Microcosmos: Moi growth in hatchery

Cultivation of Moi Larvae

Raising finfish from eggs in hatcheries has only been made possible by breakthroughs in research as recently as the 1960's. Traditional aquaculture was successful at raising fish for food, but it was neccessary to stock the ponds by capturing young fry from the wild. Today, the cultivation of larvae from the eggs of captive broodstock in hatcheries has been achieved for many species. Growing larvae requires carefully controlled conditions and techniques, because the newborn are extremely small and delicate. A small mouth size and incomplete digestive system limits what the newly hatched larva can eat at various stages. Larval species that are carnivores require a mixture of living microorganisms as food during their larval development. OI's studies have refined methods to produce a wide variety of live feed items, thus overcoming a primary obstacle to developing new breeding programs for difficult species. Fish grown in hatcheries are valuable both for commercial aquaculture and for restocking depleted natural environments.

Moi (Pacific Threadfin) is the finfish that we have chosen for the larval examples in our diagram. Moi is a marine shoreline fish often found in the near shore surf. A dull silvery white carnivore, it can reach a length of nearly three feet. At one time, it was reserved only for Hawaiian royalty (Ali?i) due to its delicate flavor. Overfishing, however, has decimated its population. Today, OI has developed hatchery techniques to make moi available for commercial aquaculture and for restocking depleted natural environments. We have successfully cultured over 1.5 million moi since 1993. Nearly 500,000 have been tagged and released into waters off O?ahu and the Big Island of Hawai?i. Others have been provided to local farmers for growout. OI is currently evaluating the use of a submerged cage off the Ewa coast on O?ahu to raise juveniles to market size.

The embryo develops in the egg for 16 - 24 days until it hatches. The eggs of cultured marine fish are typically 1 mm in diameter, and the eggs develop independently, that is, without care from the adults. In fertilization, the sperm enters and activates the egg, two nuclui fertilize, and a succession of cell divisions follows. The chorion or outer membrane hardens, while the yolk provides nourishment for the developing embryo.

The larva is the term that refers to the stages between egg hatching and the completion of metamorphosis (approximately day 8). From day 0 to day 3, the larvae continue to feed on the remains of the yolk.

Larvae must begin to eat live prey by day 2 - 5 as the yolk diminishes. The change in eating is not immediate, but is a continuum whereby the larvae begin to eat a little prey as well as yolk, and are gradually weaned until only prey is being eaten. During larval development, there are changes in shape and size, as well as behavior, locomotion, respiration, and digestion. Most changes are gradual, but some occur very quickly, such as the beginning of exogenous feeding (food from outside the organism). This time of abrupt adaptation is termed a 'critical period' and may be a time of increased mortality.

Rotifers are the first microorganisms that the larvae eat, because rotifers are small, under 2mm in length. Larvae do not have a functional stomach with enzymes for digestion. Their existing short, elementary digestive tract needs easity digestible nutritious food, thus tiny live organisms seem to be most effective. The developing larvae do not have good eyesight with which to detect food, but live prey like rotifers attract the notice of the fish larvae by swimming around nearby, providing an opportunity for the larvae to catch them.

A juvenile is the terminology for a larva after metamorphosis, approximately day 8. Metamorphosis is the term that describes the point at which the larvae begins to look like the adult of its species, with the skin taking on color, and the appearance of scales and fins. At this stage its digestive stomach is more developed, and it is able to eat larger microorganisms. Artemia are introduced into their diet, and for the next few days, they will continue to be fed rotifers while they become familiar with the artemia diet. After day 12, rotifers are withdrawn.

From day 14 to day 24, the juveniles begin to eat artificial formulated diet. It is more cost effective to wean the juveniles off live foods. It is also more environentally sound to feed artificial food rather than take live organic food from the ocean reserves. After day 24 they will leave the hatchery and go to the nursery as fingerlings.

Cultivation of Rotifiers

Rotifers can be raised at high densities, and reproduce rapidly, therefore it is economical to grow them. 1 billion rotifers are raised daily to feed 100,000 moi larvae. Rotifers become adult in just a day and their life span is about 3 ? 4 days. The females have both asexual and sexual reproduction cycles.

The newly hatched rotifer looks like the adult. Its body is divided into three parts consisting of the head, trunk, and foot. The head has retractable corona which rotate for locomotion and catch food particles such as algae. This rotation is the basis for the rotifer's name. Rotifers are grown in a tank with algae. Because algae is lacking in some nutrients, fish oils and other nutrients are added.

Cultivation of Artemia

Artemia are purchased as cysts. Adult artemia produce both eggs and cysts. Cysts have a hard shell and grow only in high brine areas. The Great Salt Lake in Utah, USA is a primary source of commercial artemia cysts (90% of the market), and therefore must be carefully protected environmentally. The floating cysts are harvested by net, cleaned and dried for commercial sale. Cysts are crustaceans and have a hard shell. They can remain metabolitically inactive if the shell is dry and may live under adverse environmental conditions for years. Dry cysts absorb veryquickly. When the cysts are incubated in seawater for 24 hours, they hydrate, swell and become spherical, then the outer membrane of the cyst bursts, and the embryo appears.

In the umbrella stage, the embryo hangs underneath the empty shell and develops in a transparent hatching membrane. At hatching the membrane breaks open and the artemia nauplius is born. It goes through instar stages: instar l lasts from hatch to 24 hours and lives on its yolk reserves. It molts into instar ll, the stage that the artemia nauplii are fed to the fish larvae. If left to mature, the larva would go through about 15 molts. Half a billion artemia nauplii are grown daily. Like rotifers, they can be grown at high density. They do not contain all the nutrition that the fish larvae need, so nutrients such as fish oils are added to the water containing larvae and artemia.