Knowing how long fish live is vitally important to managing fish stocks and protecting them from overfishing by commercial or recreational fishers. Overfishing is the practice of harvesting fish at a rate that causes a dramatic decline in the fish stock-perhaps to the extent that the fishery collapses. In essence, the rate of harvest exceeds the maximum harvest that would allow for replacement of the population through reproduction. Long-lived animals grow slowly, reproduce infrequently, and take a long time to recover from overfishing. Agencies responsible for protecting and regulating fish populations must therefore know the life span of fish that are being harvested.
Orange roughy, a deep-water fish caught near New Zealand, was presumed to live a maximum of about 20 to 30 years and catch limits of the industry were set accordingly. Within five years, the fishery collapsed due to overfishing. Scientists discovered that the orange roughy actually lives more than 100 years (up to a maximum of about 150 years), is very slow growing, and does not reach sexual maturity until about 30 years of age. It is likely that recovery of orange roughy stock will take decades.
A long-used indirect method of estimating fish age is to measure the length of large numbers of fish of the same species caught in nets. From these data, scientists estimated age based on models of how fast the particular fish species grow. Another method of age determination is to raise fish in tanks to determine directly the relationship between size and age. Neither method is entirely satisfactory, however.
A more accurate technique is to count the annual growth rings in the fish's ear stone, the otolith. This calcareous concretion is up to about the size of a nickel. Bone is deposited on the outside of the otolith as the fish grows, thereby forming a banded bone that can be sectioned and studied much like the growth rings of a tree. Some fishes produce annual bands; others take longer to form a band. By cutting thin sections of the otolith and counting the bands, it is possible to determine a fish's age accurately. Sufficient samples of many fish of the same species give a reasonable estimate of life span.
Furthermore, chemical analysis of the individual bands in otoliths permits scientists to determine the various environments where the fish has lived during its life cycle (e.g., lakes, estuaries, and the open ocean). Also, oxygen isotopic analysis of the otoliths enables scientists to reconstruct the water temperature when a particular band formed. (We describe the oxygen isotope analysis technique in Chapter 12 of your DataStreme Ocean textbook.) Finally, these detailed analyses can be used to determine which animals lived together when the bands formed.
Bony fishes have scales on their skin that grow by forming rings. These scales can be studied under a microscope and the rings counted to determine the fish's age. This technique is especially useful in tiny tropical fishes whose otoliths are too small to study conveniently. In general, these smaller faster-growing tropical fish have shorter life spans than larger slower-growing fish living in colder waters.
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Prepared by Joseph M. Moran, Ph.D.,
© Copyright, 2008, The American Meteorological Society.