Beyond population size: long-term changes in free-roaming cat population structure during TNR implementation in Seoul

Free-roaming cats (Felis catus) in urban environments are commonly managed through trap-neuter-return (TNR) programs, yet the long-term population effects of these measures remain inconsistently evaluated, particularly when considering both population size and structural dynamics.

This study aimed to assess long-term changes in free-roaming cat populations and population-structure patterns observed in the context of public TNR program in Seoul, Republic of Korea, using multiple demographic and structural indicators, specifically neutering rate, kitten proportion, and apparent retention.

A longitudinal observational study was conducted across 17 fixed survey sites from 2017 to 2025. Repeated field surveys were performed biennially, with at least five visits per site. Population size, neutering rate, kitten proportion, and apparent retention were calculated via non-invasive photographic individual identification. Negative binomial and binomial mixed-effects models evaluated temporal trends and land-use differences. A logistic mixed effects model assessed apparent retention by baseline neutering status, and within-year encounter histories were used to estimate detection probability.

Overall population size and density showed a decreasing temporal pattern, while neutering rates generally increased and kitten proportions generally decreased. At the survey-year level, higher neutering rates coincided with lower kitten proportions. After adjustment for covariates and site-level clustering, cats neutered at baseline had significantly higher odds of apparent retention than non-neutered cats (OR = 3.06, 95% CI: 2.07-4.53). The pooled single-visit detection probability was 0.299, indicating that apparent retention remained subject to imperfect detection. In exploratory site-level comparisons, sites maintaining high neutering rates showed patterns consistent with lower kitten proportions and higher apparent retention, whereas sites with declining neutering rates showed patterns consistent with greater apparent turnover and higher kitten proportions.

These findings describe population-structure patterns observed during implementation of Seoul's public TNR program, including reduced kitten counts and increased neutering probability over time. Given the observational nature of the study and the absence of untreated controls, these findings cannot demonstrate the causal effects of TNR. Nevertheless, structural indicators may complement simple procedure counts to identify sites requiring further intervention in urban cat management.