Biological Conservation 54 (1990) 61-78
Status, Conservation and Management of the Scarlet Ibis
Eudocimus ruben"in the Caroni Swamp, Trinidad,
West Indies
Keith L. Bildstein
Department of Biology, Winthrop College, Rock Hill,
South Carolina 29733, USA
and
Belle W. Baruch Institute for Marine Biology and Coastal Research,
University of South Carolina. Columbia, South Carolina 29208, USA
(Received 28 November 1989: accepted 13 February 1990)
ABSTRACT
A lthough scarlet ibises Eudocim us ruberJbrnwrly bred in the Caroni Swamp
in northwestern Trinidad, the)' have not done so since 1970. Even so, they
continue to /~,ed and roost in the swamp outside q/the breeding season. Dmqng
the breeding season, however, most adults leave the island, presumably to nest
at colony sites in South America. Although numerous causes, including
changes in habitat, have been suggestedJor the lack qf nesting, none has been
linked directO" to the consistent failure q[" ihises to Breed at the site.
Freshwater wetland losses in the region have been extensi~e. Almost all
seasonally)qooded wetlands east qZthe swamp have been ch'ained or are in rice
production, and nwst./brmer.lEeshwater wetlands within the swamp are m m
brackish. Here, this paper (1) reports on the status q[" ibises in the Caroni
Swamp; (2) documents the extent o[" saltwater intrusion in the area:
(3) describes a link between the loss of J?eshwater Jeeding sites and the
lack of ibis breeding; (4) suggests a strategy for re-establishing a breeding
population of ibises in the Caroni Swamp; and (5) makes recommendations
eoncerning other coastal populations o f ib{s'es.
INTRODUCTION
Scarlet ibises E u d o c i m u s ruber are b r i g h t - r e d c o l o n i a l - n e s t i n g w a d i n g birds
with long, tactilely sensitive, d e c u r v e d bills. T h e y b r e e d in b r a c k i s h - w a t e r
61
Biol. Conserv. 0006-3207/90/$03'50.4"~ 1990 Elsevier Science Publishers Ltd, England. Printed
in Great Britain
62
Keith L. Bildstein
coastal lagoons and large seasonal inland freshwater wetlands in
northeastern South America (Palmer, 1962; Spaans, 1975). The world
population of scarlet ibises has undergone substantial fluctuations since the
early 1970s, especially in coastal areas, where numbers have ranged from less
than 5000 to more than 35 000 breeding pairs (Luthin, 1985a; Spaans, 1990).
Development pressures in this region are intense (Luthin, 1985a), and many
human activities have the potential of disrupting the mangrove ecosystems
used by breeding ibises (Pannier & Dickinson, 1989).
The scarlet ibis is similar morphologically, ecologically (Palmer, 1962; P.
Frederick & Bildstein, unpublished data), and behaviorally (Ramo & Busto,
1985) to the predominantly North American white ibis E. albus. The two
forms interbreed, not only in captivity (Rutgers & Norris, 1970; Archibald et
al., 1980), but also in the wild in areas of range overlap (Ramo & Busto, 1982,
1987). Recently, researchers have suggested that the two named forms are
indeed color morphs of a single species (AOU, 1983, Ramo & Busto, 1987).
Both feed mainly on aquatic invertebrates, especially crustaceans and
insects, and occasionally on fishes, and nest in large colonies of up to 20 000
pairs (Palmer, 1962; Spaans, 1975; Kushlan, 1979a; Bildstein et al., 1990).
Although nonbreeding coastal populations of scarlet and white ibises
typically feed near their roosts in tidal areas on brackish water prey such as
fiddler crabs Uca. spp., during the breeding season parental ibises raising
nestlings feed almost exclusively on freshwater prey, even when this entails
flying long distances inland to secure it (ffrench & Haverschmidt, 1970;
Bildstein et al., 1990, and references therein). Recently, experiments with
nestling white ibises have demonstrated that this habitat and dietary shift is
the result of a physiological bottleneck: nestlings are salt-stressed when
raised on brackish water prey, and are capable of normal growth and
development only when they are fed relatively low-salt freshwater prey
(Johnston & Bildstein, 1990).
The scarlet ibis is the Republic of Trinidad and Tobago's national bird,
and it appears on the country's coat-of-arms, as well on its one-dollar bill.
They formerly bred regularly in the Caroni Swamp in northwestern
Trinidad (ffrench & Haverschmidt, 1970), but no longer do so, even though a
nonbreeding population continues to use the swamp outside of the breeding
season (Hislop & James, 1988). The Caroni Swamp has undergone
considerable habitat modification in this century (Bacon, 1970; Gerald,
1985). Here, this paper (1) reports on the historic and current status of scarlet
ibises in the Caroni Swamp; (2) documents the extent of recent habitat
changes, especially those that have increased the degree of saltwater intrusion into the swamp; (3) describes how habitat changes have resulted in a
lack of ibis breeding at the site; (4) suggests a conservation strategy aimed at
re-establishing Trinidad and Tobago's national bird as a breeding species;
Scarlet ibis in Trinidad
63
and (5) recommends that other coastal reserves in the region include
substantial tracts of both brackish water and freshwater wetlands.
METHODS
During 17 days in late January and early February 1989 I observed the
feeding and roosting behavior of scarlet ibises in the Caroni Swamp,
Trinidad, West Indies. During each feeding observation I recorded habitat
type, location within the swamp, group size, and, whenever possible, type of
prey. To document ibis use of areas outside the swamp, I recorded the
numbers and age classes of ibises flying north past the mouth of the Madame
Espagnol River from feeding sites south of the swamp to roost sites within
the swamp on 31 January and 8 and 14 February. I made similar counts on 1,
6 and 12 February at the northeastern corner of the swamp to document ibis
use of areas east of the swamp. Counts were initiated in late afternoon, and
continued until nightfall. I spent one day watching ibises feed in the South
Oropuche, a wetland located along the Gulf of Paria in southeastern
Trinidad, approximately 45 km south of the Caroni Swamp. I recorded the
salinity of surface water with a hand-held refractometer at 19 locations
within the swamp on 5-9 days during January and February, sampling
throughout the entire tide cycle (Fig. 1). I also recorded surface salinity at 10
ibis feeding sites within the swamp. I used an osmometer to determine the
osmolarity (salt content) of mangrove tree crabs Aratus pisonii and two
species of fiddler crabs (Uca rapax and U. maracoani) collected at and
around ibis feeding sites in the swamp.
THE CARONI SWAMP STUDY SITE
The Caroni Swamp is a 5611-ha mangrove-dominated wetland in
northwestern Trinidad (10 ° 36' N, 61 ~ 28' W), about 3'5 km southeast of the
capital of Port-of-Spain (Bacon, 1970) (Fig. 1). Approximately 85% of the
swamp is state-owned and was declared a National Park in 1979, but has not
been developed as such. The swamp is bordered on the north by the Caroni
River, on the south by the Madame Espagnol River, and on the east by the
Uriah Butler Highway, and is approximately 9"6 km from north to south and
7"2 km from east to west. The site, which is near the center of Trinidad's
growing population of over 1 100000 people, contains more than threequarters of the country's mangrove swamps (Pannier & Dickinson, 1989).
The swamp consists of four formerly natural river-fed estuaries associated
with the Caroni, Blue, Guayamare and Madame Espagnol rivers (Bacon,
64
Keith L. Bildstein
Caroni River
Blue
River
No. 9 Drain
4
"The
Gulf of
3
Paria
Reeds"
e.
.0
Q
I
River
L
I
2k .
I
Agricultural
Lands
Fig. i. Map of Trinidad depicting Port-of-Spain, the Caroni Swamp, and the South
Oropuche, together with a map of the Caroni Swamp showing The Reeds area, collection sites
for water samples ( x ) and ibis prey (numbers) (see Table 3), and the locations of man-made
structures mentioned in the text.
1970). Most of the freshwater entering the swamp drains from the Caroni
River catchment basin of 675 km 2. At least 157 species of birds have been
seen in the swamp, almost half of which have been recorded nesting there
(ffrench, 1978). Extensive surveys of the birds of Trinidad and Tobago's
wetlands conducted by the Wildlife Section of the government's Forestry
Division during the winter of 1983-84 revealed that 18 species of wading
birds (Ciconiiformes) used the site, 14 of which reached their highest island
Scarlet ibis in Trinidad
65
populations there, and 2 of which, the striped-backed bittern Botaurus
pinnatus and reddish egret Dichromanassa ru(escens, occurred only in the
Caroni (James et al., 1984).
The western two-thirds of the swamp, which drains into the Gulf of Paria,
consists of a Rhizophora-Avicennia-Conocarpus-Laguncularia mangrove
association typical of neotropical estuaries (Bacon, 1970; Pannier &
Dickinson, 1989). Before largescale habitat manipulation began in the area
early this century, the eastern third of the swamp, known locally as "The
Reeds', was an herbaceous freshwater wetland, the species composition of
which is uncertain. Elsewhere in Trinidad such natural freshwater wetlands
are dominated by giant herbs of the Cyperus-Gynerium-Montrichardia
association (Bacon, 1970). In the early 1920s, the Cipriani reclamation
scheme was initiated in an effort to convert approximately 900 ha of the
natural freshwater wetland into rice production. Nine east-west canals
were cut joining a north-south drain that was excavated along the eastern
edge of this large lagoon (Bacon, 1970), connecting two tide-exclusion sluices
(Fig. 1). Embankments separating the polders and fronting the north-south
drain were constructed from peat from the swamp that was placed atop a
foundation of mangrove branches.
The scheme failed in its attempt to establish a long-term rice culture at the
site, possibly because peat compression made draining difficult. By the early
1960s ~water lilies cover(ed) much of the surface of the water' in the
impounded area (Herklots, 1961i. Tile reclamation project was "officially"
abandoned in 1954, but even before then the north-south embankment and
the east west canals were in disrepair. By 1966 the Caroni River had broken
the northern sluice gate, and the north-south embankment, even though
repaired in 1968, is currently open to tidal flow flom the Gull" of Paria
(Bacon, 1970). A second rice scheme south of the Cipriani involving the
Guayamare River drainage, which was initiated in the 1940s, is also poorly
maintained (E. Heesterman, pets. comm.). An Institute of Marine Affairs
series of aerial photographs taken of the swamp throughout most of this
period reveals the expansion of brackish-water adapted plants, especially
red mangrove Rhizophora spp., into the former freshwater swamp, as well as
the death of large mangrove stands as a result of increased salinities west of
the north-south drain (Bacon, 1970).
Salinity readings taken over a complete tide cycle in late January-early
February 1989 reveal that water from the Gulf of Paria--the salinity of
which at that time of the year is usually 35 ppt (Kenny & Bacon, 1981)regularly enters the eastern third of the swamp, and that even within several
hundred meters of the Uriah Butler Highway (the swamp's current eastern
border) little dilution has occurred (Table 1: Fig. 1). The only low salinity
readings (i.e. <~ full-strength seawater) that I recorded within the swamp
66
Keith L. Bildstein
TABLE 1
Salinities within the Caroni Swamp, January and February 1989
Location
N
Salinio' (ppt)
Salinity sample sites east of the N-S embankment
Near Park headquarters
9
Canal 9 mid-point
9
North-south ditch at:
Canal 9
9
Canal 8
9
Canal 7
9
Canal 6
9
Canal 5
9
Canal 4
9
Canal 3
9
Canal 2
9
Canal 1
8
South sluice gate
9
Guayamare River at:
Madame Espagnol River
9
Mid-point
6
Uriah Butler Highway
9
Feeding basins near Park headquarters
Site 1
9
Site 2
9
Site 3
5
Site 4
9
0.17 ±
0.22 ±
17'8 ±
18'7 ±
Ibis feeding sites
East of the N-S embankment
West of the N-S embankment
Crab collection sites
21"9 + 5"08
23"7 ± 5'61
23.2 _+3.54
4
6
6
18.2 _±5.36a
17.4 + 6.27
23.7 +
24.6 ±
23.3 ±
24.1 ±
22"8 ±
22.0 +
20-2 ±
20"1 ±
22-5 ±
21.4 ±
8.97
6.02
7.16
588
5-33
5.24
4.97
4.26
2.20
4-75
17"7 ± 5.00
16"5 ± 1.52
10"4 ± 2"55
0-25
0.36
3'19
3'28
Locations include salinity sample sites east of the north-south
embankment, ibis feeding sites, and crab collection sites. Locations are
indicated in Fig. 1.
" Mean ± SD.
were f r o m a > 20-ha f r e s h w a t e r s w a m p directly s o u t h o f P a r k h e a d q u a r t e r s
a n d east o f the p o w e r line r i g h t - o f - w a y that runs a l o n g the western side o f
the U r i a h Butler H i g h w a y . T h e salinities I r e c o r d e d in the f o r m e r f r e s h w a t e r
h e r b a c e o u s s w a m p d u r i n g the latter stages o f an exceptionally p r o l o n g e d
r a i n y season, at a time w h e n salinities in the s w a m p are relatively low (Bacon,
1970), are similar to those r e c o r d e d for the same general area in 1977 fully
o n e m o n t h into the d r y season in that year ( D e o n a r i n e , 1980). M y readings
are also similar to those r e c o r d e d d u r i n g the late 1960s for several areas
s e a w a r d o f the n o r t h - s o u t h d r a i n (Bacon, 1970).
Scarlet ibis in Trinidad
67
Recently, saltwater intrusion into the freshwater reaches of the swamp
has been further exacerbated both by the construction of the Uriah Butler
Highway and by channelization and diking of the Caroni River (Pannier &
Dickinson, 1989). The four-lane highway, which was completed during the
1970s, replaced a more eastward two-lane multi-bridged north-south road
connecting Trinidad's two largest cities, Port-of-Spain and San Fernando.
River channelization and diking, which continue regularly, are designed to
restrict the flooding of agricultural and residential areas in the Caroni
floodplain east of the swamp (E. Heesterman, pets. comm.). Prior to
construction of the highway and channelization of the river, overflow
drainage from the Caroni River floodplain during the rainy season flowed in
a broad front from east to west into the swamp. Currently, because of
channelization and diking, most of the rainy-season drainage remains
within the banks of the river and is dumped directly into the Gulf of Paria
with minimal circulation through the swamp. Even on the rare occasions
when the river's banks are breached and water does flow onto the flood
plain, the highway acts as a dike to such flow, rediverting fresh water around
the swamp. In addition to reducing freshwater input into the swamp, these
habitat modifications also affect the swamp by reducing nutrient input and
disrupting normal sedimentation patterns (Pannier & Dickinson, 19891.
S C A R L E T IBISES IN T R I N I D A D
Historic and current status of the population
The scarlet ibis is mentioned as a regular or even c o m m o n resident of
Trinidad in several early works (Joseph, 1838; Leotaud, 1866; Belcher &
Smooker, 1934; Junge & Mees, 1958); however, it receives only passing
reference in others (Roberts, 1934; Devas, 1950), and details of its breeding
are scant prior to 1953 (Junge & Mees, 1958; ffrench & Haverschmidt, 1970).
Scarlet ibises are known to have bred in Trinidad only in the Caroni Swamp,
although it has been suggested that they may also have bred in the South
Oropuche (Belcher & Smooker, 1934; ffrench & Haverschmidt, 1970). Ibises
often nest in large colonies, usually in red mangrove, often in association
with small herons and egrets (ffrench, 1984). Between 1953, when the government of Trinidad accorded increased protection to scarlet ibises, and 1970,
the last time the species is known to have bred in Trinidad, ibises bred in the
Caroni Swamp in all but two years (ffrench & Haverschmidt, 1970). During
this period, breeding populations usually numbered in the thousands, but
declining in the late 1960s (ffrench, 1985). Occasional unconfirmed reports by
local fishermen of breeding since that time most likely represent scarlet ibises
68
Keith L. Bildstein
TABLE 2
Numbers of Scarlet Ibises Flying to Evening Roosts in the Caroni Swamp along Major
Wading-bird Flight Lines near the Southern (Mouth of the Madame Espagnol River) and
Northern (National Park Headquarters) Corners of the Swamp, January and February 1989
Location
Date
Time
watched
Numbers of
scarlet ibises
(h)
Mouth o f t h e
Madame EspagnolRiver
National Park
Headquarters
Juveniles or
second-year birds
(%)
31 January
8 February
14 February
1715-1830 a
1645-1830
1730-1830
554 (7.4)b
109 (1.0)
151 (2"5)
I February
6 February
12 February
1530-1830
1615-1830
1630-1830
0
0
0
93
75
83
Too dark to continue observations.
h Total number seen (number per minute of observation).
roosting near active cattle egret Bubulcus ibis nesting sites (Hislop & James,
1988). From 1969 through 1975, the numbers of ibises seen in late December
in the swamp ranged from 1500 to 5601 (ffrench, 1984). Currently, the
nonbreeding population in the Caroni Swamp peaks at about 10 000 birds in
late fall and winter, before dropping to between several hundred to several
thousand birds during the breeding season (Hislop & James, 1988), when
most birds leave the island, presumably to nest nearby in coastal Venezuela,
or elsewhere on the South American mainland (ffrench, 1984; Spaans, 1975).
The current seasonal fluctuations in population size are almost the reverse of
those that occurred when ibises bred in the swamp in the 1950s and 1960s
(ffrench, ! 984). Although most of the current population roosts and feeds in
the Caroni Swamp, surveys by the Wildlife Section conducted between 1985
and 1987 suggest that as many as 300 birds feed regularly in the South
Oropuche (Hislop & James, !988), a view supported by my evening
observations of ibises flying to roost sites in the Caroni Swamp from areas
further south (Table 2). By comparison, few, if any, birds fly east across the
Uriah Butler Highway to feed (Table 2). There are also recent reports of small
numbers of ibises ( < 25) in freshwater swamps in eastern Trinidad (Hislop &
James, 1988).
Feeding ecology
Scarlet ibises feed in a variety of habitats on the island, including tidal mud
flats, flooded pastures, cultivated rice areas, herbaceous swamps, aquaculture ponds, and sewage lagoons (Hislop & James, 1988). Currently, most
69
Scarlet ibis in Trinidad
feeding occurs in mangrove swamps, especially the Caroni Swamp, and to a
lesser degree, the South Oropuche (G. Hislop, pers. comm.; Bildstein,
personal observations). In mangrove swamps, they feed mainly on fiddler
crabs (Uca. spp., especially U. rapax, thayeri, and maracoani), but they also
take mangrove tree crabs A r a t u s pisonii, mangrove land crabs Ucides
cordatus, blue crabs Callinectes sapidus, and large sandworms N e r e i s succinea
(ffrench & Haverschmidt, 1970; Bildstein, unpublished data). Historically,
when ibises bred during drying periods following the rainy season, parental
ibises switched from feeding in brackish-water swamps to foraging in
... freshwater marshes, rice fields, and flooded savannas when feeding
young in the nest'. (ffrench & Haverschmidt, 1970). Although it has been
suggested that this shift was simply a response to diminished prey in
brackish areas (ffrench, 1984), support for this suggestion is lacking.
Furthermore, recent experiments demonstrate that nestling white ibises are
incapable of normal growth and development unless they are raised on lowsalt freshwater prey (Bildstein et al., 1990; Johnston & Bildstein, 1990). These
studies strongly suggest that the switch in feeding habitats noted by ffrench
and Haverschmidt was due to a physiological bottleneck, rather than to
seasonal differences in relative prey abundance. A similar switch in feeding
habitat during the breeding season occurs in coastal populations of white
ibises (Bildstein et al., 1990).
Currently, ibises that feed within the Caroni Swamp on both sides of the
n o r t h - s o u t h drain are feeding in brackish water habitats (Table 1) on crabs
and polychaete worms, prey that are relatively high in salt (Table 3). Even the
relatively small portion of the ibis population that leaves the Caroni each
morning to feed elsewhere flies south toward brackish-water mangrove
TABLE 3
Osmolalities of Prey Species taken by Scarlet Ibises in the Caroni Swamp, January and
February 1989
Collection site~
1
2
3
4
5
Guayamare river
Medford Crossing
Ditch 6 & N-S Canal
Roost Sanctuary
Headquarters' feeding pond
Prey species
N
Osmotic concentration
(mOsm k'z- t )h
Uca rapa_v
Aratus pisonii
U. maracoani
U. maracoani
Nereis succinea
16
11
16
40
879_+ 10.8
1045 _+47.6
1 096 +_97.8
932 _+77.2
610'
° See Fig. 1 for locations within the swamp.
b Milliosmols per kilogram of water. Full-slrength seawater (i.e. 35 ppt) is approximately
1200 mOsm kg 5; ibis serum is approximately 335 mOsm kg ~.
c Estimated based on the salinity of the site (17'8ppt) and the fact that nereid worms are
osmoconformers.
70
Keith L. Bildstein
swamps such as the South Oropuche, where individuals feed on fiddler crabs
(personal observations). Ibises apparently do not fly across the Uriah Butler
Highway inland to or from the Caroni River drainage, probably because
little undisturbed wetland habitat remains in the area. (Although thousands
of cattle egrets, which regularly roost together with ibises within the swamp,
do cross the highway, these birds fly mainly to agricultural fields (personal
observations), presumably to feed on insects, and not to wetland sites.) Thus,
overall, the present ibis population on Trinidad feeds almost entirely in
brackish water habitats on high-salt prey.
Legal status
Scarlet ibises were regularly hunted in Trinidad during the 1800s, as well as
earlier this century (ffrench & Haverschmidt, 1970). Indeed, as recently as
1986 they were considered gamebirds during Trinidad's five-month hunting
season, when as many as five birds a day could be taken legally (Hislop &
James, 1988). When Trinidad received its political independence in 1962, the
scarlet ibis was declared its national bird, and in the fall of 1965 the species
was listed as totally protected. Unfortunately, the species was mysteriously
added to the list of game species during the 1981 reprinting of the laws of
Trinidad and Tobago, and in 1983 the Forestry Division, Wildlife Section
estimated that as many as 600 game birds, many of them scarlet ibises, were
shot in the Caroni Swamp on the first day of the hunting season (James et al.,
1984). Ibises were removed from the list in 1986. Currently, they are
protected in Trinidad by both the Forest Act and Conservation of Wildlife
Act.
Several roosting and nesting sites in the Caroni Swamp, which was
declared a Forest Reserve in 1936, were set aside in 1953, 1958, 1960 and
1966 as sanctuaries for breeding ibises, and the entire swamp became a
National Park in 1979. In 1987, the entire area south of the Blue River and
the No. 9 Drain was declared a Prohibited Area 'offering full protection to
the habitat and fauna', with access limited by a permit system (Hislop &
James, 1988) (Fig. 1). Even so, the current number of personnel in the park is
hopelessly inadequate, and despite patrolling by a well-trained and
enthusiastic staff of wildlife biologists and game wardens, poaching
continues in the area. Indeed, I witnessed incidences of the killing of both
scarlet ibises and cattle egrets during my brief four-week stay in the swamp
in 1989, and BBC camerman M. Richards reported to me that an ibis roost
he was filming during my stay was fired upon at close range during the same
period. Luthin (1985b) reports a similar incidence during his brief visit to the
swamp in 1983, and Linblad (1969) provides vivid descriptions of poaching
during the late 1960s.
Scarlet ibis in Trinidud
71
DISCUSSION
'What is the future of the Scarlet Ibis in Trinidad? I see no reason to be
pessimistic, provided no one decides to drain all the swamps.' (ffrench, 1966).
In coastal South America scarlet ibises often breed in different colony sites
in different years, and in some years they forgo breeding, apparently
depending on local ecological conditions (Spaans & de Jong, 1982; Spaans,
1983, 1990). These movements are similar to those reported for white ibises
in Florida (Kushlan, 1976). Nevertheless, in both white and scarlet ibises,
certain colony sites, especially those in coastal areas, are often traditional,
and are used regularly by varying numbers of birds over long periods of time
[Spaans, 1975; Bildstein et al., 1990). The Caroni Swamp was such a
traditional site, at least from 1953, when accurate records of ibis breeding
activity at the site were first kept, through 1970 (ffrench, 1984).
Parental white ibises, which are incapable of rearing young on diets of
brackish-water prey (Johnston & Bildstein, 1990), forgo breeding in large
numbers when abundant freshwater prey are not available as food for
developing nestlings (Bildstein et al., 1990). Here, I have presented historic
and recent data indicating that considerable saltwater intrusion has
occurred during the past 30 years within the formerly freshwater portion of
the Caroni Swamp, known locally as The Reeds. Observations of the
foraging ecology of ibises in the Caroni Swamp and elsewhere in Trinidad
(ffrench & Haverschmidt, 1970; Hislop & James, 1988; this study) lead me to
conclude that a major, if not the major, reason why ibises no longer breed
there is that the freshwater feeding sites necessary for successful breeding are
no longer available to potential breeders. Adult ibises apparently relocate
and breed in Venezuela, possibly on Cotorra Island in the Orinoco River
delta (Spaans, 1975; ffrench, 1984).
My findings, however, do not preclude the possibility that other factors,
including long-term changes in climate, have also played a role. Seasonal
changes in rainfall patterns, especially droughts, are known to influence ibis
nesting in coastal South Carolina tBildstein et al., 1990). Droughts, however,
did not occur at the time that ibises stopped breeding in Trinidad (Berridge,
1981). Nor was annual rainfall at the Royal Botanic Gardens near the site
(Anon., 1979) significantly less between 1971 and 1977, years in which ibises
did not breed at the Caroni, than it had been during the period of verified
breeding between 1953 and 1970 (t = 1.35, p > 0"051.
Additional human impacts are unfortunately more suspect. Water
pollution in the lower Caroni River is visually obvious (Kenny & Bacon,
1981). A secondary waste-water treatment facility for the city of Port-ofSpain is located directly north of the Caroni River in the Laventille Swamp.
This facility is overloaded and in poor repair, and it dumps sewage regularly
72
Keith L. Bildstein
into the Caroni Swamp (E. Heesterman, pers. comm.; personal observations). Dredging and channelization of the Caroni River have increased
siltation in the swamp, and considerable industrial and agricultural effluent
flows into it (Rooks, 1985). As a result, organochlorines, in use in the area
since 1946, and their metabolites are now found in organisms living in the
swamp, as are PCBs (Deonarine, 1980). Bacon (1970) attributed fish kills in
the swamp following heavy rains to insecticide runoff from agricultural
fields. Pesticides do not, however, appear to be responsible for decreases in
scarlet ibis populations in other countries in the region (Anon., 1984).
All of the swamp is within 20 km of the Piarco International Airport, and
directly beneath its principal approach and departure flight lane, which is
used by more than 20 scheduled jet flights daily. The extent to which lowflying commercial jet aircraft affect ibis activity at the site is unknown.
Colonial nesting gulls are sometimes flushed from their nests by low-flying
commercial jet aircraft (Burger, 1981); however, white ibises at mixedspecies colony sites in Florida merely looked up from their nests during lowlevel flyovers by military jets (Black et al., 1984), and Kushlan (1979b) found
~no countermanding evidence against the use of helicopters' during his
surveys of such colonies. Nevertheless, additional testing, involving lowflying commercial aircraft, is needed.
Poaching, together with the disturbance of roosting ibises by tour guides
for the benefit of tourists, both of which are currently receiving attention
from Park staff, are certainly potential threats to the current nonbreeding
population of ibises (ffrench, 1984; Luthin, 1985a, b). It does not appear,
however, that these factors alone were responsible for the cessation of
breeding at the site. Indeed, during the 1950s and 1960s, when ibises
successfully bred in the Caroni, the species was legally hunted each year
between October and March (ffrench, 1985).
Specific recommendations for the Caroni Swamp
I have argued that in scarlet ibises successful breeding in coastal populations
requires the presence of freshwater habitats with an abundance of prey. In
most instances, seasonally flooded riparian areas adjacent to estuarine
island colonies provide the necessary combination of a safe nesting site
together with adequate feeding habitats. Currently, the Caroni Swamp
National Park provides, at least marginally, the former ingredient, but fails
to provide the latter. Before the swamp can support a stable breeding
population of ibises, adequate freshwater habitat will need to be made
available to potential breeders.
One possible approach to securing freshwater foraging sites for ibises
would be to re-establish the formerly herbaceous freshwater eastern portion
Scarlet ibis in Trinidad
73
of the swamp, The Reeds. To do so, would require the construction of
operational sluices at the north and south ends of a refurbished north-south
embankment, together with the diversion of freshwater from the Caroni
River into the area to compensate for the loss of sheet flow from the former
flood plain of the river east of the Uriah Butler Highway. This proposal
appears to be physically feasible; indeed most of it has already been carried
out (but not maintained) as the Cipriani Scheme earlier this century. For it to
be politically feasible, however, such a scheme will need to include financial
incentives beyond those associated with increased tourism alone. Therefore,
I recommend that the Government of Trinidad and Tobago investigate the
potential for developing a portion of the proposed reclaimed area for
aquaculture. The giant freshwater prawn Macrobrachium rosenbergii, a
species that has been successfully farmed at similar coastal sites in
Colombia, Puerto Rico, and Jamaica (Provenzano, 1973; Hanson &
Goodwin, 1977; New & Singholka, 1985), is a likely candidate for such an
endeavor. The possibility of establishing mixed aquacultural ponds using
giant prawns and mullet, similar to those that have been developed in coastal
Colombia (New & Singholka, 1985), or using native freshwater species
(Bacon, 1969), should also be investigated.
The likelihood of aquacultural ponds being used by ibises and other
wading birds, of course, raises the question of a potential conflict between
aquaculturalists and conservationists. Throughout the region, wading birds,
including ibises, frequently forage in commercial prawn and crayfish
Procambarus clarkii ponds during the breeding season, especially when
water is removed from the ponds at the end of seasonal harvest (Provenzano,
1973; Martin & Hamilton, 1985; New & Singholka, 1985). Where this
phenomenon has been studied in detail, however, pond use by wading birds
has been shown to have little, if any, effect on commercial harvest in
adequately designed ponds (Martin & Hamilton, 1985). Nevertheless, the
issue of wading bird predation at these sites would need to be addressed.
The provision of adequate feeding habitat alone will not ensure the reestablishment of a breeding population of ibises in the Caroni Swamp.
Increased protection of current and historic roosting sites, along with
historic breeding sites, is urgently needed. Round-the-clock warden patrols,
together with consistent sentencing of poachers by the judicial system, is
essential. Tour guides should be prohibited from flushing birds to permit
more spectacular photographic opportunities for their passengers. The
protection of existing roosting and potential breeding sites in the brackishwater portion of the swamp could be achieved most easily within an overall
plan that concurrently stressed the economic, as well as the ecological, value
of the Caroni Swamp mangrove ecosystem (James, 1985; Pannier &
Dickinson, 1989). Considerable opportunity exists for the development of
74
Keith L. Bildstein
sustainable estuarine fisheries including conchs, oysters, mussels, crabs,
shrimps and fin fishes (Bacon, 1969; Ramdial, 1980; Kenny & Bacon, 1981).
The easiest way to accomplish this goal would be for the Government of
Trinidad and Tobago to carry out the plans for the Caroni Swamp National
Park published in 1979 (and appropriately modified to enable the
reclamation of The Reeds area of the swamp). These plans called for
'...protection of the ecosystem, environmental education, research,
facilitation of recreation and tourism, and accommodation of traditional
productive uses such as fishing and oystering' (Forestry Division/OAS,
1979a,b). If taken, such action also would increase substantially the value of
the swamp as a tourist attraction (Pannier & Dickinson, 1989).
I suggest that the following measures be taken immediately:
(1) The Government of Trinidad and Tobago should become a member
of the Ramsar Convention, and should declare the Caroni Swamp its
first protected wetland site under that agreement.
(2) An attempt should be made to reduce sewage and industrial effluent
flowing into the Caroni and Blue rivers at the north end of the
swamp, and plans should be developed to reduce the biological
impacts (Jackson et al., 1989) of any oil spills occurring nearby in the
Gulf of Paria.
(3) The impact of low flying jet aircraft on ibis feeding and roosting
behavior in the swamp should be investigated.
(4) Commercial tour operators and guides should be trained formally by
the Forestry Division, or each tour guide should be accompanied by
a Forestry Division Biologist while in the swamp. Such action would
reduce the amount of misinformation currently given to tourists
(Forestry Division/OAS, 1979b, personal observations), as well as
increase the conservation message during each tour.
General recommendations for species conservation
A recent workshop on scarlet ibis conservation revealed that little is known
about the ecology of coastal populations of this species (Frederick et al.,
1990). Although its current world population is relatively large, it appears to
be fluctuating dramatically, especially in coastal areas (Spaans, 1990).
Colonial species such as the scarlet ibis are notoriously prone to extinction,
and a coherent range-wide conservation strategy based on its biology is
certainly desirable.
At least one conservation group has called for the 'urgent protection of
remnant mangrove sites' in coastal South America (Anon., 1984). Although
this is certainly needed to ensure breeding in coastal populations, mangrove
Scarlet ibis in Trinidad
75
swamps are open systems (James, 1985; Pannier & Dickinson, 1989), and
their protection alone will not prevent the loss of local breeding populations
of ibises. Freshwater feeding sites also need to be protected. Therefore, I
recommend that newly formed coastal reserves include adjacent or nearby
freshwater wetlands to enhance the likelihood of maintaining current
breeding populations of scarlet ibises.
ACKNOWLEDGEMENTS
I thank the Wildlife Section of the Forestry Division, Ministry of Food
Production, Marine Exploitation, Forestry and Environment for its
hospitality and essential logistic support during my stay in Trinidad. 1
especially appreciate the personal efforts of C. James, G. Hislop, J. Farrier, S.
Madoo, J. M. Clarke and R. Singh in this regard. Professors J. Agard and
J. S. Kenny of the University of the West Indies, St Augustine, offered me the
lab space and materials necessary to determine the osmolality of ibis prey. E.
Heesterman, a civil engineer, spent several hours providing me with an
overview of the hydrology of the region. My research in Trinidad was
supported by the Southern Regional Education Board, the American
Philosophical Society, and especially, by the Whitehall Foundation.
Winthrop College provided a sabbatical for my research. Finally, I thank L.
Baptista, P. C. Frederick, J. W. Johnston, J. A. Kushlan and E. S. Morton for
useful comments and suggestions on earlier drafts of this paper. This is
contribution number 806 of the Belle W. Baruch Institute for Marine
Biology and Coastal Research.
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