Bonneville cisco Prosopium gemmifer are endemic to and found only in Bear Lake, Utah–Idaho. As part of an investigation into life history characteristics of this species of special conservation concern, we examined survival of Bonneville cisco eggs by measuring changes in egg density under natural environmental conditions. Eggs were vacuumed off the lake bottom over a month period in 1992 and 1993. This survey indicated that egg loss was high: fewer than 4% of the eggs remained after 30 d of an incubation period lasting more than 100 d. Thirty percent of this egg loss could be explained by unsuccessful fertilization or improper embryonic development...