Fish and Game studies Dworshak Reservoir productivity

by Eric Stark--Idaho Department of Fish and Game

It is widely known that reservoirs are typically very productive during their first few years after construction and filling and then become increasingly sterile with age. This phenomenon, termed oligotrophication, has been occurring for decades at Dworshak Reservoir. What this means for fish and fishing is that the reservoir's ability to grow fish to a large size and in great abundance has been diminishing over the years.

Kokanee feed almost exclusively on microscopic crustaceans called zooplankton in the open water of the reservoir, and zooplankton feed on phytoplankton, which rely on nutrients in the water. But there is a limited amount of zooplankton in any body of water, so when there are many fish there is less food available per fish. As a result, they grow slower and don't get as big. This principle is commonly referred to as density dependency. Yet, the relationship between density of fish and their resulting growth and ultimate size is determined by the amount of nutrients available in the reservoir.

This spring, Fish and Game resumed monthly limnological sampling throughout Dworshak Reservoir as we had done during 2004, with funding from the U.S. Army Corps of Engineers. This research is being conducted to assess the relationship between the kokanee population and nutrient concentrations in Dworshak Reservoir. Specifically, we want to re-evaluate how changes in reservoir productivity (nutrients and zooplankton) and dam operations influence the kokanee population (density, length-at-age, growth, and survival), especially compared to historic data.

A total of nine limnological stations are being sampled monthly from April through November. Three stations in each of three sections of Dworshak Reservoir (lower, mid, and upper-reservoir). At each station, dissolved oxygen, turbidity (water clarity), and temperature measurements are taken with a water quality meter. In addition, both zooplankton and water samples are collected. Water samples are analyzed for nitrate nitrogen; orthophosphorus and total phosphorus; and chlorophyll a. In the laboratory zooplankton are identified and counted.

Nutrient concentrations in 2004 were generally higher than we expected yet still relatively low. Our values are fairly typical of oligotrophic (nutrient poor) reservoirs in the west, although more enriched than the very sterile waters of Lake Pend Oreille for example. As expected reservoir nutrients have continued to decrease or have stabilized. We also found fewer of the larger types of zooplankton that kokanee prefer than were found historically. This year's data will help to further define the relationship between reservoir productivity and kokanee density and growth, and ultimately help us determine the benefit of enhancing reservoir nutrients.

Fish watching on the North Fork Clearwater River
Each fall, kokanee migrate up the tributaries of the North Fork Clearwater River above Dworshak Reservoir to spawn. And like many other fish and wildlife species, kokanee go through major life changes as they migrate and mate. But few transformations are as eye-catching and dramatic as what happens to kokanee, Idaho's freshwater salmon.