July 21, 2006

A bacterial infection, likely caused by high water temperatures and stress from the turbid spring flows of the Clark Fork, led to a die-off of caged fish below the Milltown Dam, according to an analysis by the Montana Dept. of Fish, Wildlife and Parks.

In update from FWP’s David Schmetterling, the agency received results from a histological evaluation – a post-mortem analysis — of fish collected from the Clark Fork River downstream of Milltown Dam at two locations between June 26 and July 1. A US Fish and Wildlife Service analysis concluded that the fish ultimately died from a systemic bacterial infection, though the cumulative stress of turbidity (sediment-caused murkiness) and high water temperatures left the fish vulnerable to infection.

FWP resumed monitoring caged fish on July 11 at six locations in the Blackfoot, Clark Fork and Bitterroot rivers. Among the new fish, there have been 2 mortalities in the Blackfoot River, and none in the Clark Fork River at Turah or downstream of the dam.
But with temperatures heading this week into triple digits, the agencies warn that more fish mortalities are possible.

Also see today’s Missoulian coverage.

Read more from the FWP update from David Schmetterling:

[T]he bacterial infection, though the ultimate cause, is just a symptom of underlying systemic and cumulative stress that made this fish susceptible to the bacterial infection coupled with high water temperatures. As I mentioned earlier, it is not temperature alone that killed fish, since water temperatures were higher or similar at control locations, though mortality was much less. Gills of the fish were in good shape, and it dies not appear that there was physical trauma to the epithelial cells, as we have seen in fish in the Blackfoot River or downstream of the Blackfoot river in the past because of a high amount of sediment. Livers and kidneys of the fish from the Clark Fork River downstream of the dam showed deterioration consistent with poor water quality and, as in the past, showed deterioration over time with exposure.

Similar to the contention that systemic stressors are responsible to the mortality of caged fish downstream of Milltown Dam. Fish are readily feeding (there stomach were full of insects), yet they had no muscle mass or fatty deposits. This speaks to poor condition and systemic stress. As an aside, we feed fish in all cages and all location with the same amount of fish food daily.

Based on histology and physical evidence of fish in field chemical super-saturation may be at least partially responsible for fish heath and mortality. Several fish have had external symptoms of gas bubble trauma, and histological studies confirm. However, at this time, it is not known if it is nitrogen super-saturation or other. In addition, Myxobolus cerebralis (whirling disease) was found in fish downstream of the dam, but the parasites were not well established, and were not responsible for the mortality of caged fish. The whirling disease parasites were also found in fish from these same location in 2005, and there was not mortality associated with them either (the majority of mortality occurred in the Blackfoot River).

One of the more interesting results from this years’ monitoring was the large amount of mortality of the free-roaming, wild trout implanted with radio transmitters. Interestingly about 30% died about 6-8 days prior to caged fish and prior to the dramatic increases in water temperatures. It is likely that these fish died, although they were larger and older, in response to a systemic stress, and similar to the caged fish, but it happened much earlier because of the chronically depressed condition of wild fish in the river downstream of Milltown Dam. Although it may be counterintuitive, it is likely that the caged fish are not more sensitive to changes in the environment as older wild fish since the caged fish have not been exposed to conditions in the Clark Fork River. As a result, wild fish may have much lower thresholds to a perturbation (like we saw this year) and the caged fish may take longer to respond to changes. The benefit of the shorter exposure time to conditions in the Clark Fork River is also evident in consistent declines in organ condition and fish health over time.

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