SURVEY OF LIMU TANGA'U (Cladosiphon sp.) IN TONGATAPU, HA'APAI AND VAV'AU ISLANDS IN
TONGA
1
EXECUTIVE
SUMMARY
2
INTRODUCTION
3
METHODS
3-1.
Field
Survey
3-2.
Growth
Assessment
4
RESULTS
4-1.
Distribution
of Cladosiphon sp.
4-2.
Sites
of General Survey
4.2.1
Tongatapu
and adjacent areas
4.2.2
Ha'apai
group
4.2.3
Vava'u
group
4-3.
Aerial
Photography of the Sites of Known Cladosiphon Occurrence
4.4
The
Aerial Extent of Seagrass Habitats
4.4.1
Thalassia
hemprichii and Syringodium isoetifolium as habitats for Cladosiphon
sp.
4.4.2
Comparison
of Cladosiphon habitats: Estimate of areas, known harvest and field
sample
4.5 Growth Study
5
DISCUSSION
5-1.
Life
Cycle
5-2.
Field
Appearance
5-3.
Seasonality
5.3.1
Potential
effect of temperature on the life cycle of Cladosiphon sp.
5.3.2
El
Nino - La Nina cycles and regional water temperature fluctuations
5-4.
Potential
Consequences of Continuous Harvest
6.
CONCLUSIONS
7.
ACKNOWLEDGMENTS
8.
BIBLIOGRAPHY
APPENDIX
LIST OF FIGURES
Figure
1: Chart
of the Kingdom of Tonga with its South Pacific location (inset)
Figure
2a: The
area of initial harvest on Tongatapu off Navutoka (located to the right in the
photo). Another site of harvest is to the right of Manima and Oneata ls. See
Fig. 2b for the charted area
Figure
2b: Charted
area of Fig.2a shown in the bold lined section
Figure
3a: Tonga's
most extensive Cladosiphon habitat in the bay south of Atata and Toke l.
See Fig. 3b for detail of the chartered area
Figure
3b: Charted
area of Fig. 3a shown in the bold lined section
Figure
4a: The
seagrass beds adjacent to Uiha l., Ha'apai which potentially host commercial
quantities of Cladosiphon The charted area of Uiha and adjacent islands
are shown in Fig. 4b
Figure
4b: Charted
area of Fig. 4a shown in the bold lined box in the center of the page
Figure
5a: Aerial
view of reported Cladosiphon habitats in the seagrass beds west of Koloa
l. And north of Oloua l. See Fig. 5b for the charted details of the area
Figure
5b: Charted
area of Fig. 5a shown in the bold lined box in the center of the page
Figure
6(a-c): Reported
Cladosiphon habitats from off-shore reef areas:
-
Dark
areas show the extent of the seagrass and mixed algal assemblage between and
around Onevao and Onevai ls. on the barrier reef north of Tongatapu l. at
the top of the photo. A sandy reef flat is more characteristic of the
barrier reef environment.
-
Aerial
photo of the reef flat habitat around Fukave and Nuku ls. which is
representative of the remainder of the barrier reef area to the east of Fig.
6a.
-
The
reported occurrence of Cladosiphon sp. At Nukupule l., Ha'apai group,
shows the limited nature of the seagrass habitat by the dark area to the
west of the island.
Figure
6d: Areas
shown in Fig. 6 a, b on the barrier reef north of Tongatapu l.
Figure
7(a-o): Cladosiphon
lifecycle, habitat and experimental design
(a,b) Dispersal of Cladosiphon involves two
modes:
-
Vegetative
propagation whereby grown filaments travel by current and are subsequently
entangled by bottom relief such as a clump of hard coral (Pocillopora
damicornis) or a stiff upright algae such as Halimeda spp. and
subsequently colonise the surrounding area.
-
The
settlements of spores on hard surfaces such as the skeletal material of Halimeda
spp.
-
Settlement
of the gametophyte stage on a glass surface induced by the collection of
sample material indicating sporulation in October.
-
Cladosiphon
sp. In a common association of Syringodium isoetifolium and Galaxeura
sp. This specimen has a clear, smooth outer sheath and golden brown colour
characteristic during a period of vital growth. This condition is less
common late in the season.
-
A
more translucent nature of the outer sheath with an irregular surface is
characteristic of colonies late in the season.
-
Late
in the development of Cladosiphon, the sheath becomes ragged and more
opaque with a light appearance.
-
The
Cladosiphon colony is often matted amongst the Syringodium
seagrass forming dense layers.
-
Mounds
of Cladosiphon are rolled up in the sandy areas within the seagrass
meadows as well as on the beach following strong westerly winds. This
seasonal feature heralds the end to the algaes presence and the end of the
sporophyte phase.
-
Generally,
the seagrass Syringodium with its stiff blades provides the habitat
for Cladosiphons intertangling growth. In mixed seagrass beds and on
the periphery of the intertangling growth. In mixed seagrass beds and on the
periphery of the Syringodium patches in deeper water, Thalassia
hemprichii is also colonised.
-
In
deeper water (5–7 m), the detached pieces of Cladosiphon become
entangled on the upright thalli of algal colonies. The migration of this
material into deeper water is the result of strong winds causing the
material to remove the algae from the seagrass beds.
-
The
tank culture of the algae was conducted at the Ministry of Fisheries
aquaculture unit. The design was unsuccessful presumably due to high
temperatures and/or caused by unshaded conditions.
-
Tank
culture in trays.
-
The
rope culture design in the field at Navutoka, Tongatapu.
-
The
irregular fringing reef coast line along the western shore of Foa l.,
Ha'apai. This area is characteristic of the unsuitable habitats for
seagrasses and Cladosiphon in all but a small area of the Ha'apai
Group.
-
Intertidal
seagrasses adjacent to Koloa l, Vava'u Group. In deeper water, this habitat
is reported to host the Cladosiphon algae.
Figure
8a: Life Cycle of Cladosiphon okamuranus (adapted from Okanamura 1936;
Shimura, 1977; and Okinawa Pref. Fish. Exp. Station, 1978)
Figure
8b: Seasonal
diagram of Cladosiphon okamuranus rectified for Tonga for comparison
Figure
9: Comparative
temperature data from Tongatapu and Vava'u l.
Figure
10: Equatorial Pacific sea surface temperature anomaly (C°) for the area
between 160°E and 150°W and 5°N&S of the equator for a 20 year period
Figure
11: El Nino and La Nina periods - variation in the standard deviation for the
Southern Oscillation Index as a five month running mean for the last twenty
years
Figure
12: Time/longitude sections of sea surface temperature (C°) for the past 24
months. Data derived from moored time series sample between 2° N/S Latitude
Figure
13: Predicted
sea surface temperature anomalies at 6, 9 and 12 month lead times
TABLES
Table
1: Summary
of information on the sites surveyed
Table
2: Comparison
of Cladosiphon habitats
Table
3: Growth
rate assessment: Rope and tray culture
