April 17, 2008

From David Schmetterling, fisheries biologist for the Montana Dept. of Fish, Wildlife and Parks comes this update on fisheries monitoring on the Milltown project. In a nutshell, so far so good. Caged fish have shown no greater mortality than normal nor have the radio tagged fish. Read more after the jump.

Caged fish
We have been checking the caged fish 6-7 times/ week since the dam was breached on March 28. Since the breach, mortality of caged fish in the Milltown Section (downstream of Milltown Dam) is not any greater that the upriver controls. Since the breach, there has been very little mortality of fish at the sites in Turah, Milltown, and the Bitterroot near Missoula. There have been no mortalities in out spring creek control site near Clinton. The levels of mortality in the Blackfoot location are similar to past years and we expect they will continue for the next couple of weeks and then subside.

However, there has been much mortality in the cages near Alberton. These mortalities occurred largely over a discrete period between April 5 and April 9 where over 20% of the caged fish died. This was significantly greater than expected (based on observed mortality at our upriver sites) and was different than we have observed in pervious years. Given this situation, on April 7 we collected fish from each site and preserved them for histological evaluation by Beth MacConnell at the USFWS Bozeman Fish Heath Center to determine the cause of death.

Although we collected fish from all six locations, Beth examined fish from the Alberton site and the control in the Clinton Spring creek first. Tissue sections showed no parasites or bacteria. There was no evidence of any infectious disease or trauma. No severe lesions were seen in either group of fish.

Based on her evaluation of the central nervous system, blood, gills, tissue, and organs the condition suggests acute toxicity as the cause of mortality. Intact gill tissue, erythrocyte breakdown and degenerative changes in central nervous system are indicative of a pesticide exposure via ingestion or absorption through the skin. In discussions with her, because of the timing of mortality and relatively protracted nature of the mortality and histological changes in the fish, it appears that the fish were affected by a neurotoxin that was likely derived from the insects they were eating. A toxin that would not have killed insects, but is lethal to fish is likely the cause.

This is disturbing, because not only does it interfere with and potentially confounds results from this monitoring study, but also it shows the harmful effects of pesticides in the environment. The source of the toxic agent is unknown, and it is likely impossible to determine at this time. Ironically, this monitoring site (at the Mineral County line) has gotten a lot of attention recently because of the potential deleterious effects of the dam breach on guides and outfitters. Although mortalities have increased in the Alberton site since the breach, it is clearly not because of the breach.

Radio telemetry
Since the breach, we have been tracking the radio tagged fish 6-7 times per week to determine mortality and movements in relation to the increased turbidity and decline in water quality following the breach. Since the breach, we have had no mortalities and there has not been any movement in response to water quality changes. However, we have had several fish (including some of last years’ fish) move upstream on apparent spawning migrations. Two fish that were tagged in the Milltown Section have moved upstream into the Blackfoot River and two fish from the Milltown Section have moved upstream into the Clark Fork River and upstream through the bypass channel.

Because of continued lower than normal flows, we will not radio tag and transport fish to evaluate fish passage through the powerhouse area and bypass channel until at least the end of next week (April 23) or early the following week (week of April 28).

For related coverage, see the Missoulian’s recent article on fish passage through the restored confluence.

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