Fish eggs, the encapsulated embryos of fish, do not consume external food. The nutritional needs of the developing fish are entirely self-contained within the egg casing. Active feeding begins after the fish hatches, when it is referred to as a larva or fry. The initial survival of the hatchling depends on a rapid shift from internal reserves to the capture of microscopic organisms.
Nutrition During the Egg Stage
The developing fish embryo is sustained by a built-in nutrient package called the yolk sac. This sac offers all the necessary energy and building blocks for the initial stages of life. The yolk is rich in high-quality proteins and lipids, which serve as the primary fuel source for the growing embryo.
The yolk sac also provides necessary micronutrients, including high concentrations of vitamins such as Vitamin E and Vitamin A. These vitamins are transferred from the mother and play roles in tissue differentiation and antioxidant protection. The time the embryo spends relying on this internal supply depends on the species and the surrounding water temperature.
This internal feeding phase, known as endogenous feeding, can last from a few days to several weeks. During this stage, the embryo passively absorbs nutrients, fueling development until hatching.
The Transition to Active Feeding
The shift from the yolk sac to hunting for food marks a significant period in a young fish’s life. After hatching, the larva often retains some yolk sac, leading to a brief overlap called the mixed feeding period. The larva must successfully capture external prey before the yolk reserves are fully depleted.
This transition phase is associated with high mortality rates, sometimes exceeding 90%. Survival requires the rapid development of functional capabilities, including pigmented eyes for visual feeding, a fully formed jaw, and the ability to swim horizontally to pursue prey.
The larva must also develop a functional digestive system, including the differentiation of the stomach and intestine. The challenge is intensified because the larva’s small size makes suction feeding less effective against the viscous resistance of the water. If the larva fails to ingest sufficient food during this narrow timeframe, known as the “critical period,” starvation and death will follow. The timing of this moment determines year-class strength in wild fish populations.
Specific Diets of Fish Larvae
Once larvae begin active feeding, their first diet consists almost entirely of microscopic aquatic organisms, known as zooplankton. The initial prey must be small enough to fit the tiny mouth gape of the newly hatched fish. Rotifers, such as Brachionus plicatilis, are a common first food due to their small size and slow swimming speed.
Larvae also feed heavily on the larval stages of copepods and other zooplankton. These prey items are valuable because they provide highly unsaturated fatty acids (HUFAs), such as DHA and EPA. These fatty acids are necessary for the development of the fish’s nervous system and cell membranes.
As the fish grows, its diet undergoes an “ontogenetic shift,” meaning it changes with development. Larvae move from smaller prey to progressively larger food items as their gape size and swimming ability improve. A larva may shift its preference from small copepods to larger zooplankton like cladocerans or Daphnia species.
This dietary change reflects the fish’s increasing physical capability and the development of specialized feeding structures. Different species follow various paths, with some becoming planktivorous or carnivorous, while others consume algae or plant matter as they enter the juvenile stage.