The common belief that fish stop growing once they reach maturity, much like humans and other mammals, is a misunderstanding. Once a person reaches full physical maturity, skeletal growth ceases, establishing a fixed adult size. For most fish species, however, the answer is generally no. These aquatic creatures continue increasing in length and mass throughout their entire lifespan, which explains why the largest individuals of a population are typically the oldest.
Indeterminate Growth: The Biological Reality
The ability of fish to grow continuously is known as indeterminate growth, contrasting sharply with the determinate growth seen in mammals and birds. Determinate growth involves the eventual closure of growth plates in long bones, permanently halting skeletal elongation. Fish lack these fixed growth endpoints, meaning their tissues retain the capacity for expansion well into adulthood.
This persistent growth is supported by the continuous production of new muscle fibers (hyperplasia), in addition to the enlargement of existing fibers (hypertrophy). Unlike mature mammals, fish retain populations of muscle stem cells that continue to proliferate and differentiate into new muscle tissue over time. While the rate of growth slows significantly once a fish reaches sexual maturity, the potential for increasing size never entirely switches off.
Environmental and Resource Limitations
If fish can grow indefinitely, the natural question is why we do not see massive fish everywhere. The practical size a fish achieves is heavily regulated by external environmental conditions, which dictate the rate of growth, even if the capacity is infinite.
Temperature
As cold-blooded organisms (ectotherms), a fish’s metabolism is directly controlled by the temperature of the surrounding water. Growth rates accelerate in warmer water, up to a species-specific optimal point. This occurs because increased temperature boosts the speed of all biological processes, including food digestion and tissue synthesis.
Resource Availability
Resource availability is a powerful factor that limits realized size. When food is abundant, fish convert surplus energy into growth, but a scarcity of caloric intake leads to a significant reduction in growth rate, sometimes referred to as stunting. High population density, such as in an overcrowded aquarium or a small pond, compounds this issue through increased competition for limited food and oxygen.
Stress and Hormones
Physical and social stress caused by overcrowding or poor water quality directly impacts the fish’s endocrine system. Stressors can suppress the production and action of growth hormones, such as the Growth Hormone/Insulin-like Growth Factor (GH/IGF) axis, effectively halting growth. This hormonal suppression is why fish in small tanks or dense wild populations often remain smaller than their counterparts in expansive, resource-rich habitats. The environment acts as a practical filter, ensuring only fish with the best conditions and longevity reach truly immense sizes.
How Scientists Track Growth
Scientists rely on calcified structures to accurately determine a fish’s age and analyze its growth history, similar to counting the rings of a tree. The most precise method involves examining otoliths, which are small, calcium carbonate ear bones located behind the brain. These stones grow continuously by depositing layers of material, with distinct rings (annuli) forming annually due to seasonal changes in growth rate.
By cutting and polishing an otolith, researchers count these concentric rings under a microscope to establish the fish’s exact age. The width of each ring provides historical data: a wider ring indicates fast growth, while a narrower ring suggests environmental hardship. A less invasive technique analyzes the scales, which also develop growth rings known as circuli. Although less consistently defined than otolith annuli, circuli allow scientists to reconstruct the life history and verify the pattern of continuous, indeterminate growth.