REPRODUCTIVE ISOLATING MECHANISMS BETWEEN THE PEAMOUTH, MYLOCHEILUS-CAURINUS, AND THE REDSIDE SHINER, RICHARDSONIUS-BALTEATUS, AT STAVE LAKE, BRITISH-COLUMBIA, CANADA

Since at least 1952, the peamouth, Mylocheilus caurinus and the redside shiner, Richardsonius balteatus, have engaged in extensive hybridization (5.7-14.1% of the combined gene pools) in Stave Lake, British Columbia. A research project initiated in 1966 and continued to 1991 demonstrated that genetic swamping of the two species has not occurred. A study was conducted to determine the strength of premating and postmating isolating mechanisms between these two species. Temporal isolation is weak because both species had spawning periods in Devils Creek that overlapped by approximately 1.5 months. Also, both species spawned at night, and only on nights preceded by a sunny day. Both species were group spawners and both spawned on the first riffle of Devils Creek. Hybrid sterility is not well developed. While male F-1 hybrids have low fertility, female F-1 hybrids are as fertile as females of either species and would allow swamping if other postmating mechanisms were not operating. Hybrid inferiority was examined by laboratory experiments in which we measured survival of fingerling hybrids. While F, hybrids displayed survival equal to that of M. caurinus, hybrid backcrosses to M. caurinus showed a 20% lower survival rate than M. caurinus. Also, hybrid backcrosses to M. caurinus included a significant proportion (8.4%) of deformed individuals.