A LONG-TERM STUDY OF HYBRIDIZATION BETWEEN THE PEAMOUTH, MYLOCHEILUS-CAURINUS, AND THE REDSIDE SHINER, RICHARDSONIUS-BALTEATUS, AT STAVE LAKE, BRITISH-COLUMBIA

Hybridizing populations of the peamouth, Mylocheilus caurinus, and the redside shiner, Richardsonius balteatus, from Stave Lake, British Columbia, were examined three times over a 26-year interval to determine (i) whether swamping or introgression was occurring, (ii) whether the hybrids were being continually produced, and (iii) whether the composition of the hybrid population was constant. The hybrids were present at all three sampling periods with a rate of hybridization of 5.7% in 1966-1967, 14.1% in 1982, and 6.7% in 1991. F1 hybrids comprised no less than 80% of the hybrids at each sampling period. The remaining hybrids were mostly hybrid backcrosses to M. caurinus. Analysis of mitochondrial DNA (mtDNA) revealed that F1 hybrids were produced by females of both species, although not in equal proportions each year. Hybrid backcrosses to M. caurinus were more abundant as fingerlings than as adults, suggesting that selection is operating against them. After a 26-year interval, no evidence of swamping was detected when diagnostic morphological characters were employed. An analysis of allozyme data suggests that introgression is not occurring. The continued production of the hybrids over the three sampling periods suggests continuity in the environmental factors responsible for hybridization. Likewise, the greater proportion of F1 hybrids than backcrosses over the three sampling periods suggests a continued selective disadvantage of the backcrosses.