Seasonal Length–Weight Relationships of Mnemiopsis leidyi and Its Predator Beroe ovata in the Southern Black Sea

The Black Sea pelagic food web has been reorganized by biological invasion while regional warming and marine heatwaves are altering the seasonal windows for gelatinous consumers. Length–wet weight relationships (LWRs) are therefore needed to convert rapidly collected size data into biomass, but they are sensitive to body form, season and data handling. 2018 dataset from the Sinop coast for the invasive lobate ctenophore Mnemiopsis leidyi and its ctenophore predator Beroe ovata. Seasonal daytime plankton tows covered 1–45 m depth; live total length and individual displacement volume were recorded. A total 1,187 M. leidyi and 569 B. ovata positive length–weight were recorded. Mean length and wet weight were 2.981 ± 0.053 cm and 6.621 ± 0.211 g for M. leidyi, and 3.895 ± 0.071 cm and 5.893 ± 0.247 g for B. ovata. Pooled log-linear models gave W = 0.7503L¹·⁶⁷⁵¹ (95% CI for b: 1.636–1.714; R² = 0.859) and W = 0.2081L²·²⁵¹⁹ (2.181–2.322; R² = 0.874), respectively; the slopes differed (ANCOVA interaction: F₁,₁₇₅₂ = 129.64, p < 0.001). The equations provide a reproducible southern Black Sea biometric baseline, not evidence of climate causality. Future monitoring should combine with temperature, salinity, dissolved oxygen, prey, tow-water volume resolve climate-driven changes in ctenophore phenology and biomass.