Assessing Trout Growth and Longevity
by Jim Habera
Coldwater Fisheries Biologist
Tennessee Wildlife Resources Agency

Trout Spawning Gravel
A typical scale from an age-3 wild rainbow trout. The three annuli are indicated.
In addition to estimates of abundance, trout fishery managers also typically want to know something about how fast the fish in a particular population grow and how long they live. This information is crucial for determining what angling regulations are appropriate (or inappropriate) and for evaluating the success of certain management strategies. For example, if trout rarely live past age 3 (regardless of angler exploitation) and require two years to reach a length 8 inches—as do wild rainbows in southern Appalachian Mountain streams—a 12-inch minimum size limit would make little sense.

Fish never completely stop growing, although growth rates for mature fish are much slower than for younger fish. Various calcified structures can be used to age fish, but scales and otoliths (“ear stones”) are most commonly used. Scales have been in use since the late 1800s, beginning with carp in Germany. Concentric rings (circuli) are laid down on scales as a fish grows—like tree rings—and as growth slows (typically in the fall or winter) these circuli crowd together and form an annulus, indicating one year of age. However, scales are generally considered less reliable than otoliths because they may be lost and re-grown at some point (regenerated) or may be partially reabsorbed during times of stress. Both of these occurrences can cause the loss of some of a scale’s annuli, resulting in age underestimation. Another problem is that scales from older fish are often very troublesome to read as the annuli become crowded and difficult distinguish near the margin.

Trout Flows
A typical otolith from an age-4 wild brown trout. The four annuli are indicated.
Otoliths are small, bony structures located in the inner ear of fish and their utility for ageing has been known nearly as long as that of scales. The largest of the three pairs of otoliths (the sagittae) function in hearing and equilibrium maintenance. Because they reliably form concentric translucent and opaque zones that correspond to periods of faster and slower growth each year, they can be used to obtain accurate ages. The downside is that unlike with scales, a fish must be sacrificed to obtain its otoliths. Various preparation methods are used, but trout otoliths are relatively small (only a few millimeters in length), so they are often mounted in epoxy blocks from which a thin cross-section is cut from the center, polished, and examined under magnification. The validity of otolith (or scale) ages can be verified by marking a group of known-age fish (e.g., clipping the adipose fin of some young-of-the-year), then recapturing some of these a year later and checking the appropriate structure to ensure that an annulus has formed.

Brown Trout from AR
Large wild brown trout inhabit several east Tennessee mountain streams. Otoliths provide much more accurate age determinations for these fish than do scales.

For many years, the longevity of large wild brown trout (up to 25 inches) in Tennessee mountain streams was essentially unknown.  Scale analyses indicated that some of these fish could be up to age 6 or 7, but the potential inaccuracy of scales was always a concern.  When otoliths were finally collected from a range of populations and analyzed, they revealed that these fish lived as long as 12 years and that the discrepancy between ages derived from scales and otoliths from the same fish could be as much as five years.  Additionally, scale/otolith agreement beyond age 5 fell to 50% or less.  Obviously, natural mortality of brown trout in these populations is lower than previously estimated, thus scale-based age and growth information in such cases could lead to improper management strategies that focus on larger (older) fish.

Maximum age for rainbow trout is about 11 years, although they more commonly live 4 to 8 years throughout most of their range.  Maximum size varies with genetics and habitat, although the current world record, set in 2007 by a fish from Saskatchewan, is over 43 lbs.  Steelhead (anadromous or “sea-run” rainbow trout) can exceed 50 lbs.  Maximum age for brown trout approaches 40 years, although their life spans typically do not exceed 18 years in most populations.   The world record brown trout is just over 40 lbs. and was taken in the Little Red River (tailwater) in Arkansas. 


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