The longevity of fish spans an astonishing range in the vertebrate world, from lifecycles measured in weeks to existences spanning multiple centuries. This immense variability reflects a wide spectrum of life history strategies adapted to diverse aquatic environments. The lifespan of a fish, from tiny coral reef species to deep-sea giants, is dictated by genetics and surrounding conditions. Understanding this spectrum requires investigating the scientific methods used to determine age and the environmental factors that govern whether a species lives fast or slow.
How Scientists Determine a Fish’s Age
Researchers determine a fish’s age by counting annual growth rings on calcified body parts, similar to counting rings on a tree. The most reliable structure is the otolith, or “ear stone,” a small, hard calcium carbonate structure found in the inner ear of bony fish. As the fish grows, the otolith adds layers. Periods of fast growth (warmer seasons) deposit a wide, opaque band, while slower growth (winter) adds a narrow, translucent band. Counting these paired bands, known as annuli, provides a precise record of the fish’s age.
While otolith extraction often requires sacrificing the specimen, other structures can be used, though they are less accurate, especially in older fish. Scales, fin rays, and vertebrae also exhibit annuli, and counting the rings on a scale is the least invasive method. However, scales can be shed and regrown, leading to inaccurate age estimates, and the rings become difficult to distinguish in old individuals. For cartilaginous fish like sharks, which lack bony otoliths, scientists use radiocarbon dating of the eye lens nucleus or count growth bands in the vertebrae.
The Range of Fish Lifespans
Fish lifespans range dramatically, from species that complete their existence in a single season to those that predate human civilizations. At the extreme short end is the tiny Sign Eviota (Eviota sigillata), a coral reef fish documented to have the shortest lifespan of any vertebrate, living a maximum of only 59 days. Other short-lived species include many tropical and aquarium fish, such as guppies and mollies, which live for only one to five years. The annual killifish, adapted to temporary ponds, lives for about one year in the wild, rapidly reproducing before its habitat dries up.
Many common marine and freshwater species fall into an intermediate range, with lifespans between five and twenty years. Species like trout, salmon, and popular aquarium fish such as Oscars and Angelfish often live for a decade or more under optimal conditions. Koi, a species of carp, typically live between 25 and 35 years, although historical accounts claim specific individuals have reached over two centuries. This intermediate grouping represents a balance between growth rate, reproductive effort, and environmental stability.
The record-holders for longevity are found in cold, deep-sea environments, where life proceeds at a glacial pace. The Rougheye rockfish (Sebastes aleutianus), a deep-dwelling species in the North Pacific, can live for over 205 years. The longest-living vertebrate known is the Greenland shark (Somniosus microcephalus), an Arctic species that can live for an estimated 250 to 520 years. These extreme ages are linked to a life history strategy involving slow growth and delayed sexual maturity; Greenland sharks do not reach reproductive age until they are over a century old.
Biological and Environmental Drivers of Longevity
The primary environmental factor influencing fish longevity is water temperature, which directly affects an ectotherm’s metabolic rate. Fish living in cold, deep, or polar waters experience a slower metabolism, which slows growth, development, and the rate of cellular damage. This explains why Arctic species like the Greenland shark and deep-sea fish like the Rougheye rockfish exhibit extended lifespans; their biological processes are down-regulated in near-freezing temperatures. Conversely, fish in warm, tropical waters have higher metabolic rates, leading to faster growth but shorter lives.
Evolutionary pressure from predation also plays a role in determining a species’ life history strategy. In environments with high predation risk, natural selection favors early maturation and rapid reproduction, leading to shorter lifespans, as seen in many small tropical species. This “r-selected” strategy prioritizes high offspring numbers over individual survival time.
In contrast, environments with few predators, such as the deep ocean or remote Arctic habitats, allow for a “K-selected” strategy, favoring delayed maturation and extended lifespans. These species invest more energy into survival and growth, only reproducing once they are large and less vulnerable to threats. While body size is not a universal predictor, larger fish live longer, as increased size often correlates with lower mortality rates and a greater ability to withstand environmental stress.