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.
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.
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.
although nutrient enhancement was the best predictor of growth for both
species, it had an opposite effect on Largescale suckers than it did on
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.
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
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.
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.”
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.
of Fish Population Dynamics to Mitigation Activities in a Large Regulated River”
was published online May 11, 2017, in Transactions of the American Fisheries
co-authors are Tyler Ross, fisheries research biologist, IDFG; Michael Quist,
associate professor, University of Idaho; and Ryan Hardy, principal fisheries
research biologist, IDFG.
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?
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
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.
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.
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.”
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.
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.”
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.”