A strong and fast moving storm front last Sunday night appears to have triggered a kokanee salmon die-off in Lake Koocanusa August 26 and 27. Fisheries biologist Mike Hensler estimates there were at least 10,000 dead juvenile kokanee measuring 8”-10” long scattered from Big Creek to the Canadian border.
This kill is similar to a die-off in 2005 and several others in the 1990s and earlier. The mortality appears to be triggered by a period of calm, hot weather that lets the reservoir stratify and allows algae to bloom. The storm front, accompanying winds and drop in atmospheric pressure allows the algae to mix with deeper layers. The kokanee ingest the algae, which includes some blue-green algae which is toxic to the fish, as they are feeding. They become disoriented, come to the surface, their air bladders expand and they are unable to dive back to depth so the warm surface water kills them.
The algae is not abundant enough to affect humans and has now been dispersed by continuing winds. Likewise, the die-off only lasted a few days and only affected a small portion of what would be next year’s adult salmon. There are no health concerns with eating a fish healthy enough to hit a lure.
Contact : Mike Hensler, Libby Field Satation, 406-293-4161, ext. 204
<The following is a detailed explanation by Fisheries Biologist Mike Hensler>
Kokanee fish-kill
Place: Koocanusa Reservoir, Montana. Time: August 26, 27, 2013.
A major fish kill occurred on Lake Koocanusa in 2013, involving juvenile kokanee salmon. Although the number of fish involved in the kill could not be determined due to the extensive area, the total loss of fish was in the thousands. The dead fish were observed from Big Creek up-reservoir as far as Young Creek. The fish kill involved only Juvenile kokanee salmon. We neither found nor heard that any other species of fish were affected. No other age classes of kokanee appeared to be affected.
Following is information identified through interviews, survey and analysis of the event.
Die-off (event) occurred between August 26 and August 27, 2013.
Physical Conditions at the reservoir prior to and during the event:
– For several to many days preceding the event, weather was calm and hot (temperatures in the 90’s)
– August 24, 25 a fairly violent storm occurred near the reservoir although USFS info showed only two reservoir strikes.
Other noteworthy information from anglers at the reservoir prior to and during the event:
– There were brown greasy scum (but not boat oil) on the surface of the reservoir in the form of fairly large “slicks” but that were not reservoir-wide.
– We did get a water sample and did take a plankton tow in the area on August 28 no results yet.
– Prior to event anglers were marking kokanee at 50ft to 120ft with large kokanee deeper than the juveniles.
– Kokanee (juvenile sized) were rising throughout the area where the die-off occurred (not all kokanee died)
– Only next year’s kokanee were killed (no adults and no fingerlings)
Information from sampling that occurred on August 28:
– Conditions were calm and clear.
– I estimate over 10,000 dead kokanee spread over 21 miles of reservoir between Big Creek and the Canadian Border.
– Kokanee found on the surface were all dead; this appeared to happen over at least two days (based on state of decay)
– We sampled at least 40 kokanee and got lengths/weights visuals from 20, all were juveniles ranging from 215 mm to 240 mm TL (eerily similar lengths to the adults found during the 2005 die-off) with undeveloped gonads
– No other age class of kokanee or other species of fish were found
– All kokanee had good body fat and otherwise appeared to be healthy just prior to death.
– No obvious signs of spinal damage. We x-rayed several kokanee after a similar event in 1996 and found no evidence of spinal damage, deformities or compression fractures.
– All kokanee had over inflated gas bladders. Internal organs were forced forward but no hemorrhaging.
– Gills appeared to be bright and in good condition.
– In the “scum” sampled from the surface we found a fairly large number of copepods (we don’t know if they died and floated to the surface or were already at the surface).
What Happened?
Fish kills occur from periodically across Montana and involve a variety of causes. Sometimes the causes of fish kills are obvious (i.e. winterkill from lack of oxygen), and other times the specific cause is never determined. Often the cause of a fish kill involves a variety of contributing environmental circumstances that were difficult to understand, since by the time the fish kill was detected, the environmental conditions that contributed to the kill were no longer present. Kokanee die-offs have occurred in Lake Koocanusa in the past (last in 2005).
Investigations of fish kills like the one that occurred on Lake Koocanusa around August 26, 27, often lead to a process of elimination of what did not cause the kill. There were no obvious signs of physical damage (spine, internal hemorrhaging) so it appears that no dramatic physical event like lightning (only two strikes hit the reservoir during that event) or super turnover caused the die-off. The possibility of a water-borne pollutant is always considered, and is also difficult to detect, since we usually have no way of knowing what type of chemical to which the fish may have been exposed. However, in this case, the kokanee showed no obvious sign of exposure to a water-borne pollutant. In fact, the gills of the fish were in excellent condition, which would not have been the case had a pollutant or chemical been involved. Also, when chemicals or other water borne pollutants are involved we would expect to see a variety of species of fish involved with the fish kill. Interestingly, examination of the fish did reveal that the fish had ingested something that resulted in a deterioration of the intestinal lining, typically caused by ingestion of a toxin.
So, with all the information (and unfortunately some incomplete information), we were left with this explanation: This kokanee kill showed all the same signs as the one in 2005 which we determined was likely the result of a complicated series of environmental events involving a mixing of the surface layers down to 40 or 50 feet in the lake, elevated water temperatures (> 72o F at the surface) and likely some toxin possibly released from blue-green algae (cyanobacteria). The toxin (eaten as kokanee were feeding on their normal food source: zooplankton) caused the lining of the intestine to deteriorate (kokanee got sick). It is unclear how much this condition contributed to the death of the fish, but it certainly resulted in stress that weakened the fish and left them unable to deal with the environmental changes that occurred in the lake.
The combination of the localized weather event which caused the much warmer water to be pushed deeper and kokanee being in this weakened state caused their air bladders to expand (higher temperature and lower atmospheric pressure cause gases to expand). Kokanee ascended in the water column and the rise to the much warmer surface which caused their swim bladders to over-inflate. Once the swim bladders were over-inflated, kokanee had neither the ability to purge air from their swim bladders nor the strength to swim deeper to equalize pressure in their swim bladders; hence, could not escape the high surface temperatures. We believe high surface temperatures (as high as 76o F) actually killed the kokanee. In fact, 72o F is lethal to most kokanee by 24 hours if they are not able to escape to cooler water).
Why didn’t other sizes of kokanee or other species of fish die?
Other species of fish in Lake Koocanusa (northern pikeminnow, peamouth chubs, suckers, and smaller rainbow trout) typically live near shore and avoid open water and so avoided the cascade of conditions that caused the kokanee die-off. Because the cascade of effects may have occurred due to ingested toxins and dramatic temperature changes, the larger predators (rainbow trout and bull trout) were less likely to be affected because even if they ate “sick” kokanee they would not have felt the effects of the toxin immediately because of slower digestion and metabolism. Also, they would ingest much less toxin because they were swallowing fewer bites of toxic water (one to several kokanee vs. hundreds of zooplankton). They also may have been more effective at avoiding the higher water temperatures. So, other fish might have gotten “sick” during the event but because they didn’t experience all of the effects, they weren’t subjected to the lethal surface water. It is possible that we missed some other fish (low numbers of smaller rainbow trout have on occasion been caught while trolling for kokanee) that were similar size to and mingling with kokanee, but we saw none.
In this case, we feel the reason only juvenile kokanee salmon died during this event was likely because adult kokanee tend to congregate in similar areas and depths of the reservoir and will force younger kokanee to lower quality space, in this case shallower, the adults are also moving north to spawn and no kokanee appeared to have died in the British Columbia portion of the reservoir (although some have floated that direction). The juvenile kokanee that died were all in the same area of the lake, feeding on the same food organisms, and all experienced the same temperature changes and environmental circumstances during the event.
Mike Hensler 385 Fish Hatchery Road, Libby, MT 59923, (406) 293-4161, mhensler@mt.gov
(Report by Montana FWP; Cover Photo: wikimedia.org)
