Baby Catchers: A Novel Tank Design to Dramatically Improve Fry Recovery of Live-bearing Fishes

Live-bearing fishes (Poeciliidae) are increasingly used as laboratory models, but their utility is hampered by high rates of filial cannibalism, which reduces fry recovery and increases husbandry demands. Traditional strategies, such as mesh tank inserts (TIs), rely on fry actively seeking refuge and are often ineffective. Here, we present a novel tank design called Baby Catchers (BCs) that use water flow to passively and automatically separate newborn fry from their mothers, fitting seamlessly into standard recirculating rack systems. We tested the efficacy of BC tanks compared to open tanks (OT) and traditional mesh TIs using Poecilia parae, a live-bearing species that is notorious for intense filial cannibalism. In short-term trials, BC tanks yielded significantly higher initial fry counts on day 0 and day 2, while mesh inserts performed no better than OT. Over 10 days, negative binomial generalized linear models predicted substantial progressive fry loss in OT (∼60% decline) and inserts (∼68% decline), but no significant change in BC, consistent with high fry survival. These results provide the first quantitative evidence that automated water-flow separation of fry dramatically reduces filial cannibalism in live-bearing fishes. The BC design offers a scalable, low-cost, and humane method for fry recovery, reducing technician effort, minimizing animal stress, and lowering barriers for research and breeding programs involving live-bearing fishes (e.g., swordtails, mollies, Poeciliopsis, mosquitofish, halfbeaks, etc.).