Brown trout Salmo trutta are the largest and potentially oldest stream-dwelling salmonids in the southern Appalachian mountains. The limited amount of growth, longevity, and mortality information for brown trout from populations in Tennessee and elsewhere in the region has been based primarily on scale analyses. However, the accuracy of scale-derived ages for southern Appalachian brown trout is unverified. We have previously identified brown trout to age 7 with scales, but annuli crowd together near the margin and are poorly defined, making scales from older fish difficult to read. Because fish can live for many years with little or no growth, validation is necessary to determine that fish are not actually older than estimated with scales. Consequently, inferences about growth, longevity, and other population characteristics for Tennessee brown trout remain questionable because of uncertainty about the reliability of scale-derived ages.
Otoliths ("ear stones") are small bones that float in a fluid-filled capsule located near the base of a fish's skull. They function in equilibrium maintenance, but can be used to age fish and are generally considered more accurate than scales (particularly for older specimens). Scale ages of larger Tennessee brown trout have typically not been verified with otoliths, probably because it requires sacrificing the fish. However, if otolith data indicate that brown trout are substantially older than has been determined by scales, then current growth and mortality rates are overestimates. Such errors could affect management strategies in some cases.
The continued lack of this basic biological information was considered more costly than the loss of some large fish, so an otolith collection project was initiated in 1996. Our primary objectives were to obtain otolith ages for wild brown trout from Tennessee streams (particularly large specimens), assess the comparability and accuracy of scales for aging these fish, and validate both structures for as many age groups as possible by recapturing marked (known-age) fish. This information will be used to review current brown trout growth and mortality estimates, as well as the suitability of management strategies now in place.
A total of 227 brown trout ranging from 94-647 mm (total length) have been collected from 11 east Tennessee streams (no tailwaters). Fish up to age 8 were identified using scales. Precision between the two readers who examined the scales was 80% through age 4, then declined to 50% for fish older than age 4. The average difference when the readers disagreed was 1.4 years and the largest difference was 3 years (four cases). Brown trout up to 12 years old were identified with otoliths. Five fish were older than age 8 (the oldest scale age). Precision between readers for otoliths (89%) was higher than that for scales (76% overall). The average difference when the readers disagreed on otoliths was 1.0 years and the largest difference was 2 years (one case).
Corresponding scale and otolith ages were in agreement for 168 of the 227 fish (74%). Agreement was 81% for ages 1-5, but declined to 21% for older fish. There was no agreement beyond age 7. Scale age was an underestimate of otolith age in 40 of the 59 cases (68%) where the age of each structure did not agree. At age 3 and beyond, scale age underestimated otolith age for 98% of the cases where there was disagreement. Otolith age exceeded scale age by as much as five years and the mean difference for ages 9-12 was 4.6 years.
There was much variability in length (i.e., growth) for fish of a given age (based on otoliths). The five fish in the oldest age groups (9-12) were exceeded in length by nine younger specimens (age 4-8). Additionally, 457-598 mm (18-20") fish ranged from 4-12 years in age.
Scale samples were obtained from 90 marked (known-age) brown trout at ages 1-5 and otoliths were also collected from three of these specimens (ages 4-5). Scale and otolith ages determined for marked fish were consistent with the corresponding known ages in all cases.
Obviously, there are wild brown trout inhabiting Tennessee streams that are several years older than previous thought. Based on verification using otoliths and known-age fish, scales are relatively accurate for aging younger brown trout (up to age 4 or 5), but are essentially useless for older fish. If a clear-cut relationship existed between age and length for larger (older) fish, a model could be developed that could be used to estimate age given a particular length. Unfortunately, the substantial variability observed in growth rates means that the largest fish are not necessarily the oldest fish and that the necessary relationship between length and age does not exist when fish from all streams are pooled. However, more data may permit the development of "age-length keys" for individual streams. For now, the age of any brown trout with more than five scale annuli is considered unknown unless otoliths are examined.
Average brown trout length at age (growth) begins to plateau at age 7-8 (around 500 mm), so it generally does not appear to require 10-12 years to produce the larger fish we see now. Most fish in the older age classes probably just do not grow much the last few years of their lives. Earlier growth is better and on average, brown trout exceed the 229-mm (9-in.) size limit applicable to several streams at age 2. Therefore, given the current levels of angling pressure, we don't perceive the need to alter management of streams containing large brown trout at this point. The difficulty most anglers encounter in catching larger brown trout is also beneficial in this respect. Of course, a large increase in angling pressure could require some changes.
Scales and otoliths are also now being collected from a number of rainbow trout Oncorhynchus mykiss populations and marked (known-age) fish are at large in several streams so that a similar evaluation can be completed for this species. Previous analyses of rainbow trout scales have identified fish up to age 5 (which average 308 mm in length), although most are age 3 or younger. We may find a few fish older than age 5, but we do not expect to see any that approach the longevity of brown trout.
