Study in the ecosystem structure and trophodynamics in the Kuroshio-Oyashio Extension area

The Northwest Pacific (NWP) is not only the region with the highest fishery production in the world, but also the location of the Kuroshio-Oyashio Extension (KOE). Regular monitoring and assessment of marine ecosystem are essential for maintaining its stability and the sustainability of fishery products. Based on the survey data from the high-seas fishery resources survey in the KOE in 2023, the trophic structure and characteristics of open ocean ecosystem in this area were analyzed using the Ecopath with Ecosim model. A total of 25 functional groups were defined based on their ecological characteristics. The field sampling information was used as the biomass data to maximize the quality of the model. The results indicated that the trophic levels of these groups ranged from 1.00 to 4.35. Chub mackerel is a major economic species in the Northwest Pacific Ocean, and the ecotrophic efficiency of chub mackerel was lower than in previous studies (0.74 in 2019), indicating that the proportion of its production contributing to the ecosystem energy might have decreased. Mixed trophic impact results showed that the main interactions between functional groups are primarily predation and feeding competition. Total primary production/total respiration (TPP/TR), and total primary production/total biomass (TPP/TB) indicated the system maturity. Compared to previous studies in the KOE and its adjacent area ecosystems (the total system throughput was 3529.54 t·km^-2·year^-1 and the TR was 486.42 t·km^-2·year^-1), both the TST (1766.22 t·km^-2·year^-1) and the TR (151.27 t·km^-2·year^-1) were relatively lower, which revealed the low productivity characteristic of the KOE ecosystem. Combined with the high TPP/TR (7.31) and the low system omnivory index (0.20), these overall ecosystem characteristics suggested that the KOE ecosystem was structurally simple and vulnerable to disturbance from external activities. Sensitivity analysis indicated that the main output parameters of the model were highly sensitive to the changes in the biomass of the functional groups. This study provided a reference for ecosystem-based fisheries management in the open ocean waters of the KOE.