Do China's ecological civilization advance demonstration zones inhibit fisheries' carbon emission intensity? A quasi-natural experiment using double machine learning and spatial difference-in-differences

China's National Ecological Civilization Demonstration Zone (NECDZ) policy has a significant role in ensuring national ecological security, and it is essential to investigate how the NECDZ policy affects the carbon emissions intensity of fisheries (CEIF) to advance China's commitment to reducing carbon emissions. This study evaluates the CEIF in 30 Chinese provinces from 2007 to 2021 using ecological civilization demonstration areas as a quasi-natural experiment and double machine learning (DML)to examine the impact and internal mechanisms of NECDZ implementation on the CEIF. We also explore spatial spillover effects using a spatial difference-in-differences approach. The results reveal that NECDZ implementation has a significantly negative impact on the CEIF and this effect continues over time. NECDZ policy potentially affects the CEIF through technology development, industrial structure improvement, and reduced energy consumption. Further investigation reveals that NECDZ implementation has spillover effects and inhibits the CEIF in surrounding regions. Therefore, it is essential to focus on developing the NECDZ policy to enhance fisheries' industrial structure, encourage low-carbon innovation in fishery technologies, and increase energy consumption efficiency. This could be supported by facilitating exchanges and cooperation with other areas, considering regional disparities, and assigning common but distinct responsibility for reducing the CEIF.