Fishing Pressure on Brown Trout Populations in Northern New Mexico Streams

Part of the Young Naturalist Awards Curriculum Collection.

by Nate, Grade 9, New Mexico - 2009 YNA Winner


Waking up at 6 in the morning, I grab my gear and drive up a winding mountain road. The sun is rising over the mountains, parting the mists surrounding the ancient pine trees towering overhead. I round a corner and gaze down into the deep chasm that is a remnant of a supervolcano that erupted 1.4 million years ago. This majestic region is now the 89,000-acre Valles Caldera National Preserve, located in the heart of the Jemez Mountains in northern New Mexico.

Nathan hikes to his research area.

I meet the research team that I’m accompanying at the gate to the preserve. They are doing their own research project but have agreed to take me to a sampling site so that I can do my research alongside them. We begin to drive across the huge flat plains of grass in front of us. Herds of elk, thousands strong, surround us. A lone coyote gazes at us. As we proceed down the rough dirt road, I see men on horses, dressed in chaps and cowboy hats, driving cattle into huge trucks for the Valles’ fall roundup. We drive for another hour, through misty hills, surprising elk and coyotes as we round bends in the road, until we reach our first collection site. I walk through the damp knee-high grass to the edge of a small pristine stream no more than four feet wide. I see flashes of silver and red as I look down the length of the stream. These are the brown trout that I have come to sample. I am here. The work begins.

The purpose of my study was to determine the effect of managed fishing on the brown trout (Salmo trutta) populations in two northern New Mexico streams. I hypothesized that limited fishing would decrease the average age of the fish found in the stream, but would increase the overall health of the remaining fish as measured by a trout condition factor (K = 105*W /L3). I chose to test my hypothesis by looking at brown trout from the streams in the Valles Caldera National Preserve.

San Antonio River sampling area.

The Valles Caldera is an 89,000-acre preserve that was created by the Valles Caldera Preservation Act of 2000 and is home to many plant and animal species (Anschuetz, 2004). One of the management principles for the Valles is, “We will strive to achieve a high level of integrity in our stewardship of the lands” (Valles 2001). Knowing how angling affects fish populations is important when managing a stream system, and in particular the streams in the Valles. Fisheries biologists often use a “condition factor” to help determine whether the fish in a stream are healthy (Milewski 1994). Healthy fish contribute to a more productive ecosystem and affect all other organisms in the ecosystem.

Brown trout, although not native, have inhabited streams in the Jemez Mountains since the 1800s and now play a vital role in that environment as a key element in the food chain. Any knowledge that improves the population’s health will greatly benefit the preserve. For example, eagles depend on brown trout as a primary food source, and brown trout also limit insect populations. Brown trout were imported to the United States by fish culturist Fred Mather. Mather was appointed as a representative of the U.S Fish Commission to the Berlin Fish Cultural Exposition in Germany. Mather was so impressed by the fish in the Black Forest, he made plans to import them. The first shipment of eggs arrived in 1883 at a New York State hatchery. The shipment consisted of 20,000 brown trout eggs from streams and 60,000 eggs from lakes. This diversity allowed the trout to adapt

A medium wide shot of a small stream running through a field of brown grass. The grass growing directly next to the water is green.
East Fork of the Jemez River sampling area.

to their new home successfully. Brown trout rarely live longer than three to four years in small streams, but can live up to 12 years in large rivers and up to 15 years in large lakes. Brown trout feed primarily on invertebrates but become piscivorous (eat fish) as they grow (Behnke 2002).

Scale sampling has been used in some fish populations to age fish. Different researchers have argued about whether aging fish by looking at the otoliths, the fin rays, or the scales is the most accurate (Hubert 1987). Aging fish using the otoliths or fin rays requires killing the fish. Aging fish using scales requires that the fish be caught and a few scales removed for examination.

Materials And Methods

This is a scale from 1 1/2 year-old brown trout from the San Antonio River. Circuli are the fine dark lines (thin arrows). The groups of circuli that are spaced close together are the annuli. One annulus (circuli under red line) indicates one winter season. Circuli that are spaced farther apart (circuli under green lines) indicate summer season. Scales grow out from the center.

I sampled fish from both the East Fork of the Jemez River (24 fish) and the San Antonio River (21 fish), two beautiful, crystal-clear rivers, both in the Valles Caldera, during the fall of 2008. The San Antonio River has been fly-fished for 90 to 100 days each year since 2003. The East Fork was not fished until the summer of 2008. It was lightly fished this past summer (personal communication, Dr. Bob Parmenter).

I accompanied fisheries biologists from Aquatic Consultants Inc. They humanely stunned the trout, caught them and measured them (for a study of their own), and then placed them in a holding tank for me. I weighed and measured the fish, then removed five to 10 scales from the fish’s lateral line. Stunning and scale collection are accepted techniques that cause only momentary discomfort to the fish (Guidelines for the Use of Fish in Research 2004).

I used a microscope and a digital camera to examine the circuli on individual scales. On some samples, I used a dye to increase the contrast of the image. I photographed three scales from each fish under the microscope. I then downloaded the pictures to my computer and enlarged the photos. I aged the fish by
counting the number of annuli per scale, as described by R.P. Ericksen (1999). Examples of the circuli, the annuli, a “normal” scale, and a regenerated scale are shown at right.

To find the health of the fish, I used a formula called the condition factor, K=105*W/ L3, where W is the weight of the fish in grams and L is the length of the fish in millimeters (Barnham 1998). There are a number of condition factor formulas, and relative weight to standard weight formulas that have been developed for brown

A regenerated brown trout scale from the San Antonio River. The center of the scale (arrow) is very large, has no circuli, and looks “pebbly”, so this is a regenerated scale. This indicates that the original scale fell of (from a predator or abrasion) and a second scale re-grew in its place. This scale can’t be used to accurately age the fish.

trout and other fish species (Milewski 1994). Since most of these formulas use the standard weights of trout living in habitats that are very different from the small
stream system of the Valles, I chose to use a relatively simple and straightforward formula that has been used to describe the health of brown trout (Barnham 1998). I calculated the standard deviation (75% to 95% confidence levels) for all of my results.


Brown trout in the San Antonio ranged from 0.5 years old to 2.5 years old. The majority of fish sampled (71%) were 1.5 years old. Brown trout in the East Fork ranged from 1.5 to 3.5 years old. The majority of fish sampled (67%) were 1.5 years old.

Brown trout in the San Antonio ranged from 7.5 cm to 26 cm in length, with an average length of 18.6 cm. Brown trout in the East Fork ranged from 6 to 27 cm in length, with an average length of 20.5 cm.

Brown trout in the San Antonio ranged from 10 g to 220 g, with an average weight of 95 g. Brown trout in the

The average age of brown trout was no significantly different between the two streams. Error bars are at the 75% confidence level.

East Fork ranged from 5 g to 230 g, with an average weight of 126 g.

The average age, length, and weight of fish in both streams was not significantly different, even when the values were analyzed at the 75% confidence level.

There is less variability in the condition factor of brown trout from the East Fork than from the San Antonio. Some fish in both the East Fork and the San Antonio (peaks over 1.5) are in excellent condition. Fish with a condition factor less than 1.2 are in poor condition.

The San Antonio did have more fish with a condition factor over 2, but this again was not significant at a 75% confidence level even when an outlier value was removed The condition factors of the fish were much more consistent in the East Fork than in the San Antonio. However, there was no significant difference in the average condition factors between the East Fork (1.26) and the San Antonio (1.57).


There is no significant difference between the length of brown trout in the two streams.

My results are challenging to interpret as this was a field study and many variables were at work. However, my results show that:

Scale reading is a non-invasive technique that will work to calculate the age of brown trout in northern New Mexico.

Fishing pressure does not appear to affect the length, weight, age, or condition of brown trout in northern New Mexico streams.

The overall condition of brown trout in the Valles Caldera National Preserve is fair to excellent, as compared to brown trout in other studies.

Using scales to age brown trout is fairly accurate in the mountains of northern New Mexico. There is enough of a change in seasonal water temperature to form annuli in brown trout fish scales. For all of the samples that were collected, when I discarded the results from regenerated scales, I had 96% reliability (East Fork) and 95% reliability (San Antonio) in aging the individual scales. Any more accurate means of aging the fish (looking at the otoliths) would have resulted in the death of the fish. This agrees with a previous study done on brown trout (Rifflart 2006). Some authors have suggested that scale reading becomes more unreliable with the increasing age of the fish, but that does not appear to be a problem in the streams of northern New Mexico, since the fish do not live to a very old age.

There is no significant difference between the average mass of brown trout in the two streams.

Scientists disagree on whether fishing has a positive or negative effect on fish health in an ecosystem. Certainly, allowing humans to fish until all fish are removed from a stream would disrupt the whole ecosystem. But allowing fish to breed and overpopulate a system can also cause problems, especially for the fish themselves. In some cases when there are too many fish in a stream, the condition factor of the fish declines.

In this instance, the question was whether or not limited managed fishing hurt the health of fish populations in small streams in the Jemez. One previous study has looked at relative weights of brown trout in the Valles (DuBey 2008) and concluded that the fish had a low condition factor. These researchers did not report the value of the condition factor they found, so I can’t compare my results to theirs. Other studies have reported average condition factors ranging from 1.1 to 1.5 for brown trout and 0.88 to 2.22 for salmonid fish in general (Baxter 1998, Barnham 1998). Barnham gives K values for salmonids for extremely poor (0.8), poor (1.0), fair (1.2), good (1.4), and excellent (1.6) conditions. The condition factor of the brown trout in the Valles, in either the San Antonio (1.57) or the East Fork (1.26), seems to be in the same range as trout in other ecosystems. The averages for the two streams put the fish between fair and good condition (East Fork) and good to excellent condition (San Antonio).

Brown trout in both the San Antonio and East Fork Streams are in equally good condition.
Four plot graphs.
There is less variability in the condition factor of brown trout from the East Fork than from the San Antonio. Some fish in both the East Fork and the San Antonio (peaks over 1.5) are in excellent condition. Fish with a condition factor less than 1.2 are in poor condition.


The work presented in this study disproves my original hypothesis. However, three important conclusions can be drawn from this study.

There is enough seasonal variation in water temperature in streams in northern New Mexico to cause annuli to form on trout fish scales. This means that scale reading can be used as a non-invasive, non-lethal method to age trout in northern New Mexico.

Selective managed fishing in the Valles Caldera does not appear to significantly affect either the age or condition of the brown trout populations in the San Antonio and East Fork streams.

The overall average condition of the brown trout in the small streams of the Valles ranged from fair to excellent.


Anschuetz, K.F., and T. Merlan. More Than a Scenic Mountain Landscape: Valles Caldera National Preserve Land Use History. Gen. Tech. Rep. RMRS-GTR-196. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station (2004): 277.

Barnham, C., and A. Baxter. “Condition Factor K for Salmonid Fish.” State of Victoria, Department of Natural Resources and Environment—Fisheries Notes (March 1998): 1-4.

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DuBey, R., M. Anderson, and C. Caldwell. “Grazing Effects on Habitat, Macroinvertebrates, and Fishes in Streams on the Valles Caldera National Preserve.” Retrieved from the World Wide Web on 12 October 2008.

Ericksen, R.P. Scale Aging Manual for Coastal Cutthroat Trout from Southeast Alaska. Alaska Department of Fish and Game, 1999.

Guidelines for the Use of Fishes in Research. American Fisheries Society, 2004.

Hubert, W., G. Baxter, and M. Harrington. “Comparison of Age Determinations Based on Scales, Otoliths and Fin Rays for Cutthroat Trout from Yellowstone Lake.” Northwest Science 61 (1987): 32-36.

Milewski, C., and M. Brown. “Proposed Standard Weight (Ws) Equation and Length-Categorization Standards for Stream-Dwelling Brown Trout (Salmo trutta).” Journal of Freshwater Ecology 9 (1994): 111-116.

Parmenter, Bob. Interviewed by Nathan Clements, December 2008.

Rifflart, R., F. Marchand, E. Rivo, and J. Bagliniere. “Scale reading validation for estimating age from tagged fish recapture in a brown trout (Salmo trutta) population.” Fisheries Research 78 (2006): 380-384.

Valles Caldera Trust Management Principles. 13 December 2001. Retrieved from the World Wide Web on 5 November 2008.