When a fish egg hatches, the young aquatic animal is often called a baby fish, but its scientific and common names change as it grows. This progression of names reflects distinct biological stages, marking the transition from a larva to a fully functional juvenile. The specific terminology used depends on the fish’s size, internal development, and ability to feed itself.
The Most Common Names for Young Fish
The most widely recognized term for a newly hatched fish is “fry.” This designation applies when the young fish is actively swimming, feeding on its own, and is no longer dependent on a yolk sac for nutrition. The fry must forage independently, often on small invertebrates and plankton, to survive and grow.
As the fry grows, it reaches a larger size known as a “fingerling,” a term common in aquaculture and fish stocking programs. A fingerling is named for its approximate size, roughly the length of a human finger (four to six inches long, or 10 to 15 centimeters). At this stage, the fish has developed protective scales and fully functional fins, making it more robust than a fry. Hatcheries often release species like trout or bass as fingerlings because they have a higher chance of survival in the wild.
Developmental Stages and Specific Terminology
The life cycle of many fish species, particularly salmon and trout, involves precise names that track distinct biological changes. Immediately after hatching from the egg, the creature is called an “alevin,” sometimes referred to as a “sac fry.” This name describes the fish while it still has a prominent yolk sac attached to its underside, which provides all its nutrition for the first few weeks of life.
During the alevin stage, the fish remains largely hidden within the gravel beds, relying on the yolk sac until it is fully absorbed. Once the sac is depleted and the fish swims up to begin feeding externally, it transitions into the “fry” stage. In salmon, the fry will then grow into a “parr,” developing vertical markings or spots on its sides for camouflage in the river environment.
The parr stage is followed by the “smolt” stage in migratory fish like salmon. Smolting is a physiological process that prepares the fish for its journey from freshwater to saltwater. This involves changes in body chemistry, color, and scale structure. Following these stages, the fish is referred to as a “juvenile” until it reaches full sexual maturity.
Names for Babies of Other Aquatic Creatures
Many aquatic animals have unique names for their young that differ from those used for true fish. Baby sharks, which are born fully formed and independent, are known as “pups.” This designation is shared with the young of several marine mammals, such as seals and sea otters.
Eels have a specialized naming convention due to their long, multi-stage metamorphosis. They hatch in the open ocean as tiny, transparent, leaf-shaped larvae known as “leptocephali.” These larvae drift for months before transforming into “glass eels,” which are still transparent but shaped more like an adult eel.
As the glass eels migrate into freshwater rivers and begin to develop pigment, they are called “elvers,” the final juvenile stage before they become a mature eel. In the invertebrate world, the offspring of jellyfish are called “ephyra” after they detach from the polyp stage, resembling a tiny, free-swimming disc.