Effects of jellyfish and black seabream releasing on marine ecosystems: A mass balance approach for the coastal area of southern Zhejiang, China

Abstract

This study evaluated the potential effect of stock enhancement on a coastal ecosystem. Based on data collected from the 2020 bottom trawl survey and literature information, an Ecopath model was constructed containing 19 functional groups representing the coastal area of southern Zhejiang, China. Jellyfish (Rhopilema esculenta) and black seabream Acanthopagrus schlegelii were selected as target-enhancing species based on their extensive juvenile release records. The maximum carrying capacity of these two species was estimated by static Ecopath and the potential ecological effects of enhancement activity for these two species were evaluated by the Ecosim routine. The trophic level (TL) of the functional groups in the coastal area of southern Zhejiang ecosystem ranged from 1 (primary producers) to 4.56 (sharks). The mean transfer efficiency of the coastal area ecosystem of southern Zhejiang was 8.71%, in which the relatively low transfer efficiency from TL2 to 3 was considered the main bottleneck that limited the biomass of higher TL species in this ecosystem. System characteristics indicated that the coastal area ecosystem of southern Zhejiang was vulnerable to disturbances and had low recovery ability. Enhancement simulation results indicated that the enhancement of jellyfish and black seabream had a partial impact on the functional groups of the ecosystem. Both jellyfish and black seabream enhancements adversely affected most groups of fish, cephalopods, and crabs, while jellyfish release stimulated the biomass of crabs. Ultimately, the present study indicates that the enhancement of black seabream decreased the total fish biomass via complex trophic interactions in the food web. Additionally, this study shows that recapture rates might not be sufficient to evaluate the net effect of stock enhancement, and therefore highlight the importance of ecosystem-based assessments for stock enhancement.