Page 820 - The Toxicology of Fishes

800 The Toxicology of Fishes

Fish Avoid Natural Waters That Are Metal Contaminated, Affecting Availability of Habitat
Behavioral avoidance of unfavorable concentrations of metals such as copper and zinc may be an
additional cause of reduced fish populations in natural systems. Behavioral avoidance is particularly
important because it can occur at concentrations lower than those causing effects on survival and growth.
It may limit fish populations by displacing them from preferred habitats. Copper in the Clark Fork River
is at and above the concentrations that cause avoidance by trout. In experiments, brown trout and rainbow
trout avoided conditions simulating the presence of copper and other metals in the river (Hansen et al.,
1999; Woodward et al., 1995). Rainbow trout were more sensitive than brown trout. Even after 45 days
of acclimation to a metals mixture that simulated the Clark Fork River, rainbow trout preferred clean
water over water with elevated concentrations of metals. This finding may also favor the presence of
brown trout over rainbow trout in the upper river.
Although detection and avoidance of undesirable substances have been documented in the laboratory,
wild populations of fish are influenced by numerous other factors. Goldstein et al. (1999) used radiote-
lemetry and a migrating population of Chinook salmon to determine the response of a wild population
of fish to the presence of metals. Salmon were transferred from their natal stream to a point 2 miles
below the confluence of the North Fork and the South Fork of the Coeur d’Alene River. The South Fork
has elevated concentrations of metals (mainly cadmium, lead, and zinc with some copper) from historical
mining activities, and the North Fork is relatively pristine. The fish were tagged with radiotransmitters
and allowed to move upstream. Although other influential variables were operative—flow, temperature,
general water quality, and cover—the higher metals concentration in the South Fork tributary appeared
to be the factor of greatest difference. The majority (70%) of the fish tagged selected the North Fork.
The results of this experiment, coupled with a companion laboratory study showing that fish avoid the
metal mixture in the Coeur d’Alene River, support avoidance as a factor in reduced fish abundances.

Conclusions

In this chapter, we show that large-scale dispersion of sediment and metal contamination is a consequence
of base-metal mining operations that do not carefully manage their wastes. The ultimate effects of such
mining activities on fish populations are determined by the types of wastes, how those wastes are
contained, and the hydrologic and ecological characteristics of the affected river system. In the Clark
Fork, circumneutral contaminated sediments are dispersed for hundreds of kilometers, contributing
contamination to the water column and the food web.
It is a substantial challenge to document metal dose in nature and to show unambiguously that metal
dose is related to biological effects. Simple, linear dose–response curves are not common in field studies
of fish effects because of the multiple metal exposure routes, complicated spatial exposure regimes on
scales smaller than hundreds of kilometers, temporally variable exposures, and confounding stressors.
Traditional measures of metal effects have yielded ambiguous results in the Clark Fork River; for
example, toxicity tests that rely on surrogate species and water exposures do not suggest toxicity in the
river. Pulses of metal input seem quite feasible but are difficult to document in conjunction with toxicity
(other than in historic fish kills). Nevertheless, the body of evidence, integrated with observations from
the river itself, consistently shows that mine wastes are a primary influence on fish in Clark Fork.
An intuitively obvious agreement exists between fish distributions and metal contamination in all
compartments of the environment. Lower fish diversity and fewer numbers of fish, compared to carefully
documented reference areas, accompany the occurrence of mining-related metal and sediment contam-
ination. Other characteristics of the fish community are also consistent with metal exposures and what
is known about metal effects. Above Rock Creek, brown trout dominate the community. Laboratory
studies show that brown trout acclimate more effectively to metal contamination and are better able to
detoxify metals via metallothionein induction than are rainbow trout. They also are less sensitive in their
avoidance response.
The dietary pathway of metal exposure (e.g., via contaminated invertebrates) appears to be especially
important for Clark Fork fish. Reduced contamination of sediments and invertebrate prey indicates that