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910 Environ. Toxicol. Chern. 20,2001 1.0 Q) II) 5 0.8 0- Il) & 0.6 '+- o >- 0.4 ~ :.a ro 0.2 .0 o .... a.. 0.0 Control 30 100 Copper Concentration (.ug/L) o 300 Fig. 1. Probability of response to skin homogenate for Colorado pikeminnow exposed to copper for 24 (circles) or 96 h (squares). The mean response for all replicates at each exposure concentration is shown. Probability 1 = response; 0 = no response. fLg/L total copper; EC50s were 43.3 (28.5, 69.0) and 56.0 (39.3, 86.6) fLg/L total copper, respectively. Comparing these estimates with total copper concentrations in the Yampa River (Fig. 2) revealed that the 24-h EC1 was equaled or exceeded by about 52% of the field samples. The 96-h EC1 was equaled or exceeded by about 14% of the samples. For dissolved cop- per, the 24- and 96-h EC1s were equaled or exceeded by about 57 and 2% of field samples, respectively. The EC50 values were approached or exceeded on one or two occasions during the 17 -year period of record. Scanning electron microscopy The SEM surveys always detected one or more ciliated receptor cells in control fish but did not detect receptor cells immediately after exposure in fish in the other copper treat- ments examined (Table 3). Observations confirmed that Col- orado pikeminnow regenerated olfactory receptor cells after 96-h exposure to 60.0 fLg/L copper. Ciliated receptor cells were detected in the olfactory epithelium of all three fish from this experimental treatment after the 14-d recovery period. DISCUSSION Mechanism for variable sensitivity We exposed Colorado pikeminnow to a gradient of copper concentrations and evaluated the influence on olfaction by Table 2. Results of behavioral assays after exposure to copper for 24 or 96 h or after a 14-d recovery period; values are number of positive behavioral responses/replicates assayed 24-h exposure 96-h exposure 14-d recoverya Concen- Concen- Concen- Positive tration Positive tration Positive tration re- (fLg/L) responses (fLg/L) responses (fLg/L ) sponses <lOb 10/10 <lOb 10/10 <lOb 9/9 16.6 5/9 15.0 10/10 15.0 8/9 33.3 6/10 30.0 8/9 30.0 9/9 66.5 4/10 60.0 5/10 60.0 8/9 133 2/10 120 0/6 NDc ND 266 0/10 a Fish were held in control water for 14 d following 96-h exposure to copper concentrations. b Control. c ND = not determined because few fish recovered from copper exposure. D.W. Beyers and M.S. Farmer 80 70 60 50 40 30 >- 20 (,) c Q) 10 :J 0" 0 ~ u.. +-' 80 c Q) (,) 70 .... Q) a.. 60 50 40 30 20 10 Total Copper . Dissolved Copper 10 15 20 25 30 35 40 45 50 55 60 Concentration (f.lg/L) Fig. 2. Frequency of observation of copper concentrations in the Yam- pa River near Maybell, CO, USA, for the period of record from September 1974 to September 1991 (n = 29 for total copper, 66 for dissolved copper). Histograms represent number of observations less than or equal to the label value in the interval between each category. BD = below detection. challenging fish to detect and respond to a skin homogenate assumed to contain fright pheromone and by determining pres- ence or absence of ciliated receptor cells in exposed fish. In general, olfactory ability declined with increasing copper con- centration. However, an unexpected result was that the re- sponse to copper varied with duration of exposure. For copper concentrations less than 66 fLg/L, olfaction was more sensitive to exposure at 24 h than at 96 h. This response is inconsistent with the general principle of toxicology that, for a given con- Table 3. Results of scanning electron microscopy examinations after exposure to copper for 24 or 96 h or after a 14-d recovery period; values are number of fish with one or more ciliated receptor cells/ fish examined 24-h exposure 96-h exposure 14-d recoverya Concen- Concen- Concen- tration Receptors tration Receptors tration Receptors (fLg/L) detected (fLg/L) detected (fLg/L) detected <lOb 3/3 <lOb 3/3 <lOb 3/3 266 0/3 60.0 0/3 60.0 3/3 120 0/3 NDc ND a Fish were held in control water for 14 d following 96-h exposure to copper concentrations. b Control. c ND = not determined because few fish recovered from copper exposure.