264
The Biology of Sea Turtles, Vol. II
1688 and 1730 (Jackson, 1997). By 1790, green turtles had become scarce in Cayman
waters and soon could not support a fishery, so Cayman turtlers went to the waters
of southern Cuba (Williams, 1970; Smith, 2000). By 1830, green turtles off south
Cuba had diminished, so Cayman turtlers went to the Miskito Cays, off the coasts
of Nicaragua and Honduras (Williams, 1970). By 1890, concerns were expressed
over the growing scarcity of turtles in the Miskito Cays (Hirst, 1910). In 1901,
Duerden (1901) urged the government of Jamaica to establish artificial hatching and
rearing facilities for green turtles and hawksbills because of “the diminution in the
supply [from the Miskito Cays] which is now being felt.”
Although the Cayman green turtle story is the best known, it is far from being
the only extirpation of green turtle populations in the Caribbean. Early historical
accounts report “vast quantities” of sea turtles in areas where few, if any, sea turtles
exist today. For example, the pirate John Esquemeling, in his account of the activities
of buccaneers in America, described turtles that “resort in huge multitudes at certain
seasons of the year, there to lay their eggs” on the Isle of Savona off the coast of
Hispaniola, as well as on the west coast of mainland Hispaniola (Esquemeling,
1684). Neither area supports such sea turtle nesting today.
The pattern of overexploitation of green turtles is clear from these accounts.
However, how many green turtles lived in the Caribbean before humans began
harvesting them? Jackson (1997) used the Jamaican exploitation records described
above to estimate the preexploitation number of adult green turtles in the Caribbean.
Jackson’s estimates ranged from 33 to 39 million adult green turtles.
If preexploitation green turtle populations were regulated by food limitations,
the carrying capacity ( K ) of Caribbean seagrass beds for the green turtle would be
a maximum estimate of population size. The seagrass Thalassia testudinum is the
primary diet of green turtles in the Caribbean (Bjorndal, 1997), and the green turtle
is one of the few species that consumes Caribbean seagrasses as a major part of its
diet (Thayer et al., 1984) after the extinction of the diverse dugongid fauna before
the Pleistocene (Domning, 2001). Populations of large herbivores are often “bottomup” regulated by food limitation rather than “top-down” by predators (Sinclair, 1995;
Jackson, 1997), so green turtle populations in the greater Caribbean may well have
been controlled by food limitation (Bjorndal, 1982; Jackson, 1997), and densitydependent effects would have regulated productivity of green turtles (Bjorndal et al.,
2000). Jackson (1997) used an estimate of the carrying capacity of the seagrass T.
testudinum for green turtles from Bjorndal (1982) and generated an estimate of 660
million adult green turtles in the Caribbean. Bjorndal et al. (2000) estimated a range
of carrying capacities of T. testudinum for green turtles based on three estimates of
intake and two estimates of T. testudinum productivity (Table 10.2). The estimates
ranged from 122 to 4439 kg of green turtle per hectare (ha) of T. testudinum , or
16–586 million 50-kg green turtles. This range nearly encompasses the range of
33–660 million adult green turtles of Jackson (1997). The estimates of K vary by
an order of magnitude based on the two productivity levels of T. testudinum measured
in areas heavily grazed and more moderately grazed by green turtles (Table 10.2).
This variation is not surprising. The biomass, rate of production, and quality of
seagrasses are all affected by grazing (Thayer et al., 1984). In grazing systems,
highest plant productivity is often associated with light to moderate grazing
1123 book.book Page 264 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
1688 and 1730 (Jackson, 1997). By 1790, green turtles had become scarce in Cayman
waters and soon could not support a fishery, so Cayman turtlers went to the waters
of southern Cuba (Williams, 1970; Smith, 2000). By 1830, green turtles off south
Cuba had diminished, so Cayman turtlers went to the Miskito Cays, off the coasts
of Nicaragua and Honduras (Williams, 1970). By 1890, concerns were expressed
over the growing scarcity of turtles in the Miskito Cays (Hirst, 1910). In 1901,
Duerden (1901) urged the government of Jamaica to establish artificial hatching and
rearing facilities for green turtles and hawksbills because of “the diminution in the
supply [from the Miskito Cays] which is now being felt.”
Although the Cayman green turtle story is the best known, it is far from being
the only extirpation of green turtle populations in the Caribbean. Early historical
accounts report “vast quantities” of sea turtles in areas where few, if any, sea turtles
exist today. For example, the pirate John Esquemeling, in his account of the activities
of buccaneers in America, described turtles that “resort in huge multitudes at certain
seasons of the year, there to lay their eggs” on the Isle of Savona off the coast of
Hispaniola, as well as on the west coast of mainland Hispaniola (Esquemeling,
1684). Neither area supports such sea turtle nesting today.
The pattern of overexploitation of green turtles is clear from these accounts.
However, how many green turtles lived in the Caribbean before humans began
harvesting them? Jackson (1997) used the Jamaican exploitation records described
above to estimate the preexploitation number of adult green turtles in the Caribbean.
Jackson’s estimates ranged from 33 to 39 million adult green turtles.
If preexploitation green turtle populations were regulated by food limitations,
the carrying capacity ( K ) of Caribbean seagrass beds for the green turtle would be
a maximum estimate of population size. The seagrass Thalassia testudinum is the
primary diet of green turtles in the Caribbean (Bjorndal, 1997), and the green turtle
is one of the few species that consumes Caribbean seagrasses as a major part of its
diet (Thayer et al., 1984) after the extinction of the diverse dugongid fauna before
the Pleistocene (Domning, 2001). Populations of large herbivores are often “bottomup” regulated by food limitation rather than “top-down” by predators (Sinclair, 1995;
Jackson, 1997), so green turtle populations in the greater Caribbean may well have
been controlled by food limitation (Bjorndal, 1982; Jackson, 1997), and densitydependent effects would have regulated productivity of green turtles (Bjorndal et al.,
2000). Jackson (1997) used an estimate of the carrying capacity of the seagrass T.
testudinum for green turtles from Bjorndal (1982) and generated an estimate of 660
million adult green turtles in the Caribbean. Bjorndal et al. (2000) estimated a range
of carrying capacities of T. testudinum for green turtles based on three estimates of
intake and two estimates of T. testudinum productivity (Table 10.2). The estimates
ranged from 122 to 4439 kg of green turtle per hectare (ha) of T. testudinum , or
16–586 million 50-kg green turtles. This range nearly encompasses the range of
33–660 million adult green turtles of Jackson (1997). The estimates of K vary by
an order of magnitude based on the two productivity levels of T. testudinum measured
in areas heavily grazed and more moderately grazed by green turtles (Table 10.2).
This variation is not surprising. The biomass, rate of production, and quality of
seagrasses are all affected by grazing (Thayer et al., 1984). In grazing systems,
highest plant productivity is often associated with light to moderate grazing
1123 book.book Page 264 Monday, November 11, 2002 11:11 AM