Environmental DNA (eDNA), due to its non-invasive nature, high sensitivity, and operational convenience, has shown great potential for aquatic biomass assessment and can be efficiently applied to biomass monitoring in deeper offshore aquaculture cages. However, eDNA signals are easily influenced by both biological characteristics and environmental conditions. To investigate the feasibility of using eDNA for biomass assessment of Thamnaconus modestus, a key species cultured in deeper offshore aquaculture cages in northern China, this study conducted controlled experiments on biomass, individual abundance, body-size composition, and temperature. A biomass prediction model for T. modestus was constructed based on eDNA copy number per unit biomass, and cage biomass was further estimated after environmental and temperature corrections. The results showed that eDNA concentration was significantly positively correlated with fish biomass per unit volume. Increased individual abundance promoted eDNA release, and under the same biomass conditions, smaller individuals released more eDNA per unit biomass than larger individuals, resulting in higher overall eDNA concentrations in populations with a greater proportion of small individuals. In addition, eDNA degradation followed an exponential decay model, and the degradation rate increased with rising water temperature. Model validation showed that, after correction, the eDNA-based model performed well in nearshore cages, whereas its predictive accuracy was relatively lower in deeper offshore cages. These results indicate that eDNA has potential for biomass assessment in cage-cultured T. modestus, although its quantitative application still requires full consideration of population structure and environmental conditions.

Environmental DNA (eDNA) has the potential for quantitative biomass estimation, but its application in open marine cages remains challenging due to biological variability and seawater current conditions. Here, we evaluated an eDNA-based framework for estimating biomass of cage-cultured Thamnaconus modestus by integrating controlled experiments with field calibration under different current conditions. Under controlled conditions, eDNA concentration increased with total fish biomass, whereas eDNA signals per unit biomass showed patterns associated with individual body size and population size structure. Based on five field calibration observations, pond-to-cage conversion coefficients showed a positive association with model-derived instantaneous seawater current speed. This relationship was preliminarily evaluated using three temporally separated observations from a single cage site (site B), resulting in absolute percentage errors of 11.6%, 23.2%, and 26.8% and an overall mean absolute percentage error (MAPE) of 20.5%. These preliminary results suggest that combining size-matched biological references with current-dependent calibration may provide a potential approach for improving eDNA-based biomass estimation in marine cages.