Page 819 - The Toxicology of Fishes

Mining Impacts on Fish in the Clark Fork River, Montana: A Field Ecotoxicology Case Study 799

Metals may also exert effects on fish by changing the types of prey species that are present and thus
changing the dietary sufficiency available to fish in the system. A number of metal-sensitive species are
missing from the benthic community of the Clark Fork in the uppermost river. In the dietary exposure
experiments compared above, the concentrations of essential amino acids were greater in Artemia than
in the collection of invertebrates collected from there (Farag et al., 2000). Where diet caused some of
the differences between the toxicity experiments, it may reflect deficient nutritional conditions in the river.
The indirect effect of metals on the diet of fish in a stream could be a fruitful avenue of investigation
in the future, but uncertainties about such explanations have added to the controversy regarding diet as
a source of toxicity. Dietary toxicity is a complex subject and one of some controversy in the area of
metal ecotoxicology, even though the importance of dietary exposure is unambiguous (Luoma and
Rainbow, 2005). It is difficult to discount the experiments that most closely simulate the diet of trout in
the Clark Fork, but a USEPA risk assessment (1999) concluded that the effects of dietary exposure in
the Clark Fork River were ambiguous. The Agency’s lack of experience with this exposure route was
probably a contributing factor in that decision. Despite the controversy, it is clear that assessment of
metal exposure to fish populations in natural systems must include evaluation of dietary as well as
waterborne metal contamination (Farag et al., 1999).

Pulse Inputs of Metals Can Also Affect Fish Abundance and Diversity
Marr et al. (1995a,b) performed laboratory experiments to determine the effects of pulse and chronic
exposure of metals in the water column on fish. The pulse concentrations simulated the mixture of metals
in the Clark Fork River during fish kill events (Phillips, 1995). Metal concentrations were increased over
a 1- or 2-hour period, held constant for 6 or 4 hours, then decreased over a 1- or 2-hour period. The
post-pulse mortality was monitored, and an LC for each experiment was calculated. Alkalinity, pH,
50
and conductivity were kept constant, increased, or decreased during the pulse. Marr et al. (1995a,b)
found that fry and juvenile trout were equally sensitive to the pulse exposures; fish that died during these
experiments experienced losses of potassium and calcium (≥27 and ≥33%, respectively) but not sodium.
Rainbow trout fry were more sensitive than brown trout fry when elevated metals were present with
depressed hardness and pH, conditions that mimic rainstorm events. The authors concluded that thun-
derstorm events in the Clark Fork River that supplied metals-rich acidic runoff to the river would
especially limit the survival of rainbow trout. Brown trout fry, however, were more sensitive than rainbow
trout fry to 8-hour exposures with constant hardness and pH, so simple chronic exposure alone did not
explain the dominance of brown trout in the upper river.

Acclimation Differences Explain the Reduced Diversity of the Trout Community
Acclimation of resident fish to elevated concentrations of metals may cause an increased tolerance to
metals and aid survival of some species. Marr et al. (1995a,b) tested tolerance and resistance in naïve
and metals-acclimated brown and rainbow trout. Tolerance levels were determined from 96-hour median
lethal concentration (LC ) or incipient lethal level (ILL). Resistance was measured as the median lethal
50
time (LT ) and mean time to death (TTD). Naïve hatchery rainbow trout were more tolerant to acute
50
metal exposures (greater LC ) than naïve brown trout, but brown trout that were acclimated to sublethal
50
concentrations of cadmium, copper, lead, and zinc were most resistant and survived the metal exposures
for the longest periods of time. Marr et al. (1995a,b) also documented an increase in metallothionein
(MT) in brown trout, but not rainbow trout, following acclimation. Resident brown trout from the Clark
Fork River also induced this metal-binding detoxification protein (Farag et al., 1995); thus, a better
ability to acclimate via induced detoxification of metals may also explain why brown trout survive better
than rainbow trout in the Clark Fork. Although MT induction may aid brown trout survival, it was
associated with a reduction in growth during the acclimation. Others have documented reduced growth
as a physiological cost associated with metal detoxification by metallothionein induction (Dixon and
Sprague 1981a,b; Marr et al., 1995a,b; Roch and McCarter, 1984). Such costs for survival may explain
why brown trout resident in the Clark Fork River have a smaller length at annulus compared to reference
populations (Tohtz, 1992).