Effective number of breeders and reconstructed sibships reveal low reproductive output by a reintroduced population of endangered fish

Fishes with periodic life histories are long-lived, slow to mature, and have intermittent periods of successful recruitment, limiting the ability of managers to recover their populations rapidly. Endangered razorback sucker (Xyrauchen texanus) in the Colorado River Basin, USA, is a periodic strategist whose persistence is largely dependent on hatchery augmentation. Stocking efforts have increased population sizes in the basin; however, natural recruitment remains extremely limited. Annual reproductive output of a reintroduced population of razorback sucker was evaluated over a 10-year period in a Colorado River tributary by estimating the effective number of breeders (N-b) and numbers of repeat spawning adults through genetic analysis of wild-spawned larval cohorts. Comparative estimates were also obtained for a self-sustaining sympatric periodic strategist, flannelmouth sucker (Catostomus latipinnis). Given known trade-offs between delayed maturation and long lifespan, population size and age-structure of razorback sucker was quantified to assess potential demographic constraints to reproductive output. The results showed consistently low annual N-b estimates for razorback sucker ( x over bar h = 127), compared with flannelmouth sucker ( x over bar h= 3,022) and within the same period (2013-2018) an order of magnitude more repeat spawning individual razorback sucker (n = 41) were detected compared with flannelmouth sucker (n = 4). Given the relatively young age structure of the razorback sucker population (median = age 4), the results indicate that successful recruitment may be limited by the number of spawning adults. These results highlight the importance of understanding how life history characteristics may affect the rate and success of reintroductions and where life-stage specific bottlenecks to reproduction and recruitment may occur. Given the increasing necessity of stocking for most reintroductions, management of periodic strategists may need to aim at increasing the relative age-structure of adults to bolster reproductive output in addition to increasing juvenile survival.