Incorporating Cannibalism into Fish Species Distribution Modeling: a case study of Harpadon nehereus in the coastal East China Sea

Understanding the ecological mechanisms that underpin distribution patterns is critical for the effective management of marine species exhibiting cannibalism. This study evaluated the extent to which prey- density proxies improves distribution models of a cannibalistic predator relative to models based solely by oceanographic covariates. Specifically, we incorporated conspecific juvenile density as a proxy for potential intraspecific prey availability, allowing us to examine whether documented cannibalism has a detectable spatial signature in the distribution of larger individuals. Using seasonal bottom trawl survey data for Harpadon nehereus (Bombay duck) collected in the coastal East China Sea from 2016 to 2021, we developed three model formulations: (1) a Predator-Environmental (PE) model including abiotic predictors (temperature, salinity, sea level) with year as a temporal effect; (2) a Predator-Prey (PP) model including biotic predictors, using proxies for conspecific juvenile and alternative prey; and (3) a combined Predator-Biophysical (PB) model incorporating both abiotic and biotic variables. Model performance improved markedly when prey variables were included, and the combined model performed best across all seasons. Conspecific juvenile was a consistently positive predictor of larger individual distribution and generally provided greater explanatory value than alternative prey index. The association between conspecific juveniles and larger individuals also persisted after accounting for measured environmental conditions, year effects, and repeated sampling among stations, indicating that their spatial correspondence was not attributable solely to shared responses to the measured environment. Abiotic effects varied seasonally. In summer, temperature showed a unimodal relationship with larger individual density, with an apparent upper response point near 26.5°C within the study region. Seasonal predictions indicated that larger individuals were concentrated mainly in shallow inshore shelf waters and shift southward to offshore waters during winter. Collectively, these results support a detectable cannibalism-related spatial association and show that models based solely on abiotic covariates may overlook ecologically important variation associated with conspecific juveniles availability. Incorporating biologically informed indicators of intraspecific prey availability, alternative prey, and environmental conditions may improve the ecological interpretation of distribution models and support seasonally informed management of cannibalistic fish populations.