Compared to a lot of marine fish larvae, raising salmon fry is child’s play: the larvae already measure about 20 mm in length when they hatch; they are fairly robust and, after consuming the yolk sack, they can immediately be fed on dry starter feed. In contrast, marine fish larvae are sometimes only 3 or 4 mm long and extremely sensitive. In a lot of cases there is no artificial feed mixture that would be acceptable to the tiny organisms during this tricky phase. For that reason hatcheries mostly fall back on live feed. Of course, the best would be wild plankton but this is rarely available in the required quantities and quality. That is why other organisms are used that are always available and can be produced in the desired quantity at the right time. They include for example microalgae, ciliates, rotifers or Artemia (brine shrimp) nauplii. The production of this feed demands considerable effort but it has long been routine in hatcheries. In some companies that grow marine fish and shrimp species the departments in which the live feed is produced are just as big as the sections where fish eggs and larvae are kept.
Brine shrimp or Artemia are an almost universal live feed for aquaculture. The life cycle of these shrimps is about one year. The females that have reached sexual maturity eject eggs about every six days and, given good living conditions, the nauplii will hatch immediately. Under poor conditions, for example if the salinity of the water rises above 150 ppt or if oxygen is lacking, they produce cysts that are extremely resistant and can even survive temperatures of minus 190°C or boiling water. This ametabolic state of life that is entered by an organism in response to adverse environmental conditions is called cryptobiosis. Artemia cysts can remain in this state for over two years, only to hatch within just a few hours once more favourable conditions prevail. This attribute makes the cysts particularly suited to use in aquaculture because they are available in large quantities, easily storable and relatively inexpensive. The 0.4 mm nauplii can be hatched at exactly the required time for immediate use as feed.
The potential of these tiny crustaceans for use in aquaculture was recognized in the 1970s and since then demand for them has risen continually. In Great Salt Lake (Utah, USA) alone, which covers about 90% of global requirements, 2,000 to 3,000 t of Artemia cysts are gathered per year. Other sources such as Lake Urmia (Iran), Bolshoye Yarovoye (Siberia) or some salt lakes in Turkmenistan and Kazakhstan are becoming increasingly significant, too. The biggest buyer of Artemia worldwide is China with an annual demand for 1,500 t cysts, part of which they can satisfy themselves, however, with cysts from Bohai Bay and Aibi Lake. The rise in demand for Artemia cysts led to a rise in price. Whereas in the 1980s one kilogram of cysts cost about 25 USD they can cost 80 USD or more today. The price depends heavily on the cysts’ quality. Important quality criteria are, in particular, the hatching rate (in per cent) and the efficiency (number of nauplii per gram of cysts), as well as synchronicity of hatching.
Under conducive conditions (salinity approx. 35 ppt, temperature about 25°C, pH value around 8.0-8.5) and intensive aeration (the eggs should be constantly in motion and not lying on the bottom) the incubation of the cysts rarely causes problems so that the nauplii will hatch after one to two days. The exact time of hatching depends on the temperature, the cysts’ origin, their age, and other factors. To remove the nauplii it is sufficient to switch off the aeration. The empty cyst shells rise to the surface while the nauplii gather on the bottom of the tank. Because the larvae move towards the light it is possible to concentrate them in a suitable place using a light source so that they can then be sucked in with a tube over a tight meshed sieve. In the hatcheries whole batteries of incubation vessels are usually in operation so that nauplii are available for feeding the fish larvae whenever they are needed.
Enrichment improves the nutritional value of live feed
Due to their yolk reserves freshly hatched Artemia nauplii are relatively nutritious. Depending on the species of incubated brine shimp (in practice not only Artemia salina are used but also A. franciscana, A. parthenogenetica, A. gracilis, A. sinica and some other species) the nauplii contain between 37 and 71% protein and up to 30% fat which, although it contains unsaturated fatty acids, hardly contains any EPA and DHA which are of particular importance to marine fish larvae. Added to this is the fact that the nauplii quickly eat up their own yolk reserves. The more time passes before they are fed to the fish larvae the more nutritional value is lost. Because Artemia as live feed do not contain all nutrients in the composition and quantities that the young fishes need to enable their healthy development the nauplii are often enriched with the missing substances prior to feeding. To do this the nauplii are fed unsaturated fatty acids, amino acids, vitamins, pigments, minerals and trace elements as well as, if necessary, also medicaments. After enrichment, the intestines of the nauplii thus contain all the nutrients that they otherwise lack. These are then absorbed and digested by the fish larvae during feeding.
Using this simple method (which is also called bioencapsulation) it has been possible to solve some of the dietary deficits and developmental disorders that had previously often occurred during marine fish farming. The occurrence of backbone deformation and pigment defects decreased noticeably. Of course, enrichment is only possible once the Artemia nauplii have reached a certain age and their intestines are fully developed. As a rule that is about eight hours after hatching when the nauplii are in the second larval stage and have moulted. Often it is already sufficient to feed the nauplii with phytoplanktonic algae (green water) or yeast cells. These substances contain valuable nutrients that the tiny crustaceans otherwise lack. In the meantime there are also industrially produced concentrated supplements that are exactly geared to the nutritional requirements of fish larvae. The tiny microparticles of these concentrates spread out in the water to form an emulsion that is filtered out of the water again during the non-selective feeding of the nauplii and consumed.
Already in the 1980s a technique was developed for freeing the Artemia cysts of their thick shells (decapsulation) with the help of chemicals, for example through treatment with sodium hypochloride. The embryos are then practically naked but still fully viable and vital. The decapsulation of the cysts offers the great advantage that a lot of microbial germs are also removed with the shell, thereby quasi disinfecting the cysts. This is particularly useful where sterile, germ-free conditions are necessary… for example in shrimp hatcheries that grow SPF postlarvae. Apart from that, decapsulation prevents the remains of shells from entering the tanks containing the fish larvae during feeding with nauplii. The hard shells are not digestible and can block the intestines of the fish larvae and even cause their death. When nauplii hatch under normal conditions and have to break open the thick shell of the cysts they also use up a lot of energy. This effort is not necessary in the case of decapsulated cysts with the result that the nauplii have 30 to 55% more energy which makes them much more nutritious for the fish larvae.
A further option for the use of Artemia is to feed the adolescent, sexually mature organisms to suitably sized larger fishes. Adult males of brine shrimp reach a length of about 8 to 10 mm, the females are somewhat larger at 10 to 12 mm.
Rotifers – optimal starter feed for a lot of marine fish larvae
Despite their tiny size Artemia nauplii are still too big as a starter feed for a lot of fish larvae. If this is the case rotifers are usually used. The word "rotifer" is derived from a Latin word meaning "wheel-bearer", due to the crown of hair-like cilia around the mouth that when in motion resembles a wheel which is used to waft feed into the mouth cavity. Some rotifers are only half the size of Artemia nauplii. Of the approximately 2,000 rotifer species that are known up to now it is mainly two species that are used in marine and brackish water aquaculture: Brachionus plicatilis (“L”-strain, size = approx. 0.20-0.36 mm) and Brachionus rotundiformis (“S”-strain, size = approx. 0.12-0.22 mm). Rotifers constitute an optimal live feed for smaller marine fish species because they are very small, too, have a soft body and are highly digestible. They spread out evenly in the water and swim so slowly that they easily fall prey to the fish larvae. Apart from that, they reproduce quickly and can be fed easily with microalgae in special cultures.
From one single rotifer a population of several thousand rotifers can develop within just a few days. Under ideal conditions the individual number in a rotifer population can grow by 50% per day. Under practical conditions in hatcheries the growth rate is mostly lower but even there considerable rates of 20 to 30% are attainable. These reproduction rates are achieved through parthenogenesis, an asexual reproduction mode for which no male organisms are needed. Under favourable conditions (22-28°C, pH 7-8.5, oxygen content over 4 ppm, salinity 10-35 ppt) the females simultaneously eject up to 7 eggs from which only females will develop that will in turn eject their own eggs after 12 to 18 hours. In this way the number of individuals in a population can rise astronomically within just a very short time. If the environmental capacity is exhausted and living conditions deteriorate the rotifers can also reproduce sexually and, like Artemia, produce very resistant cysts.
When growing rotifers for use as live feed in hatcheries either seedstocks of parthenogenetic rotifers or cysts are used. Two methods are commonly employed for the cultures. The first is to use batch cultures, discontinuous techniques in which the rotifers are usually produced at four to five day intervals. On the first day rotifers are added to the culture and they develop quickly with daily algal feed and partial water exchange. At the end of the cycle when the rotifers have reached a density of about 500 organisms per millilitre the whole population is harvested (one part will be added to the next cycle), fed to the fish larvae and the culture restarted. The second method is continuous cultures. These provide rotifers daily over a longer time period but require much more effort and are more susceptible to failure. Continuous cultures work on the principle of a recirculation system in which circulating water is repeatedly cleaned with particle and bio filters and protein skimmers and the germ count reduced with ozone or UV light. The more stably the water quality can be kept at a level that is conducive to rotifers the more productive the culture will be in general. Algae pastes are mostly used for feeding. These are dispensed from machines. In well adjusted continuous cultures rotifer densities of 1,000 to 5,000 and more organisms per millilitre are possible.
Before they are fed to the fish larvae the rotifers are rinsed several times over in clean salt water. In order to ensure high survival rates and good growth the fish larvae should be “swimming in” feed. To grow red seabream (Pagrus major) successfully the feed density should be about 2 to 4 rotifers per millilitre. For black seabream (Spondyliosoma cantharus) 5 to 10 rotifers / ml or even more are necessary.
Like Artemia nauplii, rotifers have a relatively low nutritional value and should thus be enriched with unsaturated fatty acids, particularly EPA and DHA and other nutrients before being fed to larvae in the hatcheries. You could almost say that rotifers are only as nutritious as the feed they have just consumed. As filterers, rotifers waft almost any feed that arrives in front of their hair-like crowns and fits into their mouths. Particle sizes usually range from 0.001 to 0.010 mm. As a rule, their feed today consists of microalgae that are mostly put into the water in aquaculture in the form of highly concentrated algae pastes (green water technique). This saves the effort of preparing additional algae cultures and is very efficient because the algae concentrates mostly have a high quality. On average about 1 to 1.5 ml of algae paste per million rotifers is required daily. As soon as the algal density decreases more has to be added since the rotifers must not be without sufficient feed for longer than a few hours.
Aquafeed industry looking for acceptable alternatives
The mouth cavities of some fish larvae are so small, however, that even “normal” rotifers would be too big for them. For example, the fact that some grouper species are very difficult to raise is not least due to problems concerned with feeding. Ciliates are often fed to these fish species, single cell wheel animals whose best known representative is probably Paramaecium caudatum. With a length of almost 0.3 mm its size is similar to that of the rotifer but due to its slender body shape it can be picked up much better by a lot of fish larvae. Apart from that, the group of ciliates comprises about 6,000 species so that the choice of potential live feed is very large. For feeding marine fish larvae tintinnides and species of the genus Euplotes are often used. Because ciliates have a low nutritional value the diet is changed to enriched rotifers as quickly as possible once the fish larvae are large enough to pick these up. Like Artemia and rotifers ciliates can also be produced quite easily in mass cultures. Most simply this can be achieved by pouring water onto hay which in a short time leads to the mass development of various ciliate species.
Cromaris farms seabass and seabream
In-house control over hatchery feed production safeguards quality
A Croatian producer of seabass and seabream Cromaris has its own broodstock and hatchery for the production of the fish larvae. In common wih other hatcheries for these species Cromaris produces its own live feed (rotifers) to feed the seabream larvae once these have consumed the yolk sac. In the case of seabass the first feed given to the larvae is artemia. Irrespective of the species, this is a delicate stage in the life cycle of the fish and hatcheries want complete control over all the parameters including the feed to keep mortality rates within tolerable limits, says Gordana Sarucic, the hatchery manager. Cromaris has invested significantly in trained personnel, equipment, and space to produce rotifers and the phytoplankton on which they feed.
The larvae feed on the live organisms for about 40 days in the case of seabream. Rotifers are added to the tank at regular intervals to ensure a density that is high enough for the fish larvae to have easy access to their food. Phytoplankton is added to the tank in the form of a paste that provides the rotifers with nutrition. The presence of phyoplankton also improves the water quality in the tank by limiting the development of bacteria and reducing nitrogen and phosphorus loads. After about 20 days the larvae are introduced to artemia as well and are gradually weaned off the rotifers. Finally, the artemia is slowly substituted with dry feed. Seabass larvae go through te same procedure except they are fed only with artemia.
We certainly remain aware of developments in the feed industry and if we find compelling reasons to switch to dry feed we will certainly consider them, says Ms Sarucic. For the moment, however, we find that the fish do well with live feeds and being able to control the whole production process is an added advantage that we will not dispense with lightly.
Ciliates reproduce by cell division. Under normal temperature conditions of around 20°C they divide about once in 24 hours. By raising the temperature to 26°C division frequency can be double or even triple that.
The feed industry is going to great effort to develop high-quality alternatives to the use of live feed in aquaculture. This would be an effective contribution towards lowering effort and costs of raising and stabilising production of fry. In the meantime it has been possible to bring forward the point in time at which larvae can switch from live to dry feed (weaning) for some species but apart from occasional successes the industry is still a long way from finding a solution to this problem. At present live feed cannot be replaced in the early phase of farming of a lot of fish species. Although its nutritional value does not always match the exact needs of the larvae live feed does offer some advantages. The swimming movements of the live feed awaken the larvae’s hunting instinct, it can be swallowed more easily, and it is closer to the preferred flavour of the fish larvae than is at present the case with dry feed.