Reproductive Traits Revealing the Invasion and Coexistence of Two Tilapia Species in the Jinghong Reservoir of the Lower Lancang River, Southwest China

Coptodon zillii and Oreochromis niloticus are dominant invasive fish species that have successfully established and coexisted in the Jinghong Reservoir of the lower Lancang River, Southwest China. To explore the mechanisms underlying their invasion and coexistence from the perspective of reproductive traits, we conducted monthly sampling from January to December 2025 to compare key reproductive parameters, including size at first sexual maturity, sex ratio, breeding season, spawning pattern and fecundity. The results showed that the predicted sizes at first sexual maturity for females and males of C. zillii (83.4 mm and 81.7 mm, respectively) were significantly smaller than those of O. niloticus (127.7 mm and 125.8 mm, respectively). Both species exhibited early sexual maturation. The sex ratio of C. zillii was approximately 1:1, whereas that of O. niloticus was significantly female-biased. The breeding season of C. zillii lasted from April to September, peaking in May and June, while O. niloticus spawned from May to November, with a peak in August, showing a staggered temporal distribution. Both species were batch spawners; however, O. niloticus had significantly larger egg diameters than C. zillii. Absolute fecundity, length-relative fecundity and weight-relative fecundity were significantly higher in C. zillii than in O. niloticus. Moreover, the absolute fecundity of C. zillii showed stronger correlations with body length, body weight, net weight and gonadal weight. C. zillii adopted a more r-selected strategy, while O. niloticus exhibited a more K-selected strategy. The two species displayed clear divergence in their reproductive strategies, including size at maturity, breeding season, egg diameter and fecundity, which reduced interspecific competition and promoted niche separation and coexistence in the Jinghong Reservoir.