By: Ann Kunz
Today was a big day for the aquarium staff who have been hard at work growing out fish larvae from tiny eggs collected in the big aquarium exhibits.
A number of Natal moony’s, stumpnose, karanteen, spadefish, 2-bar seabream and zebra fish which were grown out from eggs harvested in the snorkel lagoon and open ocean exhibits were put on display in one of the aquarium’s small exhibits. They are all approximately 3.5/4 months old and will remain in the small exhibit until they are big enough to be moved to one of the larger exhibits.
The aquarium provides many fish with conditions that are conducive for spawning. Spawning usually takes place after dark during spring and the summer months up until the end of March.
As the eggs are neutrally buoyant, a floating egg collector net is left overnight in the exhibit and any eggs collected the following morning are taken to the fish hatchery. Growing-out these fish in sufficient numbers from our resident adults means that we will need to collect less from the ocean.
Our fish hatchery and cultures laboratory has, over the past two years, developed into a busy environment. The specific food requirements for the various species of newly hatched fish is often unknown to us, making this area of aquaculture particularly challenging and rewarding. We are currently culturing brine shrimp, rotifers, copepods and infusoria which we feed to the larvae.
Most fish breed by broadcast spawning. Fertilisation is external. Males and females swim together when the females release eggs and the male’s sperm into the water. In our tanks the eggs are very small. A big egg is the size of a pin’s head, but most are smaller. It is fascinating to watch the little embryos growing on the wall of an egg under a microscope. After 24 hours the egg starts to swim. The larva absorbs the egg after 3 days, develops eyes and mouth and starts to feed. The rotifers are free swimming in the water and are thus available to the baby fish.