Previous Page Table of Contents Next Page


5. DISCUSSION

To understand the context of this study in terms of the field survey and the growth study, it is important to be aware of the life history of Cladosiphon sp. particularly from a seasonal point of view. The taxonomy of the species occurring in Tonga is still in question and the life history has yet to be fully detailed. Insights have, to present, been gained by comparison with two other species of the genus Cladosiphon okamuranus Tokida and Cladosiphon novoe-caledoniae Kylin.

5.1 Life Cycle

It is evident from observation that the species of Cladosiphon in Tonga has the same general life cycle to the well studied Cladosiphon okamuranus. There is an alternation of generations where there is a macroalgal sporophyte phase and a microscopic gametophyte phase. The sporophyte growth occurs during the cooler months of April to December. During the latter part of this period, the growth is negative with an approximate 50% reduction in weight in two weeks (See Table 3). On maturity produces neutral spores throughout the cooler part of the season. Gametangia are produced at the latter part of the season with the increase in temperature.

The gametophyte generation occurs during the higher temperature months of December to April. It is microalgal through the season and multiplies parthenogenetically. Spore settlement was observed in late October and remained diminutive through December (Kikutani pers comm).

With the cooling of the water temperatures, the sporophyte generation begins its growth forming filamentous thalli. This growth continues until the thalli form a mat which covers the substrate between the seagrass, forming a dense layer. Latter in the season, sediment fouls the mat and in the case of the situation south of Maria Bay, is populated by littorinid molluscs and hermit crabs. In this state the environmental conditions are less than optimal with only the surface algae receiving illumination and exposure to the current. Algae observed during early December, showed a vitality only in the situation where the thalli were constantly current washed and generally in a situation in which the algae was elevated from the substrate. This appearance was observed in the seagrass tangle but was characterised by low density and an elevation of the thalli into the current.

5.2 Field Appearance

The condition of the Cladosiphon at the end of it's sporophyte generation can be categorised into four states. That which appears vital and healthy. The algae is raised from the surface, attached or tangled in an open area and is characterised by a clear sheath and good colour (Fig. 7a).

Another form is that of a densely tangled mat which occurs on the sandy substrate between Syringodium shoots (Fig. 7g). An intermediate state is that which is loosely tangled either in the Syringodium or Halimeda (Fig. 7e,f). Lastly, a common form is that of the free floating algae which is often mid-water being conveyed by the currents.

As with the algae growing in an open area, the free floating algae generally appear healthy. This mode of dispersal results from the algae breaking off from the benthic growth, whether tangled or attached from settlement. It is this material which becomes ensnared on isolated coral outcrops. If the site is washed by current, it subsequently grows with the thalli extending in the direction of the current until it breaks and the process of migration continues. If the fragment becomes tangled amongst the seagrass, then the growth is contained within the restraining shoots. In this case, the algal growth is spatially contained, proliferates and develops a mat-like nature. This mat appears less vigorous in its health with a good deal of fouling by sediment. Also characteristic of the algal mat, in the area south of Maria Bay is the preponderance of small littorinid snail which appear to be eating the algal sheath giving it an irregular surface.

Table 3: Growth Rate Assessment: Rope and Tray Culture

Experimental design-

Rope Culture:
In Tank
Date: 3/12/96
Rope weight
(gm)
Rope + Limu 6 bundles/rope
(gm)
Algal Wt. T1
(gm)
3/12/96
Rope + Limu6 bundles/rope
(gm)
17/12/96
Algal Wt. T2
(gm)
17/12/96
Weight reductionT2-T1 (gm)

14 day duration/% reduction
Rope no. 499562463---
Rope no. 5100793693---
Rope no. 6102609507---
In Field Date: 4/12/96
Rope no. 1106770664295189-475/71.5%
Rope no. 2106688582391285-297/51%
Rope no. 3127760633429302-331/52.2%
Rope no. 7102781679342240-439/64.6%
Rope no. 8112775663425313-350/52.7%
Rope no. 9106770664441335-329/49.5%
      -56.92%
      SD8.96
Tray Culture
Date 7/12/96Algal weight
(gm)
Tray and sieve
(gm)
Net T1
(gm)
Net T2 (gm)
17/12/96
Weight reduction T2-T1 (gm)
17/12/96
10 day duration
Percentage loss
Section 1: 'Onevai coll.1166776390218-17244%
Section 2: 'Onevai coll.1332783549276-27349.7%
Section 3: 'Onevai coll.1254773481162-31966.3%
Section 4: Navutoka coll.1232773459266-19342.0%
Section 5: Navutoka coll.1390772618331-28746.4%
Section 6: Navutoka coll.1310778532328-20839.0%
      -47.9%
SD9.72

Figure 8a

Figure 8a : Life Cycle of Cladosiphon okamuranus (adapted from Okanamura 1936; Shimura, 1977; and Okinawa Pref.Fih. Exp. Station, 1978)

5.3 Seasonality

Anecdotal information on seasonality indicated that Cladosiphon harvesting can occur as early as May and certainly by June. The matted or dense concentration of material that found in the grass appears old and fouled by sediment. The 3–4 tons harvested in the first week of December, is presumed to be largely of this type of material.

The quality of the material which is growing unencumbered and that which is matted are different in appearance. This may not be the case, in terms of quality, as with the process of cleaning and drying the ‘younger’ material and the ‘older’ material appear to be similar. In vivo, the more vital material is characterised by a clear sheath and a golden brown tubular thallus core. The thalli are distinct and clumped or extend along the direction of the current. By contrast, the older material is kinked and matted amongst the seagrasses and the transparent coating is less full or expanded and covered with sediment. It appears to be dying with a less turgid nature.

Anecdotal information indicates a much later harvest is available from the area south of Maria Bay. This may be due to the greater protection offered by the expansive reef and greater water depth in comparison with the reef along Navutoka, which is a fringing reef. This latter area is where 10 tons was harvested earlier in the year. It is generally considered that the end of the season here is when the northerly winds develope and the wave action removes the existing Limu which becomes piled on the beach. The next available harvest is in May when the sporophyte generation expresses harvestable vegetative growth.

By contrast the area south of Maria Bay, is thought to be harvestible until March. This contrary to the current understanding of the seasonality from Navutoka and from Okinawa. The termination of both of these localities are due to northerly winds which remove the previous seasons standing crop. In the case of the area south of Maria Bay, the area of occurrence is more protected and deeper. Recent strong winds have provided an understanding of the process here with the shallow Syringodium areas devoid of the Limu. In the deeper water adjacent, there are current rows of material piled in long ridges along the sandy bottom in 6–7 metres of water. This is the result of the strong winds that blew for 6 days during the first week in December from the northeast and then the southeast. The broad sand flat of the bay may act as a buffer allowing the algae to remain viable for sporulation after the areas of narrower reef flats have had the algae removed. Recent observation of Cladosiphon have revealed a general senescence which is more indicative of the effect of the higher water temperatures.

5.3.1 The Potential Effect of Temperature on the Life Cycle of Cladosiphon sp.:

Figure 8b.

Water temperature data was obtained from both Tongatapu and Vava'u. In the case of the Tongatapu data, it was obtained from the hatchery facility after being pumped from the sea. The temperature was recorded twice a day for 15 months from April, 1995 to June 1996. The morning reading was thought to be a more faithful record of the ambient sea temperatures. The Vava'u record was also taken at approximately the same time in the morning (9am) but was collected from the sheltered bay directly from the wharf in front of the Fisheries facility in Neiafu. This area is part of a broad, enclosed inlet that forms the harbour adjacent to Neiafu. The period of recording is from February, 1996 to December, 1996.

Though the readings are not completely synchronous with respect to the monthly record (5 month overlap), there is good agreement with respect to the overlap period and seasonal oscillation expected. There is confidence that the observed fluctuations are sufficiently accurate to allow discussion of the role that temperature might play in the life cycle of Cladosiphon in Tonga.

As the areas of Cladosiphon occurrence in Vava'u are 3°19" of latitude north of those in Tongatapu, there would be an expected increase in temperature. Vava'u and Tongatapu experience two different temperature regimes which are offset by 1.6° at the maximum and 2.6° at their thermal minimums. The consequences of the fluctuations and the effect that the difference in regimes has on the life cycle of Cladosiphon is speculative but may be correlated with field observations to provide a provisional understanding.

Figure 8b

Figure 8b: Seasonal diagram of Cladosiphon okamuranus rectifie for Tonga for comparison.

If temperature is correlated with the field observations of the occurrence of the sporophyte generation, a critical temperature becomes apparent. The period when the sporophyte generation becomes absent in the field is during January (A). Negative growth and general decline in the appearance of the in situ algae was observed in this study during the month before and probably is progressive for much longer. The reported appearance of the sporophyte phase in late March (B), interestingly, is coincident with the temperature at which it becomes absent the previous year. That temperature is 26–26.5° C. It is speculated that this is the point where the algae can no longer cope with the higher temperatures and the sporophyte phase either ceases or begins for the following year as the temperature lowers.

This relationship is supported by field reports in Vava'u where the algaes earlier occurrence (O. Moala pers comm.) had totally disappeared by December's field inspection (E). If the relationship between temperature and the algaes occurrence is correct then the sporophyte generation should occur in at the end of May (C) and continue to October (D). The higher temperature regime in the northern range of the algae effects a much shorter season than in Tongatapu. If this increase in temperature from south to north is a linear relationship then an intermediate effect on season length would be expected at Felemea Bay in the Hapa'ai Group. The seasonal growth, of the sporophyte generation, here, would be expected to begin in late April and cease in December.

Field information obtained in Felemea Bay, Ha'apai, during the second week of December noted an abundance of residual Cladosiphon which had been removed from the seagrass beds during seasonal westerly winds. Large quantities of the algae were washed ashore with the remainder concentrated as clumps in the sand areas among the sea grass. In this case, it is apparent that temperature alone was not the limiting factor in the presence of the algae. Depth of water and habitat exposure to the late season winds are other factors which limit the sporophyte phase.

Figure 9:

Figure 9: Comparative temperature data from Tongatapu and Vavau I.

*Water temperature data was obtained from the Japanese International Corporation Association (JICA) maricultural project at the Ministry of Fisheries, Tongatapu. The water temperature data from Vava'u was supplied by the Department of Fisheries in Vava'u.

5.3.2 El Nino - La Nina cycles and regional water temperature fluctuations.

El Nino years are typified by higher sea surface temperatures where as La Nina years have characteristically lower averages. As the lifecycle of Cladosiphon is temperature dependent, this Pacific wide temporal fluctuation is of interest.

Though the sea surface temperatures assessed for the 20 years period and the last 24 month record are not at the latitude of Tonga, they represent an indicator of the regional trend. During the period of previous harvest sea-level, temperatures were near normal (NOAA, 1996).

Generally, sea surface temperatures were near-normal in the central and east-central equatorial Pacific during November, 1996. Indices reflect a weak positive phase (La Nina) of the Southern Oscillation Index during this period. Mid 1993–94 was the last El Nino period and reflected higher water temperatures of 1.4°C during mid-1994. 1995. This represents a maximum period which also occurred in 1982–1983 and 1988. 1992 was also a warmer peak (1.2°C). Figure 10 illustrates the fluctuation in water temperatures around the equator within the longitude north of Tonga over a 20 year period.

Figure 11 shows fluctuations in the standard deviation for the Southern Oscillation Index (SOI) as a five month running mean for the last twenty years. Since the El Nino of 1994, the standard deviation of the SOI has increased into a weak La Nina phase. For the two years (mid-1995 to late 1996) of temperature records listed in figure 9, the SOI has been near normal. The SOI has risen from the mean to a standard deviation of +.5. Sea temperatures fluctuated during this period between +/-.5°C about the average.

Assessment of surface sea temperature in the past 24 months shows the sea surface temperatures higher at the beginning of 1995 and residing within a more uniform range through 1995 and 1996 at the longitude of Tonga (Figure 12). This trend is in keeping with the increase in SOI values for this period as shown in Figure 11.

Prediction of temperature from normal seasonal fluctuations for 3, 9 and 12 month periods from October 1996, indicate maximum seasonal anomalies to be +0.25°C. Generally, Tonga lies in an area of predicted nil variation from the seasonal norm as projected to October, 1997 (NOAA. 1996) (Figure 13).

The following figures (10–13) were adapted from National Oceanic and Atmospheric Administration (NOAA) 1996 November 1996: Climate Diagnostic Bulletin.

Figure 10

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

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

Figure 12: Time/longitude sections of sea surface temperature (C°) for the past 24 months. Data derived from moored time series samplebetween 2° N/S Latitude.

Figure 13
Figure 13
Figure 13

Figure 13: Predicted sea surface temperature anomalies at 6, 9 and 12 month lead times.

5.4 Potential Consequences of Continuous Harvest

There are two main considerations of Cladosiphon removal from the inshore system. Given that one of the objectives of commercial exploitation is to maximise harvest and that with every year this will reach greater efficiency.

1) The removal of the sporophyte generation will reduce the amount of neutral spore production. The consequences of this are unknown. It would be difficult to assess this effect prior to an obvious decline in stock over seasons given the inherent seasonal variation associated with benthic algae.

2) Conspicuous in the habitats where Cladosiphon is found on Tongatapu I. is the high diversity of algal species that often monopolise large areas to the exclusion of other algae. Progressive removal of Cladosiphon sp. may give other macroalgae a competitive edge utilising areas which were fomerly colonised by Cladosiphon.


Previous Page Top of Page Next Page