 3, 4). Occurring in the tropics at lat. 11[degrees]-12[degrees]N, this pearl oyster has a relatively long spawning season as do many tropical species. Reproduction takes place throughout the year, as shown by juveniles less than 5 mm long being present during all months, but setting is heaviest from June into November and December when water temperatures are highest. Water temperatures rise from about 24[degrees]-25[degrees]C to 26[degrees]-28.5[degrees]C during May to November, when the oysters spawn, and their condition index, determined by the volumetric method, drops from about 65-70% to 40-50%. This index is lowest during October and November. The Las Cabeceras bed (Fig. 5), the largest remaining bed, had the highest recruitment of pearl oyster spat of the beds surveyed. The oysters grow from settlement size to about 7 cm within 14 months (Leon et al., 1987).
[FIGURE 3-5 OMITTED]
Ecology of Pearl Oyster Beds
The Atlantic pearl oyster inhabits clear waters (Cervigon, 1998). It does not occur where changes in temperature, salinity, and oxygen are large or on muddy bottoms (Leon et al., 1987). Its ready-to-set larvae attach with a byssus to hard substrates including other pearl oysters, rocks, dead coral and octocorals, other molluscan shells, and barnacles (Table 1). The oysters retain this attachment with several byssal threads throughout their lives unless, while seed, they are torn free by fishermen. Empty pearl oyster shells apparently do not accumulate beneath live pearl oysters as empty shells do in beds of the eastern oyster, Crassostrea virginica, in Canada and the United States. As an example, the oysters on the centuries-old Las Cabeceras pearl oyster bed near Cubagua Island rest on a base of sand rather than a deep base of empty pearl oyster shells. In some locations, eastern oysters rest on bases of shells at least 7 m deep.
The relatively shallow regions (down to at least 20 m) surrounding the islands of Margarita, Cubagua, and Coche and those off the peninsula of La Guajira and in Chengue Bay, Colombia, become enriched with nutrients (mainly nitrates, silicates, and phosphates) from December through February each year, when extra strong easterly winds cause upwelling that brings waters from the nearby ocean depths into the shallows (Fig. 6). Water temperatures that usually are about 26[degrees]-28[degrees]C in summer become 2-3[degrees]C cooler and the salinity increases by about 2[per thousand] to nearly 36[per thousand]. The nutrients stimulate a large increase in phytoplankton that feeds fish (mainly sardines, Sardinella aurita) and mollusks (i.e. pearl oysters and mussels, Perna perna) that increase sharply in overall size but especially the sizes of their gonads throughout this period, in advance of spawning (Leon et al., 1987; Cervigon, 1998; Gaspar, 1999; Urban, 2000a). The fisheries for these resources benefit enormously (Leon and Millan Q., 1996).
[FIGURE 6 OMITTED]
The taxonomic list of mollusks associated with P. imbricata on and near the Las Cabeceras bed includes 89 species: 48 gastropods, 34 bivalves, 6 cephalopods, and 1 chiton (Fig. 7) (Leon, unpubl. data).
[FIGURE 7 OMITTED]
In 1948, northeastern Venezuela had about 76 identified pearl oyster beds, nearly all of which were too small for worthwhile harvesting. The beds were located in the same areas as they had been historically, i.e. between Margarita Island and the Peninsula de Araya on the Venezuelan mainland, in depths from 4 to 20 m; several more beds were located off the north and northeastern shores of Margarita Island. The largest beds were at least 2.5 km across, and the most productive was the Las Cabeceras bed just east of Cubagua Island (Galtsoff, 1950b).
In Colombia, pearl oysters were harvested from a bottom area that extended for 150 km just off the coast of Guajira. In 1994, a survey of the abandoned beds found concentrations of live pearl oysters that ranged from 0.05 to 2.77 oysters/[m.sup.2]. Pearl oysters larger than 5 cm (marketable size) ranged from 25% to 49% of the total oysters. In one location, of the 493 pearl oysters that were opened, 17 had pearls (3.4%), most of which were around 2.5 mm in diameter, while in another location 279 pearl oysters had 12 pearls (4.3%) about 1.5 min in diameter. The pearls lacked the shape and luster required of good quality gems and were too scarce for commercial harvesting. Settlement densities of pearl oyster spat (juveniles), nonetheless, appeared to be sufficient to support a program of pearl culture which would include collecting spat on shells and other materials (Borrero et al., 1996).
Urban (2000b) observed aspects of P. imbricata reproduction in Chengue Bay, Venezuela, in 1997 and 1998. Chengue Bay is located on the north coast of Venezuela 150 km west of the Guajira pearl oyster area described by Borrero et al. (1996) and 14 km north northeast of the city of Santa Marta. Some natural stocks of P. imbricata are present in this bay. Urban (2000b) found the highest abundances of their larvae in November 1997 (1 collection: 0.8 larvae/[m.sup.3]) and in January...
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