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Parasites Percent occurrence of the external parasite (Lemaea cyprinacea) on HBC in September was moderate, with 100 fish (22% of total HBC captures) observed carrying the parasite, carrying 1-7 parasites per infected fish. Only one f1annelmouth sucker was captured carrying one of the parasites. During October, 324 HBC were seen with Lemaea (14% oftotal HBC captures), each carrying 1-7 parasites per fish. Population Abundance Estimation We used the following criteria to define our sampled population during the fall mark-recapture effort. During September, 135 unique HBC ~ 150 mm were marked [M]. During October, 416 unique HBC ~ 150 mm were captured [C], and 21 unique HBC ~ 150 mm were recaptured [R]. The smallest HBC recaptured had a total length of 151 mm, and the largest HBC recaptured was 342 mm TL. Figures 20 and 21, suggest some differences in the length frequencies of marked, captured and recaptured fish. Using two-tailed Kolmogorov-Smirnov tests, the length distribution of marked [M] HBC was significantly different from captured [C] HBC (n1 = 135, n2 = 416, Z = 2.060, P < 0.001). However, the length distribution of marked [M] HBC was not significantly different than recaptured [R] HBC (n1 = 135, n2 = 21, Z = 0.636, P = 0.813; Figure 21). In addition, there was no significant difference (X2 = 10.60, df = 5, P = 0.06) in the mark rates of HBC within different length strata (Table 12). However, since the population of marked fish was found to be significantly different from the population of captured fish, it was necessary to stratify our abundance estimate based on length (Seber 1982). The optimal stratification is found by choosing length boundaries in a contingency table setting of unmarked and marked fish that maximize the homogeneity in mark rate among length groups (Seber 1982, Bernard and Hansen 1992). This procedure was performed and it was found that the optimal stratification occurred at 200 mm (l = 0.15). In addition, there was no significant difference (l = 1.78, df = 2, P = 0.41) in the mark rate among the three sampling reaches (Table 13). This test suggests that the abundance estimate need not also be stratified by location (Le., Salt, Coyote and Boulders reaches). Based on the above tests, it was concluded that the abundance estimate should be stratified by length (Le., those fish from 150 mm to < 201 mm and those fish> 200 mm), but did not also need to be stratified by reach. The resulting length stratified Chapman modified Petersen abundance estimate for HBC ~ 150 in the lower 13.6 rkm of the LCR was 2,565 fish (SE = 519; Table 14). Table 15 and Figure 22 show this estimate as compared against the historical estimates obtained by Douglas and Marsh (1996) for HBC ~ 150 mm during these months. 24