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Study Looks At How Artificial Nutrient Additions Influence Kootenai River’s Native Fish Growth
Posted on Friday, May 19, 2017 (PST)

Artificially adding nutrients to the Kootenai River in Idaho, downstream of Libby Dam, influences growth of two native species more than does controlling water temperature and discharge from the dam, according to a recently released study.


The study published this month looked at Mountain whitefish and Largescale suckers in the river to determine the best predictors of growth and recruitment. It found that the artificial addition of phosphorous to the river – a mitigation action that began in 2006 – was a better predictor of growth than providing cooler temperatures or more natural flows.


In fact, flow and temperature had little correlation to growth for either species, according to the study. Still, when it came to the number of fish added to the population every year, known as recruitment, neither flow, temperature nor the addition of nutrients influenced recruitment for Largescale suckers, but they all had a positive impact on Mountain Whitefish recruitment.


However, although nutrient enhancement was the best predictor of growth for both species, it had an opposite effect on Largescale suckers than it did on Mountain whitefish.


“Artificial nutrient enhancement was the best predictor of somatic growth of both species; however, the relationship between nutrient enhancement and growth was opposite between the two species (i.e. positive for largescale sucker and negative for mountain whitefish). This was a very interesting and unexpected result,” said researcher Carson Watkins, regional fisheries biologist with the Idaho Department of Fish and Game.


That could be due to how phosphorous additions impacted what each of the species feeds on. The nutrient additions increased the density and total biomass of periphyton, a food consumed by Largescale suckers, but not favored by Mountain whitefish, which feed primarily on chironomid and trichopteran larvae. Even though chironomid larvae density increased by about 50 percent after nutrients were added to the river, there appeared to be no growth response by the whitefish.


Another or additional explanation for this lack of positive response could be density dependence, the study says. Since nutrients have been added, the number of Mountain whitefish has nearly doubled and their numbers may be nearing capacity for the stream, while their size (weight and length) has diminished.


“Largescale suckers, on the other hand, did not show this pattern, but we believe that they will at some point,” Watkins said. “Suckers have positively responded to nutrient enhancement in recent history, but we are unsure about whether that trend will continue.”


“Suckers are much longer lived and later maturing compared to Mountain Whitefish, so we think that the effects of mitigation on density (and subsequently growth) have not manifested in the population yet,” he added.


“Response of Fish Population Dynamics to Mitigation Activities in a Large Regulated River” was published online May 11, 2017, in Transactions of the American Fisheries Society (


Watkins’ co-authors are Tyler Ross, fisheries research biologist, IDFG; Michael Quist, associate professor, University of Idaho; and Ryan Hardy, principal fisheries research biologist, IDFG.


The researchers ultimately wanted to answer two important questions: 1) what is the appropriate level of inference when a biologists needs to assess the influence of a management action and 2) what are the trade-offs to other species when management actions focus on just one or two species?


The impoundment backed up behind Libby Dam, Lake Koocanusa, retains some 63 percent of the total phosphorous from upriver, as well as about 25 percent of the total nitrogen, and that limits productivity of invertebrates downstream, the study says. Phosphorous was introduced to downstream reaches in 2006 to mitigate for this loss.


The two fish studied together account for 50 to 65 percent of fish caught during fish assemblage sampling. Overall, 595 Largescale suckers were sampled in 2012, and 1,627 Mountain Whitefish were sampled during September in three years – 2007, 2009 and 2012.


The two fish are important to the Kootenai River’s fish assemblage because both are prey for other fish species, such as bull trout, redband trout and Kootenai River white sturgeon, the study says.


“Given this understanding, it is likely that Largescale Sucker recruitment supports bioenergetics development in many recreationally and ecologically important fish populations at higher trophic levels,” the study says, which goes on to encourage researchers to study all fish at both the population and the fish assemblage levels “to holistically evaluate management actions.”


The most useful idea from the study, according to Watkins, is that this approach can be used to retrospectively evaluate how any management action can influence the population parameters of any fish species.


“Most studies of this nature will, ultimately, seek to evaluate how an entire fish community is influenced,” he said. “We argue that future projects (especially those evaluating environmental mitigation on fishes) may benefit from a change in scope, a change that will give researchers more power to predict what sort of community-level shifts may result from management actions and a more holistic view of how management is influencing the resource.”


There are trade-offs to management actions that can have important implications for the ecosystem as a whole as well for the studied species. For example, Watkins continued, “largescale sucker and mountain whitefish support bioenergetic development among fish species that are much higher on the food chain. So, it’s important to understand how a non-target change in a population of forage fish species might manifest in changes to the focal species. This will help resource managers optimize management actions.”

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