Brook Trout Morphology

Morphological diagram of a fish
Morphometric features were measured on each fish so they could be compared within and among populations. There were no statistical differences in fish among streams. These data indicate brook trout are not significantly different from stream to stream and do not represent different species.

Casey Weathers photo

Brook trout (Salvelinus fontinalis) are native to the eastern United States and are the only native trout species historically found within the boundaries of Great Smoky Mountains National Park. Although brook trout once occupied nearly all cold and cool water habitat throughout the park, factors such as logging, settlement, fire, and the introduction of non-native rainbow trout have dramatically altered brook trout distribution, eliminating them from nearly 70% of their historical range within the park (Kelly et al. 1980). Most of the range loss was in the low to mid-elevation streams brook trout once occupied, relinquishing them to the smaller headwaters of most streams, above natural barriers. These barriers, along with the miles of invasive rainbow trout downstream, which they cannot outcompete, further isolate headwater brook trout from other brook trout populations.

Given these human and natural disturbances, numerous populations of brook trout in the park have become isolated from other fish communities, causing them to become uniquely adapted to a very specific niche in the aquatic habitats that they occupy.

 
A fisheries crew and volunteers electrofish in a stream
Brook trout populations at Lynn Camp Prong are annually monitored by fisheries scientists. Observations made at this stream helped to spark morphological studies.

NPS photo

There are several factors that play a role in the isolation of brook trout populations in the park. Often times a physical barrier, such as a cascade or a waterfall, will limit fish passage. Other times brook trout are restricted due to various water quality parameters (i.e. too acidic). Either way, each of these physical and chemical features can fragment populations and isolate them, sometimes to the point that they cannot mix with their own species up or downstream.

Restoration projects at Lynn Camp Prong and LeConte Creek sourced brook trout from several nearby streams in the park to strengthen brook trout populations at these sites. However, brook trout collected during monitoring for genetic studies years after their introduction showed little to no breeding occurred between trout from each different stream population, despite the fact that they were all brook trout from nearby watersheds.
 
Preserved brook trout pinned to a board
Eleven brook trout from each study stream were euthanized, pinned in place, then preserved. These fish were taken back to the lab where their morphometric features could be further analyzed.

Casey Weathers photo

The lack of spawning observed between brook trout from different nearby streams spurred fisheries biologists to ask 'Why fish from geographically isolated, but nearby streams were not spawning with each other? One theory was that these populations have been isolated for so long in these headwater areas that they developed reproductive isolating mechanisms or some sort of obstructions that reduced their ability to spawn with each other. If this was true, were the fish observing behaviors that kept them from spawning with other populations? Did these fish spawn, however their offspring did not survive? Or was it perhaps that the fish saw physical differences (i.e. numbers of spots, size of fins, eye diameter, number of parr marks, etc.) that caused them not to spawn with fish from the other stream? To determine if it were visual differences, biologists decided to take a closer look at the morphometric (form of physical features) and meristic (counts of features) differences between brook trout populations in GRSM.

For this study, fisheries scientists were interested in seeing whether or not brook trout from different watersheds in the park display physiological changes that are only unique to that specific stream or region. These variations in external appearance are otherwise known as morphology. By definition, morphology is labeled as "the branch of biology that deals with the form of living organisms and with relationships between their structures".
 
A girl running genetic tests with test tubes
Additional DNA testing was conducted as a way to compare the genetic composition between brook trout populations.

Casey Weathers photo

Scientists have concluded that brook trout populations found within the park do in fact exhibit minor morphometric differences. For example, a brook trout in a high elevation stream might have a larger caudal peduncle (the part of the fish that connects the caudal fin to the body) than a brook trout found at lower altitudes. This adaptation will provide the fish with the power and stabilization it needs to swim in the swift currents commonly found at these elevations. In other cases, brook trout in northern aspect streams may have a larger eye diameter than brook trout in southern aspect streams as the larger eye diameter helps them see in the lower sunlight of the northern aspect areas.

On a larger scale, these differences are not significant in relation to the overall identification of the fish; rather these differences are considered "phenotypic plasticity" or simply the population's ability to adapt to local environmental differences from stream to stream. In other words, a brook trout caught at Cosby Creek is still classified the same species of fish as a brook trout caught at Lynn Camp Prong.

Going forward, GRSM biologists will continue to manage brook trout populations at the sub-watershed level. This data allows for fisheries managers to cater their techniques for the specific river of stream that they are focusing on. For example, if brook trout are restored in a stream located at 3,000-3,500 feet elevation, biologists would seek source stock from the closest sub-watershed in that elevation range to transplant to the newly restored section.

The collection of morphological data provides further fisheries management insights that help biologists manage the fisheries of the national park. These data can be used to enhance brook trout restoration and monitoring as well as provide excellent angling opportunities for present and future generations.

For more information on this topic, visit the following links: https://irma.nps.gov/App/Reference/Profile/2180856

Prepared by Quinn Buckley

Last updated: August 18, 2015

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