Polyculture refers to the technique of growing several species of marine organisms together. Species selected for integrated seafarming should have mutually beneficial effects which enhance production output. This chapter will introduce three integrated seafarming systems: polyculture of Laminaria and mussels, polyculture of Laminaria and Undaria and polyculture of Laminaria and scallops.
A commercial marine polyculture system in which two or more seafood species are produced may greatly improve output because of the beneficial interaction between species. Laminaria grown on rafts with mussels, for example, forms an integrated culture system which greatly improves growth and yield of both species.
Laminaria plants on kelp rafts provide shading, create sheltered areas less exposed to current flows, release oxygen as a byproduct of photosynthesis and generally improve water fertility. Overall, Laminaria plants create a “mini-ecosystem” in otherwise open shallow seawater, making conditions more favourable for commercial production of mussels and other marine organisms.
In turn, mussels and other cultured marine organisms in a polyculture system produce metabolic byproducts, especially dissolved N, P and CO2, which act as natural fertilizers to meet the nutrient requirements of Laminaria plants. This effect is especially important in seawater regions in northern China that are nutrient deficient.
In the simplest method of raft polyculture, alternating kelp ropes and mussel ropes are suspended vertically from a floating raft rope. This polyculture method is also used in China to grow other marine species in conjunction with Laminaria, such as scallops, which are suspended in cylindrical net cages about 40 cm in diameter and 1 m long. Thus a simple hanging rope raft polyculture system may alternate Laminaria ropes, mussel ropes, and hanging net cages containing scallops.
The main problem with this method is shading, because maturing downward-hanging Laminaria plants screen-out sunlight, thus decreasing photosynthesis and slowing plant growth. On the other hand some degree of shading and shelter is usually beneficial for the growth of marine organisms, because the “water trap” effect created by hanging Laminaria plants enhances trophism and thus stimulates plankton growth.
To overcome the problem of decreased illumination, an alternative raft polyculture system can be established in which the hanging culture rope method is used for mussels (or other marine organisms) and the horizontal kelp culture rope method is used for growing Laminaria plants. In this system vertical mussel culture ropes are suspended from floating raft ropes between horizontally positioned kelp culture ropes. Mussel ropes should never be suspended from the horizontal kelp culture ropes because the weight of the growing mussels would sink them and pull the floating rafts together. Hanging mussel ropes must always be suspended directly from the floating raft ropes.
Floating rafts should be positioned in the same direction as prevailing tidal currents so that plants on horizontal kelp ropes and strings of mussels on hanging mussel ropes will all tend to sway in the same direction, i.e. downcurrent, thus avoiding intertwining and twisting of mussel ropes and kelp ropes.
The following are examples of various polyculture raft systems widely used for growing Laminaria and mussels in China:
Two parallel foating raft ropes between 60–65 m long and spaced about 5 m apart can be used to support 30 pairs of kelp culture ropes. Culture ropes between adjacent rafts are suspended horizontally. Each 1.5 m long culture rope can hold approximately 34 Laminaria plants. About 40 hanging mussel ropes can be attached between the horizontal kelp culture ropes. Thus each raft rope may support a total of 70 Laminaria and mussel culture ropes spaced 0.75–1.0 m apart. The main limiting factor on cultivation intensity is the weight of mussels on raft buoyancy. Each 1.2 m long mussel seedling rope can hold 800–1,000 attached spring mussel seedlings or between 600–800 autumn mussel seedlings.
The basic polyculture system can be intensified by reducing the interval spacing between horizontal kelp culture ropes to 50–70 cm, so that 80–100 pairs of 2 m long kelp culture ropes are suspended between floating raft ropes 50–60 m in length. As usual, mussel culture ropes are suspended vertically between kelp culture ropes, with the interval spacing between mussel ropes also being reduced to between 50–70 cm.
The basic polyculture system can be altered by changing the degree of dominance of one species or the other. The ratio of horizontal kelp culture ropes to vertical mussel culture ropes may be 1:1, 2:1, 3:2, 3:4, 1:2 (see Table 8.3). For example, two parallel floating rafts may support 50 horizontal kelp ropes and up to 100 hanging mussel ropes as long as the buoyancy of the raft ropes is sufficient to bear the weight.
Much variation in the ratio between numbers of kelp and mussel ropes is possible, though the basic method of alternating horizontal kelp ropes with vertical mussel ropes remains the same. Table 8.3 illustrates that certain combinations of culture ropes may yield higher output. Generally speaking, selection of any particular combination depends on facilities available and environmental conditions at the raft site.
In integrated seafarming an entire plantation area may be subdivided into areas for separate mussel and kelp monoculture rafts. The principle of integrating culture of two or more species is the same as for a polyculture raft system, but interspecies effects are spread over the whole plantation.
For example, an ocean plantation located in a coastal bay area may be subdivided into areas for rafts holding Laminaria and rafts holding mussels. Laminaria rafts should be located in the outer areas of the mariculture plantation where there are stronger tidal currents. The Laminaria rafts then give some shelter to the inner plantation areas where mussel rafts are best located.
Marine areas selected for polyculture should have free-flowing tidal currents, with the currents not swirling or eddying but flowing in a definite direction. Water depth should be between 3–5 m with very clear water transparency. Specific gravity of the seawater, a measure of salinity, should not exceed 1.018.
When seawater temperature falls to about 20° C around mid-October in northern China, raft polyculture can begin. Mussel seedlings attached to mussel culture ropes are wrapped in loose net bags and suspended from raft ropes. The net bags prevent mussel seedlings from becoming dislodged in the initial days of transfer to raft site conditions. After two or three days the mussel culture ropes are divided into shorter sections which are again placed in smaller net bags and suspended from raft ropes. In another three to five days of growth the mussels will have attached firmly to the mussel ropes. The net bags can then be removed to improve water circulation through the mussel culture ropes.
Table 8.1 reports data from an experiment comparing output and market value of Laminaria and mussels from monoculture and polyculture systems. Laminaria yields from the polyculture systems (PI and PII) compared with Laminaria monoculture were 23% and 35% higher and market values were 27% and 31% higher respectively. Laminaria produced in polyculture was of higher quality than in monoculture, the fresh/dry weight ratio falling from 7.22 in monoculture to 6.95 and 5.84 respectively in the polyculture systems. Furthermore the proportion of first class product rose from 59.3% under monoculture to 74% and 80% in the polyculture systems. Output and market value of mussels improved by 19% and 23% respectively in the polyculture systems compared with mussel monoculture.
Table 8.2 reports experimental data showing return on investment from Laminaria monoculture and from polyculture of Laminaria and mussels. Though higher investment is required for the polyculture system, more intensive use of facilities together with the beneficial effects of integrated seafarming result in a substantially higher profit under polyculture compared with Laminaria monoculture.
Table 8.3 shows experimental data comparing market returns from various polyculture systems having different proportions of kelp and scallop culture ropes. Seafarming area is held relatively constant and identical raft rope facilities are used in each trial. (I.e. a block of 24 raft ropes occupies a seafarming area of about 5 mu.) Highest returns occur in the polyculture system (PII) where horizontal kelp ropes and hanging scallop ropes alternate in equal number (40:40) along the raft ropes. This polyculture system yields a 58% increase in market returns compared with Laminaria monoculture using identical production facilities.
The interspecies effects of an integrated polyculture system have been studied extensively. Table 8.4 reports data from an experiment on the production and use of metabolites in a mussel and Laminaria polyculture system. Results of seven trials of polyculture cultivation in seawater are presented, conducted between December and March at various seawater temperatures. The consistent results show that changing seawater temperature is not an important factor with respect to metabolite exchange in the polyculture system. The figures in Table 8.4 show base concentrations of nitrogen-N in seawater at the beginning of each experimental trial. Measurements of dissolved nitrogen-N were again taken after a 25 hour period of cultivavation. The end-of-trial measurements have not been tabulated. Instead Table 8.4 reports the difference between dissolved nitrogen-N levels over the 25 hour period, shown as an “increase” (+) or “decrease” (-) over the base concentration levels.
The data in Table 8.4 show that nitrogen-N levels rise over base concentration levels in a mussel monoculture system, as expected. Whereas nitrogen-N levels are intermediate between base concentrations and concentrations resulting from mussel monoculture in a mussel and Laminaria polyculture system. These results clearly demonstrate that mussels produce nitrogenous byproducts of metabolism which are actively utilized by Laminaria over a very short timespan. Mussels produce “natural fertilizer” which directly increases Laminaria production while eliminating the need for costly fertilizer additives.
By measuring nitrogenous wastes from mussels it has been calculated that in one night at spring or autumn seawater temperatues one ton of mussels can release an amount of dissolved nitrogen equivalent to the following amounts of chemical fertilizer additives: 141 g of NO3-N or 235 g of NH4-N.
Table 8.5 reports data from a longterm experiment on the utilization of nitrogenous metabolites in a mussel and Laminaria integrated seafarming system, covering the grow-out period from 8 December until 7 April. All four systems (ML, MM, PI and PII) were established in the same seafarming region. Though somewhat inconclusive, results generally show that dissolved nitrogen levels are higher for mussel monoculture than for Laminaria monoculture over the entire growing period and that nitrogen levels in the polyculture systems (PI and PII) are lower than levels measured in the mussel monoculture seafarming area. Again the longterm evidence from this experiment verifies the natural fertilizer hypothesis, i.e. that mussels provide Laminaria with dissolved nitrogen requirements for growth and development.
Table 8.6 shows data from an experiment designed to compare levels of dissolved O2 (DO) and CO2 in monoculture and polyculture systems. Seven trials were conducted between December and March. As before, base concentrations of dissolved O2 and CO2 were measured at the beginning of the experiment and, after a twenty-four hour cultivation period, concentrations were again measured. The differences are reported as “increases” (+) or “decreases” (I/D) in Table 8.6.
After a twenty four hour culture period DO in Laminaria monoculture shows an increase of between +0.55 and +3.92 ml/l. Whereas DO in mussel monoculture shows a decrease of between-0.24 and -3.29 ml/l (with one anomalous result of +0.12 ml/l). The increase in DO in the mussel-Laminaria polyculture system is intermediate, being between +0.32 and +1.26 ml/l, confirming that DO created as a byproduct of Laminaria photosynthesis is being absorbed by mussel respiration in the polyculture system.
Similarly, CO2 levels for Laminaria monoculture show a decrease of between -1.79 and -13.17 ml/l. Whereas dissolved CO2 for mussel monoculture shows an increase of between +0.86 and +6.35 ml/l (with one anomalous result of -.06 ml/l). Dissolved CO2 levels for the polyculture system are intermediate, between -.06 and -7.62 ml/l (with one anomalous result of +1.23 ml/l), confirming that CO2 released by mussel respiration is being absorbed by Laminaria to metabolize photosynthates.
Experimental results are as expected, showing that DO and CO2 exchange takes place between Laminaria and mussels in a polyculture system.
As kelp plant density increases on culture rafts during grow-out, water circulation is slowed and backwater conditions favourable for plankton growth are created. Increased seawater fertility results in increased plankton reproduction. Since mussels are plankton feeders, mussel production is greatly enhanced when integrated with Laminaria production.
Results of a study of plankton growth and utilization in mussel and Laminaria polyculture systems are shown in Fig. 8.1. The study compares plankton levels occurring in mussel and Laminaria monoculture and polyculture systems. Plankton growth in open seawater is used as the control against which changes in plankton levels for the other systems are measured.
Fig. 8.1. Comparison of plankton growth in Laminaria and mussel monoculture and polyculture systems.
The experimental results confirm an expected increase in plankton for the Laminaria monoculture system. Since mussels are filtering organisms, the mussel monoculture system shows a decrease in plankton, as expected. Whereas plankton levels for the Laminaria and mussel polyculture system are intermediate, indicating that increased plankton growth resulting from the presence of Laminaria is utilized as a food source by mussels.
Undaria is used mainly for human consumption in China, Japan and Korea. Production has been underway in China since the 1960's, however low market demand in past years has meant that seafarming of this species has not developed quickly. Export market demand has recently increased interest in the commercial value of Undaria. If transplanted to rafts early in November or December, Undaria can be harvested in February or March at a time of year when other vegetables at local markets in China are scarce. For these various reasons Undaria seafarming has good future potential in China.
Undaria grows faster and yields higher output per unit area of raft facilities than Laminaria.
By growing Laminaria and Undaria together a double harvest can be obtained using the same raft facilities. Undaria is harvested in February; Laminaria is harvested in July. Because Undaria can be grown using the same raft facilities and seedling-rearing stations used for Laminaria, marginal production costs are low. Most importantly, studies have shown that polyculture of Laminaria and Undaria can increase overall output and market value by 45% compared with Laminaria monoculture (Table 8.7).
Transplantation Methods
Previous sections of this manual have described the production of Laminaria seedlings in a seedling-rearing station. The production of Undaria seedlings is done using the same seedling-rearing station facilities and methods. However, differences in morphology between Laminaria and Undaria neces- sitate some differences in methods of transplantation and raft culture.
Undaria has a broader frond than Laminaria, shaped like a banana leaf with a length of 1.0–1.5 m and a width of 0.6–1.0 m. This means that more space is required for illumination. Transplantation to the hanging culture ropes can begin when Undaria sporophytes are about 10–15 cm long, spacing plants at intervals of about 6 cm. About 35 young Undaria sporophytes can be attached to each 2 m long culture rope. Undaria holdfasts are smaller than Laminaria holdfasts and are therefore more easily dislodged. Care must be taken when attaching Undaria sporophytes to culture ropes that holdfasts are well-spliced between rope strands without being too deeply enmeshed.
Undaria and Laminaria Raft Polyculture Methods
The three procedures described below for polyculture of Undaria and Laminaria are variations of the basic raft culture methods presented in chapters II and V.
Procedure I
Horizontal kelp culture ropes are suspended between parallel floating raft ropes. Hanging Undaria culture ropes are tied to the raft ropes alternating with kelp culture ropes. Undaria plants are harvested in February and Laminaria is left to continue growing until harvest in July.
Procedure II
This is a two-stage procedure. First the hanging rope raft culture method is used, followed by the horizontal rope raft culture method:
Hanging Undaria and Laminaria culture ropes are suspended alternately from raft ropes {L-U-L-U-L-U-etc.}. Rafts are spaced parallel to one another about 5 m apart. Undaria culture ropes are harvested in February.
Laminaria plants are left to continue growing with a change in grow-out method. Pairs of kelp culture ropes hanging from adjacent parallel rafts are joined together with connecting ropes, i.e. the horizontal kelp rope raft method is used.
Fast-growing Undaria plants give shading to Laminaria plants in early grow-out stages following transplantation. Initially, Laminaria culture ropes should be longer than for Undaria culture ropes, so that young Laminaria sporophytes are well-sheltered from strong illumination by the rapidly growing Undaria fronds. In later grow-out kelp plants require higher illumination and this is provided after Undaria harvest by lifting the kelp ropes into horizontal position between the floating raft ropes.
Procedure III
This three-stage procedure basically uses the horizontal culture rope raft method twice over, first for Undaria and then for Laminaria, thereby maximizing use of raft facilities:
Laminaria and Undaria culture ropes are suspended vertically on separate raft ropes, with Undaria raft ropes placed parallel to one another about 6 m apart. The density of the Laminaria ropes can be increased by about 60% over normal, because this stage is only temporary. Increased cultivation density is good in the early grow—out stage when Laminaria requires lower illumination.
After 15 days pairs of Undaria ropes between parallel rafts are joined together, thus changing to the horizontal culture rope raft method. Laminaria ropes are lifted and transferred to the Undaria rafts, where they are suspended vertically between the Undaria culture ropes. Undaria plants are harvested in February. During the growing time before Undaria harvest, kelp culture ropes may be reversed periodically so that illumination on kelp plants is evenly distributed.
Following Undaria harvest pairs of kelp culture ropes are joined together with connecting ropes, lifting them into horizontal position between the parallel floating raft ropes. Grow-out continues until harvest in July.
Controlling Water Depth of Undaria Ropes
Undaria grows best at a shallower depth. Table 8.8 shows that Undaria plants cultured between January 7 and March 21 at depths of 80 cm and 160 cm had the same rate of increase in blade length. However increase in blade width shows considerable variation at different depths. At a depth of 80 cm, width of blades increased 14.73 times original width at the beginning of the experiment. At a depth of 160 cm, width of blades increased only 8.15 times original width. When grown at a depth of 210 cm, blade width increased only 7.33 times original width.
Thus the depth at which Undaria ropes are suspended on culture rafts has a significant effect on production levels. During the initial 15 days. when Undaria plants are suspended vertically, the connecting rope joining the hanging Undaria rope to the floating raft should only be about 50 cm long. After 15 days, when Undaria cultivation is changed to the horizontal culture rope raft method of grow-out, the mid-point of horizontal Undaria ropes should first be suspended 2 m below the water surface. Over the following two weeks horizontal Undaria ropes should be gradually raised to a depth of 100 cm.
Harvest of Undaria from a Polyculture System
Undaria harvest usually occurs in March. But if Undaria sporophytes are transferred to raft culture in November, harvest can take place in January or February. The earlier harvest tends to reap better quality plants for human consumption.
Benefits of Undaria and Laminaria Polyculture
The Undaria and Laminaria polyculture system is profitable for the following reasons:
Polyculture of Undaria and Laminaria increases economic returns by about 45% compared with Laminaria monoculture (Table 8.7).
There is high demand for Undaria as an export product.
Techniques for growing Undaria are basically the same as those used for Laminaria seafarming. No new major investment in seedling-rearing stations or floating rafts is necessary. Undaria seafarming merely intensifies use of already existing equipment.
Undaria products are good for human consumption, having high iodine content (0.5 ppt dry weight) and protein content 1–2 times higher than Laminaria. If harvested in February, Undaria provides an excellent substitute for vegetables in mid-winter in northern China.
Economic returns from Undaria sales, alone, can cover costs of raft installations. Thus Undaria provides “insurance” in the event of poor Laminaria harvest. Undaria harvest in February comes before the seasonal stormy weather. If rafts are destroyed the Undaria harvest at least covers damages to raft facilities.
Scallops (Chlamys farreri) are suspended from polyculture rafts in net bags or net cages (Fig. 8.2). There are two growth stages:
When shells of seedling scallops are less than 3 cm long seedlings are contained in loose net bags. Each hanging connectinganging can support 10 net bags.
When shells of seedlings grow over 3 cm in length scallops are transferred to net cages. Net cages are about 40 cm in diameter and 1 m long. Each net cage is a net bag extended into a cylindrical shape by being held open with plastic plates fastened at different levels inside the net bag. Thus the net cage contains several (6–8) “growing floors” on which the seedling scallops are placed for grow—out. The plastic plates are perforated with holes for good water circulation. The top opening of the net cage is guarded with a drawstring which attaches to a hanging connecting rope. About 40 juvenile scallops with shell length of 3 cm or more are placed on the plates in the net cages. Where Laminaria grow-out uses the horizontal kelp rope raft culture method, connecting ropes holding scallop net cages are suspended vertically from the floating raft ropes. Connecting ropes hold the hanging net cages at a depth of about 1.5–2.0 m, each connecting rope supporting one net cage.
| Fig. 8.2 Hanging net cage used for growing scallops in a Laminaria and scallop polyculture system. |  |
| 1: connecting rope 2: plastic plates 3: side opening 4: stone weight |
A plantation area of one mu may have: (a) 500 hanging scallop ropes supporting 500 net cages, each cage containing about (5 floors X 40 scallops =) 200 scallops for a total of about 100,000 growing scallops, and (b) 7 rafts each 65 m in length supporting a total of 400 horizontally suspended kelp culture ropes, each 2.33 m long kelp culture rope holding an average of 32 kelp plants. Scallops are harvested in November. Laminaria is harvested in July.
Natural Fertilization in a Scallop and Laminaria Polyculture System
Experimental evidence indicates that byproducts of scallop metabolism, such as dissolved N, P and CO2, provide nutrients for Laminaria growth. Table 8.10 reports data showing dissolved levels of nitrates and phosphates in different seafarming areas supporting Laminaria and scallop monoculture and polyculture systems. Levels of nitrates and phosphates are compared against a control seawater area without seafarming operations. As expected, levels of nitrates and phosphates are lower in the seawater region supporting Laminaria monoculture and higher in the seawater region supporting scallop monoculture. Levels of N and P are intermediate for seawater areas supporting polyculture where Laminaria is the dominant species, indicating that Laminaria plants absorb dissolved N and P that is produced by scallops.
Table 8.10 reports data from a short term investigation of the release and utilization of N, P and CO2 by scallops and Laminaria in monoculture and polyculture systems. Levels of dissolved gases were measured at the beginning of the experiment and at the end of a twenty–four hour cultivation period. Tabulated results show increases (+) or decreases (-) in levels of dissolved N, P and CO2 over this cultivation period. As expected, levels of N, P and CO2 increase for scallop monoculture, decrease for Laminaria monoculture, and are intermediate for scallop and Laminaria polyculture.
On average each scallop with a mean shell length of 5.57 cm produces the equivalent of 66.63 micrograms of inorganic nitrogen every twenty four hours at a temperature of 10o C. If 100,000 scallops per mu are cultured the total content of released nitrogen can reach 66.63 g, equivalent to using 333 g of ammonium sulphate fertilizer. This natural source of fertilizer, released gradually and continuously, can provide kelp plants with required N and P in nutrition deficient seawater regions.
In a Laminaria and scallop polyculture plantation area the actual amounts of N, P and CO2 released by scallops may fluctuate considerably during the grow–out season. In warmer autumn and spring seawater temperatures scallop metabolism becomes more active. Oxygen demand and amounts of of N, P and CO2 discharged increase with their growth and development. If these changes are found to have adverse affects on Laminaria growth because of excessive levels of dissolved N and P in seawater, it may be necessary to reduce the number of scallop net cages hanging from rafts in the plantation area.
Conversely, as seawater temperatures fall during winter months scallop metabolism becomes more dormant, with release of lower levels of natural fertilizers. Artificial fertilizers may have to be added to the polyculture system to maintain kelp vitality. Thus care should be taken to monitor levels of dissolved N, P and CO2 in seawater throughout the growing season and to make appropriate adjustments where necessary.
Dominant Species Polyculture Systems
In a scallop and Laminaria polyculture system emphasis may be placed on growing one or the other species, depending on environmental factors at different seawater locations. Table 8.11 shows results of an experimental study on output and market value of Laminaria and scallops from monoculture and polyculture systems. In PII, 210 scallop net cages yield 111 kg shelled dry weight, whereas in PI it takes 500 scallop net cages to yield only 67 kg shelled dry weight. Results show that, at this particular seafarming location, scallop production is significantly enhanced by more intensive Laminaria production. The experimental data indicate that the Laminaria and scallop polyculture system with Laminaria as the dominant species (PI) is most cost effective and gives much higher yields and market returns for both Laminaria and scallop production.
