Spawning probability of the endangered humpback chub, Gila cypha, in the Little Colorado River, Arizona

Authors: Kristen Pearson, William Kendall, Dana Winkelman

Year: 2013 (xlv)

Abstract

Accurate estimation of population vital rates, such as survival and breeding probability, is crucial to the appropriate management of fish and wildlife populations. When estimating vital rates for migratory species monitored on their breeding grounds, it may be necessary to evaluate temporary emigration from the study site to avoid generating biased estimates of survival and breeding probability. Evaluating temporary emigration is especially important when non-annual breeding is anticipated as skipped breeders may be unavailable for detection. Population monitoring for the endangered humpback chub, Gila cypha, within the Lower Colorado River Basin (LCRB) has focused on sampling within their primary breeding grounds, the Little Colorado River (LCR). Therefore, to generate unbiased estimates of survival and breeding probability, we evaluated temporary emigration, which we equate to skipped spawning probability, using robust design mark-recapture methodologies. We found considerable evidence for skipped spawning among both males and females. On average, the probability of a current spawner failing to breed in the subsequent year is 0.4465 (estimated var = 0.3056), and the probability of a current skipped spawner failing to breed in the subsequent year is 0.5950 (estimated var = 0.0849). In long-lived species, such as humpback chub, skipped breeding may be an adaptive trait to accumulate sufficient resources prior to spawning, thereby increasing survival and future spawning success. Understanding the spawning strategy of humpback chub in the LCRB is essential to accurately estimate demographic parameters because, as our research indicates, humpback chub are not annually present in the LCR.