Evidence for skipped spawning in a potamodromous cyprinid, humpback chub (Gila cypha), with implications for demographic parameter estimates

- Mark-recapture data provide evidence for skipped spawning in humpback chub (HBC).
- HBC average spawning cycle is 2.12 years, conditional on survival.
- Markovian skipping process indicates need to build up resources prior to spawning.
- Skipped spawners are unavailable for detection during annual sampling events.
- If availability is unaccounted for, survival estimates may be biased.

Our findings reveal evidence for skipped spawning in a potamodromous cyprinid, humpback chub (HBC; Gila cypha). Using closed robust design mark-recapture models, we found, on average, spawning HBC transition to the skipped spawning state (γ¯ˆ″) with a probability of 0.45 (95% CRI (i.e. credible interval): 0.10, 0.80) and skipped spawners remain in the skipped spawning state (γ¯ˆ′) with a probability of 0.60 (95% CRI: 0.26, 0.83), yielding an average spawning cycle of every 2.12 years, conditional on survival. As a result, migratory skipped spawners are unavailable for detection during annual sampling events. If availability is unaccounted for, survival and detection probability estimates will be biased. Therefore, we estimated annual adult survival probability (S), while accounting for skipped spawning, and found S remained reasonably stable throughout the study period, with an average of 0.75 ((95% CRI: 0.66, 0.82), process variance σ2 = 0.005), while skipped spawning probability was highly dynamic (σ2 = 0.306). By improving understanding of HBC spawning strategies, conservation decisions can be based on less biased estimates of survival and a more informed population model structure.