Culture of Small Zooplankters for the Feeding of Larval Fish
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SRAC Publication No. 701 VI October 2000 PR Culture of Small Zooplankters for the Feeding of Larval Fish Granvil D. Treece1 and D. Allen Davis2 In nature, zooplankton is one of is a euryhaline species, small and tion occurs below 35 ppt. Most the primary foods of larval fish. slow swimming, with good nutri- production facilities use 10 to 20 Two of the dominant zooplankton tional value. It is well suited to ppt salinity. Abrupt salinity groups are Rotifera (rotifers) and mass culture because it is prolific changes of more than 5 ppt can a sub-class of the Crustacea, and tolerates a wide variety of inhibit swimming or even cause Copepoda (copepods). These two environmental conditions. death, so acclimation should be groups are the preferred prey for Strain selection is important done slowly and carefully. shrimp and fish and are the live because reproduction rate, size Temperature, salinity and feed feeds used most often by cultur- and optimum culture conditions concentration all affect the growth ists. The intensive larval culture of (temperature and salinity) can all rate of rotifers, but temperature is most marine fish depends on a vary with different strains and the most critical factor. The opti- large supply of zooplankton. species. Some freshwater rotifer mum temperature for most strains Brachionus plicatilis (Fig.1a), is a variation can be seen in Figure 1b. is 28 to 32 oC (82.4 to 89.6 oF). small rotifer first developed as lar- Two of the best known strains of Above 28 oC, the salinity and size val fish food in Japan in the 1950s. brackishwater rotifers were of the strain are not very critical, Since then, many methods of cul- thought to be morphotypes of B. but the density of feed is very turing it have been developed. plicatilis, and were referred to as important. Below 26 to 28 oC More than 60 species of marine the “large” (L) and “small” (S) (78.8 to 82.4 oF), the bigger strains finfish are cultured using B. pli- types. Later it was found that tend to grow faster than the catilis as live food. This publica- these are two different species smaller ones. tion will concentrate on the cul- (L being B. plicatilis and S being Rotifers have broad nutritional ture and feeding of rotifers, but B. rotundiformis). Mean dry requirements that must be met to will include information on less weights are approximately 0.33 produce stable cultures. They are used zooplankton such as clado- microgram/rotifer for the L type planktonic filter feeders, feeding cerans (water fleas), copepods and and 0.22 microgram/rotifer for on organic particles brought to tintinnid ciliates. An important the S type. The size of the S type their mouths by the movements of larger zooplanktor used in aqua- is 126 to 172 micrometers accord- their coronas. The corona is a cili- culture is the Artemia (brine ing to one source, and 100 to 340 ated organ on the head region that shrimp), which is the subject of micrometers according to another. characterizes rotifers and is their SRAC publication 702. The L type is 183 to 233 microme- means of locomotion. Rotifers ters according to one source, and ingest many types of feed, includ- Rotifers 130 to 340 micrometers according ing bacteria, as long as the size of to another. Larval fish survive bet- B. plicatilis is the species used the particle is appropriate, so a ter with L-type rotifers, probably variety of food sources can be most commonly to feed larval fish because the larvae use less energy in hatcheries around the world. It used to rear rotifers. However, to feed on larger rotifers. rotifers cultured indoors often 1 Texas A&M University, Sea Grant College Rotifers may tolerate 1 to 97 ppt require vitamin B12 and vitamin A Program salinity, but optimum reproduc- supplements. 2Auburn University Saltwater Freshwater as “semi-continuous” or the com- bined “batch/semi-continuous a. b. technique.” Nutrient sources for culturing rotifers include baker’s yeast and emulsified oils; algae (Isochrysis galbana), yeast and emulsified oil; algae alone; bacteria alone; and outdoor culture using semi-pure or wild strains of algae. The high- est reproduction rate (21 offspring per female every week) has occurred when rotifers were fed a pure diet of Isochrysis galbana (Tahiti strain) and kept at a tem- perature of 20 to 21 oC (68 to Brachionus placatilis with eggs 69.8 oF). The optimum feeding Adult 100 to 300 microns (0.003 to 0.01 rate is 105 to 107 cells of the algae inches) in length Nannochloropsis oculata per indi- vidual rotifer, or 106 to 107 cells of baker’s yeast per individual rotifer. The normal concentration of rotifers is about 100 to 200 per ml, but often reaches more than 1,000 per ml with an adequate food supply. And if there is also a pure oxygen supply instead of aeration, the number will reach Keratella spp. more than 10,000 individuals per ml. Concentrated Chlorella sp. also c. can be used for rotifer culture. No one food source contains all the nutrients required for the long- term culture of a species. Several food sources should be used for Figure 1. Rotifers cultures that are to be maintained for long periods of time. The nutritional value of rotifers tinuous and feedback culture for larval fish depends on the techniques evolved. Each system Larval culture with rotifers rotifers’ food source. Researchers has advantages and disadvan- have determined that highly tages. Batch culture is the most Rotifers usually are fed to fish lar- unsaturated fatty acids (HUFAs) reliable but the least efficient. vae as soon as the larvae have are essential for the survival and Semi-continuous is less reliable developed mouthparts. For larval growth of marine finfish larvae. than batch but more efficient; red drum (Sciaenops ocellatus), this Rotifer feeds containing DHA, however, it allows wastes to build will be on day 3 post-hatch. 22:6n-3, docosahexaenoic acid, up, which causes contamination. Rotifers are fed at a rate of three and EPA, 20:5n-3, eicosapen- Continuous cultures are the most to five rotifers per ml until larval taenoic acid, can be valuable, efficient and consistent but are fish can consume larger foods at with DHA the more essential for maintained under strictly defined about day 11 post-hatch. Larval marine fish larvae. Depending conditions and are almost always mullet (Mugil cephalus) require a upon their food source, rotifers “closed” and indoors, which lim- food density of 10 rotifers per ml, are about 52 to 59 percent protein, its the size and increases the cost when there are 25 to 50 larvae per up to 13 percent fat, and 3.1 per- of the operation. The feedback liter, through day 40. Once rotifers cent n-3 HUFA. system, developed in Japan, uses are harvested from the culture system food is often limited, so There are many methods of cul- wastes from rotifer culture (treat- ed by bacteria and the nutrients the nutritional value of rotifers turing rotifers. Some are low-den- decreases over time. It is best to sity and some high-density. An retrieved) as fertilizer for algae cultured in a separate tank. The feed them to fish at least twice a early method involved daily day, or replenish them whenever transfers of rotifers to fresh tanks Japanese consider this method the most efficient and reliable. The rotifer density drops below a des- of the same size after most of the ignated number per ml. For exam- algae were consumed. Following culture technique described in this publication is usually referred to ple, in red drum larval culture, this, batch, semi-continuous, con- replenishment should occur when rotifer density drops below 3 per gravity feed to rotifer tanks, umes may be harvested rou- ml. Since one fish larva can eat as or the algae can be pumped tinely by dropping to the 50 many as 1,900 rotifers per day, to rotifer tanks. Gravity feed percent level. Even if the from 13,300 to 57,000 rotifers are is preferred; it helps control rotifers are not needed in the needed to feed one fish larva contamination of algae tanks hatchery, the volume in the through this period (depending with rotifers. Rotifer tanks tank should be reduced, and upon fish species and rotifer size). are usually the same size rotifers discarded. Most producers estimate three (1,800 liters) as the algae 5) Drain-harvest rotifers for 1 times the amount of rotifers actu- tanks. Rotifer tanks must month unless a problem ally eaten (1,900 X 3 = 5,700 also have drains and harvest occurs such as a “crash” or rotifers per day) are fed per larva. baskets or nitex screen socks die-off. If this occurs, drain, Therefore, as many as 39,900 (48- to 60-micrometer mesh) clean, disinfect and restart rotifers (for a 7-day period) to to capture the rotifers. Both the tank. Restart the cultures 171,000 rotifers (for a 30-day peri- rotifer and algae tanks in clean tanks monthly. od) may be required to feed one should have aeration and Starter cultures of rotifers fish larva. Feeding too few rotifers illumination. should be maintained at low often results in slow growth and 2) A few days after inocula- densities and in a separate too much size variation; feeding tions the Tetrasalmis cultures facility. Densities of rotifers too many rotifers can cause the will turn a darker green and at harvest will vary, but the fish to ingest so much that assimi- cell densities will be about ranges to expect using this lation becomes a problem.