Productivity of Long Ogo (Gracilaria parvispora) in Floating Cages
Edward P. Glenn and Kevin Fitzsimmons
Environmental Research Laboratory, University of Arizona, Tucson, AZ 85706
The edible, red seaweed, long ogo (Gracilaria parvispora), was grown in floating baskets in ancient Hawaiian fishponds along the south shore of Molokai. Yields were 0.6 kg/m2/week in two ponds and 0.3 kg/m2/week in a third pond. The best long-term production was obtained from the pond with the lowest exposure to wind and wave action and the lowest measured water motion (5 cm/sec water velocity). Yields from this pond were competitive with reported, tank-culture yields of Gracilaria. On the other hand, the lowest production was obtained from a pond that had a broken retaining wall and was exposed to strong wave action. The baskets and seaweed in that pond became infested with an epiphyte (Lyngbya majuscula) that reduced growth; epiphyte proliferation may have been stimulated by the high water motion environment of the pond (13 cm/sec water velocity). Market quality of cleaned long ogo from all baskets was high and the cultures were stable over the 24 weeks of study (April - October). Basket culture in fishponds or in other sheltered, inshore locations in Hawaii appears to be a high-yielding, cost effective method for producing this species for the fresh market.
Keyword (1) Seaweed (2) Cages
In Hawaii, as elsewhere in the Pacific and Asia, seaweeds are widely consumed in fresh form. The edible seaweed "long ogo" (Gracilaria parvispora) is one of the larger red seaweeds in Hawaii, reaching lengths of 60 cm; it is not a native species but first appeared on the reefs after 1900 (Magruder & Hunt, 1979). It began to be harvested in commercial quantities from Oahu reefs by the 1930's (I. Abbott, personal communication). Until the 1970's it was the seaweed most often found in Honolulu fish markets; it was used fresh or incorporated into recipes representing Hawaiian, Korean, Filipino, Japanese and Caucasian cuisines (Abbott, 1978; Abbott and Williamson, 1974; Magruder & Hunt, 1979). Today, over-harvesting on the reefs has led to severe restrictions on the amount that can be taken for commercial sale and it is rarely found in markets. It has been supplanted by an imported species from the Florida Keys, Gracilaria tikvaheae, that is produced by several commercial growers on Oahu and Hawaii using land-based, tank aquaculture methods (Laws, 1990; Lapointe et al., 1976; Lapointe and Ryther, 1978). The substitute species is different in appearance, taste and texture from long ogo. Whereas long ogo has flexible, spaghetti-like branches, G. tikvaheae branches are short and bristly; it is sometimes sold under the name "short ogo", but this name properly belongs to G. coronopifolia, the Limu Manuea of the Hawaiians, which resembles G. tikvaheae in appearance but differs in taste and texture. Interviews with consumers, wholesale buyers and retailers confirm there would be a good market for traditional long ogo if it were once again available (Fountain, 1991). The agar from G. parvispora (= G. bursapastoris) was found to produce a gel with an excellent texture for food use in blends with the gel from G. coronopifolia (Santos and Doty, 1983), hence there may be an export market for dried long ogo. One aquaculture approach would be to introduce it into existing tank cultures. In fact, long ogo has been intermittently available from one of the growers and has been experimentally cultivated in tanks at Anuenue Fisheries Research Station. However, it is difficult to maintain in tank culture. It grows well for a variable number of weeks then fragments into small, necrotic pieces; at this point the culture must be renewed from wild stock or from healthy fragments salvaged from the crashed culture (M. Fujimoto, Anuenue Fisheries Research Station, private communication). The growth form of the seaweed in tank cultures is also different from reef material. Tank-grown thalli lack the desired fine branches that develop under natural conditions, perhaps due to excess water motion in the tanks. Furthermore, tank culture methods are expensive, requiring shoreline modifications, a pumping station and seawater discharge point; they are energy-intensive due to the need for frequent water exchange in the tanks (Laws, 1990; Bird & Benson, 1987). We attempted to develop an economic culture method for long ogo that could be practiced in the ancient Hawaiian fishponds of Molokai. These ponds represent an under-utilized resource for aquaculture (DHM et al., 1990). There are 44 Loko Kuapa type ponds and 13 Loko 'Ume'iki types, enclosing 200 or more hectares of reef water surface on this island (DHM et al., 1990). By utilizing the nutrients already present in the silt-laden water of the ponds and the natural water motion provided by the trade winds, we sought to develop a culture system that did not require pumping of seawater for high productivity. The goal was to develop a low-capitalization, low-operating-cost culture method that could be practiced as a shared activity by the coastal residents of Molokai as a part-time or family-level activity. The results apply not just to Molokai fishponds but to any sheltered inshore location on tropical reefs. In preliminary experiments carried out at Ualapue Pond we tested several methods that proved unsuccessful. We tied small vegetative pieces of long ogo to lines which were strung out between stakes. These experiments showed that the fishpond environment was favorable for long ogo, as growth rates in the range of 4-15% per day were recorded at different times of year (see previous quarterly reports to NCRI). However, the thalli tended to break loose from the lines as they grew, resulting in a poor recovery of seaweed at harvest. We also attempted to grow the seaweed in bottom culture in pens in Ualapue Pond, similar to the way Gracilaria is grown in ponds in Taiwan (Shang, 1976). However, the growth rates were low due to low light penetration and the smothering of the thalli by silt. We finally grew the seaweed in floating baskets anchored in the ponds. Similar baskets or cages have been used in short-term (2 week) nutrient enrichment studies with G. tikvaheae in Florida (Lapointe, 1985), and commercial cage culture of fish started at least 15 year ago (Landless, 1974), but to our knowledge this is the first time the method has been applied to commercial seaweed culture. Here we report production rates of long ogo over a 24 week period in three fishponds on Molokai.
Materials and Methods
Source of seaweed. G. parvispora was first introduced onto Molokai in a series of experimental outplantings of spore-bearing gravel conducted from 1983-1985 on the reef east of Kaunakakai (Doty et al., 1986). A few of the outplantings produced mature plants which apparently spread spores that gave rise to one or more naturalized populations present today. We collected approximately 2 kg from a population in the vicinity of Kaunakakai on April 15, 1991, along a 100 m stretch of beach. This material was multiplied by growing it first in a single basket in Ualapue Pond and latter in multiple baskets in Puko'o and Panahaha Ponds, from April 15 to October 9, 1991. Approximately 2 kg of additional wild material were collected in July and grown in the ponds to compare with previously collected material. Culture methods. The dimensions and design of the floating baskets are shown in Figure 1. They were made of PVC-coated wire mesh which permitted a free exchange of water through the mass of seaweed in the baskets. The floatation collar kept the basket near the water surface and the lid out of the water to prevent its fouling. The lid was covered with plastic shade fabric which reduced light levels in the baskets by 42%. Baskets were tethered to concrete blocks or stakes in the fishponds. The tether rope was attached to the basket in the middle of a long side so that the long dimension of the basket faced into the prevailing winds when the basket was allowed to swing freely in the current. Baskets were inoculated with 1-2 kg wet weight of seaweed. Each week seaweeds were removed, cleaned of silt and epiphytes by spaying with fresh water from a hose, weighed and replaced in cleaned baskets. When 4-6 kg of seaweed accumulated in a basket it was harvested back to 1-2 kg. Harvested material was either used for test marketing or to start new baskets. Measurement of water motion and other environmental variables. Water motion in the baskets and on the bottom of the ponds was estimated by the weight change of plaster of paris (calcium sulfate) clodcards over 24 hours exposure (Doty, 1971). The weight change values of clod cards placed in the ponds were related to current speeds (cm/sec) by exposing clodcards from the same batch to a unidirectional flow of seawater at equivalent temperature and salinity in a tank (Figure 2) (the method is described in Glenn and Doty, submitted). Twelve clod cards were placed at each location (6 in the baskets and 6 on the bottom) at each measurement period. Water motion was estimated once (June 18) in Ualapue, 5 times in Puko'o (3 weeks in August and 2 weeks in October) and 6 times in Panahaha (June 24, 3 weeks in August and 2 weeks in October) during typical tradewind conditions (15-25 knot northeasterly winds). Water temperatures and salinity were measured concurrently with water motion. Evaluation of market quality of seaweed. Samples of long ogo were removed and cleaned July 7 and October 7, 1991 from Puko'o Pond. They were taken to Honolulu where they were evaluated with the help of a professional seafood dealer with experience with edible seaweeds including ogo (Glenn Tanoue, Tropic Fish and Vegetable Center, Honolulu). Evaluation was subjective but consisted of visual inspection, taste testing, and preparation of six traditional ethnic dishes from the material: par-boiled; kim-chee; oil and vinegar; tuna poke (chopped seaweed with raw tuna); squid poke; and sashimi which were presented to a panel of seafood dealers for their comments. Percentage of dry matter was determined by drying several sample thalli to constant weight at 60 C; a value of 10% dry matter composition was obtained, the same as the value for G. tikvaheae (Lapointe et al., 1976).
Comparison of fishpond environments. The outer wall of Ualapue Pond had been completely restored and the gates (makaha) were closed at the
Mean yields of 0.6 kg/m2/week in Puko'o compare to values of 1.0-1.5 kg/m2/wk for G. tikvahaea grown in tank cultures at Kona, Hawaii under natural sunlight and optimal conditions of fertilization, carbon dioxide enrichment, water flow and aeration (Laws, 1990). On a dry matter basis the Puko'o baskets yielded approximately 9 g/m2/d (= 33 t/ha/yr) compared to dry matter yields of 12-17 g/m2/yr for G. tikvaheae grown in tank cultures in Florida during the same months as the present experiments (April-October)(Lapointe et al., 1976). Hence, the present low-input, low-capitalization culture method produced 50-70% of the yield of optimized tank cultures. The optimized tank systems reported above utilized full sunlight, but in commercial practice heavy shading is needed to minimize the growth of epiphytes such as Enteromorpha in tanks, and this shading limits yields (Laws, 1990). So, while accurate production data from proprietary tank cultures are not available, the present method would appear to be competitive in terms of yield when the shading factor is considered. The basket method is 3-5 times more productive than bottom methods of pond culture which produce 7-12 t/ha/yr of Gracilaria (Shang, 1976). The baskets keep the seaweeds near the surface under good light conditions and allow higher stocking densities than are normally achieved in bottom culture. The present method of growing the seaweed in fishponds has surmounted the major problem reported for this species: its instability and altered growth form in tank culture. The basket method would offer some economic advantages over tank culture even if production and quality factors were equal. The cost per unit production is much lower for baskets than tanks and there are no pumping costs. Lease payments on state-owned fishponds are much lower than land rentals (Fountain, 1991). This culture method will be practical wherever shallow, protected inshore waters are available in tropical and subtropical locations. At present the cost of baskets, while lower per unit production than tanks, still appears to be prohibitive for growing Gracilaria on a large scale for agar extraction. Larger baskets made of less expensive materials may be able to bridge that gap (Landless, 1974). Some problems remain to be solved if seaweed is to become a cottage industry on Molokai. The first requirement is a dependable source of starting material to inoculate cultures. At present, the only dependable supply of long ogo is from the experimental baskets in Puko'o and Panahaha Ponds. Further work is needed to define the fertility requirements of long ogo. Lapointe (1985) found that G. tikvaheae productivity could be increased 2-3 fold by soaking the seaweeds in aerated fertilizer solution for 5 hours each week. This is a cost-effective way to increase production. The present results should be extended to other fishponds and to other sheltered inshore locations, and carried out over an entire annual cycle. There is also a need to adapt the methodology to other economic seaweeds such as Limu Manuea (G. coronopifolia), Limu Ele'Ele (Enteromorpha) and Huluhuluwaena (Grateloupia filicina) that may also thrive in the fishpond environment.
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Table 1. Environmental conditions in Molokai fishponds in which long ogo was grown. The entries give ranges of values obtained under typical afternoon tradewind conditions, April - October, 1991.
|Water Motion, (cm/sec):
11 - 15
11 - 15
2 - 4
3 - 6
|Salinity (ppt)||28 - 30||30 - 33||33 - 34|
|Temperature, C||25 - 27||25 - 28||26 - 28|
|Water Depth (cm)||20 - 60||30 - 120||60 - 120|
|Bottom Material||Mud & Silt||Cobbles & Mud||Sand|
|Wave Height (cm)||6 - 12||8 - 12||2 - 4|
*Measured on one occasion only.