Dynamic changes in food resources in key habitat of Indo-Pacific humpback dolphin in Jiangmen

To investigate historical changes in the food resources of Indo-Pacific humpback dolphins (Sousa chinensis) in the Pearl River Estuary, focusing on the Guangdong Jiangmen Chinese White Dolphin Provincial Nature Reserve and adjacent waters, we analyzed the drift gillnet fishing data from two periods (2012—2014 and 2020), evaluated the trends in the fishery resources, and assessed the shifts in the spatio-temporal distribution of the dolphins' potential food resources. A total of 89 fish species were collected, belonging to 10 orders, 37 families, and 68 genera. Annual species richness ranged from 32 to 46, with no significant interannual differences (p>0.05). Fourteen cumulative dominant species were identified, among which demersal fish species predominate, accounting for 64.28% of the total. The annual mean values of the Shannon-Wiener Diversity Index (H') for individual years ranged from 1.39 to 1.87, with sites within the Guangdong Jiangmen Chinese White Dolphin Provincial Nature Reserve generally exhibiting higher H' values in multiple years. In 2020, the H' value at the harbor entrance of Taishan Power Plant was the lowest (0.27). Principal coordinates analysis (PCoA) shows that fish communities in 2012 and 2013 were clearly separated from those in other years, while the situation in 2020 was more similar to that in 2014, and the sites in 2020 exhibited higher spatial heterogeneity. The largest shift in community structure took place between 2012 and 2013. The annual average catch density fluctuated between 6.14 and 8.49 kg·km⁻¹·h⁻¹, with no significant variation (p>0.05). Based on published dietary data of humpback dolphins in the Pearl River Estuary, three criteria were established to classify potential food resources: body length≥80 mm (Criterion 1), body mass≥4.99 g (Criterion 2), and species from known prey families (Criterion 3). The density of potential food resources filtered by Criteria 1 and 2 was significantly higher than that by Criterion 3 (p<0.05), yet three criteria showed strong positive correlations with catch density. High-density zones of potential food resources shifted from the southeastern waters off Dajin Island to the Taishan Power Plant harbor entrance, which may be a key factor driving changes in the core habitat of humpback dolphins in Jiangmen. In conclusion, the Indo-Pacific humpback dolphins in Jiangmen waters may face risks of population decline, and it is urgent to optimize the fishery management to improve prey availability in their habitat.