Brown and Rainbow Trout Age Determination using Otoliths

[Pat Stout]

Brown and Rainbow Trout Age Determination using Otoliths ‐ Snake River Nebraska 2016

Dr. J. P. Stout provided me with 23 otolith pairs (sagitta) harvested from brown and rainbow trout in the
Snake River below Merritt Reservoir near Valentine, Nebraska in 2013 and 2015. Using a dissecting
microscope, age was determined by counting the number of annuli (alternating opaque and hyaline
layers, similar to tree rings) for each set of otoliths. Otolith pairs were delivered in individual zip‐lock
bags labeled with a unique ID number. A key was provided listing the specie, sex, length, reach and date
harvested for each unique ID number. All otoliths were aged without referring to the key so as not to
influence age determination. Age determination results are presented in Table 1.

General Observations and Comments
Both brown and rainbow trout in the Snake River exhibit relatively fast growth rates for the first 2
growing seasons, attaining lengths up to 10 inches by September of their second growing season (Age
1+). At Age 2+, growth rates start varying, perhaps in response to the location in the river they occupy or
their level of sexual maturity. For example, females that produce viable eggs in their third growing
season are likely to be shorter in length than females that wait until their fourth growing season to
produce viable eggs because of the metabolic cost of producing gonadal tissue. Because inland trout are
iteroparous, capable of multiple reproductive episodes during their lifespan, the age/length
relationships for the trout in the Snake River may become blurred after they reach sexual maturity as
individual trout may produce viable gametes every year, or alternate years. The oldest trout I identified
in this sample were in their sixth growing season (Age 5+) and ranged from 17 to 21.5 inches. I would
expect that some trout in the Snake River live 8‐9 years, perhaps longer. What I have observed in other
trout populations is that faster growing trout tend to have a shorter lifespan than slower growing trout.
With such a limited sample size, I don’t know that this is the case for the Snake River population and I
don’t know how predation and fishing pressure influence the population structure here. However, what
I can say based on the size distribution of the fish and the otolith appearance (boney mass and size) that
the trout in this population are very healthy.

Jim Nankervis
Blue Mountain Consultants, LLC

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[Jason Miller]

Very interesting data, yet kind of disturbing as well……this data is somewhat dated ( 5-7 yrs old ) BUT my understanding of Otolith testing requires the death of the fish. Maybe this data also helped with the new fisheries management plan? Over 43% of these fish are equal/greater than the current harvest size regulations – some are way, way beyond the 14″ and true trophy, older age class, fish. Glad the new reg’s are in place – hope to see the average size of the fishery continue to grow!! Good stuff.

[Pat Stout]

The exact age determination of fish is one of the most important elements in the study of their population dynamics. It forms the basis for calculations leading to a knowledge of the growth, mortality, recruitment and other fundamental parameters of their populations. Otoliths are a far better sample to accurately age trout than scales. Scales become quite useless after 3-4 years of age and thus, their limitation in trout that can be 8, 9 or more years in age, depending on species. In fact, this was noted in a scale sampling years ago from the Snake River, but the results were incorrectly interpreted out of context as “trout only get to 4 years old”. The number of trout sampled, and killed, was perhaps not statistically significant as killing 400 trout, but sufficient to recognize age groups and comparable to data from the early 1980s. Not dated, to be sure, but simply a biologic survey to check on the health of the trout from time to time. Interestingly, I was able to obtain 4 trout 18-22.5 inches on a single day by walking around the cabins and inquiring whether any fisherman had sizable fish that I could dissect for otoliths. The number of trout killed for the survey was inconsequential. They are a renewable resource and we are focused on protecting the fishery for future classes and trophy fish.

To that end, the otolith analysis was an essential part of implementing new regulations to fit the fishery and the anglers. Furthermore, the condition of the otoliths along with weights and lengths obtained during the survey indicate a very healthy trout population. This, in September, after 3-4 months of low water conditions and summer months. And…high angler presence. So, the trout are thriving, despite the sometime hysteria and sky is falling anecdotes.

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