Letter to the editor: On the Kokanee die-off in Koocanusa

Professional biologists offer an alternative explanation for the deaths of thousands of salmon.

We are responding to an article that appeared in the Monday, September 9, 2013 Townsman titled “Koocanusa algae kills thousands of salmon”. We are both registered professional biologists who have worked on Lake Koocanusa water quality and fish populations for government and in private consulting for a combined 60-plus years.

This kokanee die-off was also reported by several media sources from Montana which, similar to the Townsman article, attributed the cause to a bloom of blue-green algae. Blue-green algal blooms, more common in small lakes and ponds than large lakes, can release toxins that can kill fish and animals or humans drinking the water. Such algal blooms usually occur in warm summer months in temperate regions in response to high concentrations of nutrients, particularly phosphorus. In the early 1970’s, shortly after the construction of the Libby Dam on the Kootenai River in Montana, blue-green algal blooms did occur in Lake Koocanusa in response to large releases of phosphorus from Cominco’s fertilizer plant in Kimberley. These soon ceased to occur following the installation of pollution control measures by Cominco in 1975. Since 1987, phosphorus levels in Koocanusa have fallen to low levels and blue-green algal blooms have not occurred.

The speculation that a blue-green algal bloom was the cause of the juvenile kokanee die-off in late-August this year originated from fisheries biologists with the Montana Department of Fish, Wildlife and Parks. We agree with their assumption that the die-off was related to prevailing weather conditions with little wind followed by a violent storm. Juvenile kokanee die-offs have been reported over the past four decades in Koocanusa, Moyie Lake, Kootenay Lake and several in Okanagan Lake. A more plausible explanation considers thermal stratification (upper, middle, and lower layers of the water column) of the reservoir during a long period of hot and dry weather in concert with strong wind activity immediately preceding the die-off; the upper strata are typically warm, the middle strata represent a zone of declining water temperature (also known as the thermocline) and the lower strata are typically cool. The surface waters can warm to temperatures beyond those optimum for kokanee survival and excessive wind at the reservoir surface can create an internal wave within the water column that is ultimately responsible for deepwater movements from bottom to surface due to wave oscillation. The internal wave is called a seiche (saysh). The oscillations can lead to vertical and horizontal transport of water and profoundly alter the distribution of organisms found within lake or reservoir strata.

The reason for kokanee susceptibility to internal seiches is directly linked to their daily behaviour; under daylight conditions, kokanee move to lower strata to minimize their risk of predation. The specific strata occupied are typical of optimum rearing temperatures between 10 and 15 degrees Celsius. Following wave oscillation, kokanee can be immediately shifted to surface waters that may support water temperatures between 18 to 20 degrees Celsius due to surface heating. Evidence for this sudden shift from lower strata to surface is provided by over-inflated swim bladders in fish; the shift occurs rapidly and exceeds the ability of the fish to compensate for changes in pressure between bottom and surface. The over-inflated swim bladder prevents the fish from returning to more suitable depths and the instantaneous temperature differential places the fish in physiological/thermal stress; under prolonged stress, death is inevitable.

As a final note, phosphorus concentrations in Lake Koocanusa are far too low to produce a blue-green algal bloom and the article is further in error in reporting that kokanee feed on this algae. Microscopic algae dwelling in the water columns of lakes (and the ocean) are called phytoplankton. Phytoplankton are consumed by small, just visible crustacea called zooplankton and these form the bulk of the kokanee diet. Kokanee die-offs like the recent one in Koocanusa appear to be natural phenomena, best explained by prolonged climatic conditions, local storm events and water column oscillations.

Les McDonald, RP Bio, Spirogyra Scientific Consulting

Gerry Oliver, RP Bio. GG Oliver & Associates

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