Demography of an endangered, long‐lived fish: Informing management options in the face of cyclic and stochastic climate variation

June sucker (Chasmistes liorus) is a long‐lived, endangered fish endemic to Utah Lake, Utah. For several decades June sucker have failed to recruit sufficient numbers to the adult size classes such that the current wild population consists of a small number of old adults and it continues to decline. Vital rates of June sucker are influenced by climate‐driven variation in lake level and inflow from the Provo River. We used population projection matrix modeling to assess effects of cyclic and stochastic environmental variation on population growth trajectories of June sucker in Utah Lake. The stable stage distribution is dominated by stage 1 individuals (93% of the total population) in contrast to the current situation where old age classes are the most abundant. Total population size is highly influenced by the stochastic component of climate variation; whereas, the adult population of June sucker closely tracks the systematic drought cycle. If changes in survival of larvae and juveniles can be coordinated such that positive changes in both parameters can occur somewhat simultaneously, then each parameter would only have to be increased by a factor of about 8.8 to achieve sustainable population growth (compared to a 77‐fold increase for each parameter separately). Stochastic climatic variation has relatively little long‐term effect on population growth. However, the multidecadal cyclic pattern of lake level and river discharge imposes a similar pattern on population growth rates of the June sucker, such that during some periods, populations decline even when the long‐term trend is positive. For several decades June sucker have failed to recruit sufficient numbers of individuals to the adult size classes such that the current wild population consists of a small number of old adults and the population continues to decline. We used population projection matrix modeling to assess effects of cyclic and stochastic environmental variation on population growth trajectories of June sucker (Chasmistes liorus) in Utah Lake. If changes in survival of larvae and juveniles can be coordinated such that positive changes in both parameters can occur somewhat simultaneously, then each parameter would only have to be increased by a factor of about 8.8 to achieve sustainable population growth.