Feeding down the food web?! Trophic cascading reflected in the long-term dietary trends of endangered Indian Ocean humpback dolphins Sousa plumbea from KwaZulu-Natal, South Africa

• Sousa plumbea is a good indicator species for the coastal zone of the Indian Ocean. • A multi-method approach investigated long-term dietary trends (1972–2017). • Stomach contents and δ 13 C isotope signatures showed a diet shift over time. • Significant decrease was found in the mid-dorsal body condition index over time. • Changes are likely driven by declining prey availability. Approximately 60 % of coastal cetacean species globally are at risk of extinction, despite these organisms role as indicators of Ocean Health. Changing food availability due to overfishing and/or climate change has been identified as one stressor among cumulative impacts on cetacean populations. Using a multi-method approach of analysing stomach contents (n = 107), stable isotopes (n = 30) and blubber thickness (n = 87) data (as a proxy for condition), we investigated long-term dietary trends (1972–2017) of incidentally bycaught Indian Ocean humpback dolphin Sousa plumbea off the KwaZulu-Natal coast, South Africa. A total of 64 teleost and three cephalopod species were identified. A diet shift was observed in the stomach contents between 2001 and 2020, with new prey species being recorded, which was validated by the significant difference in prey species diversity over time ( p < 0.001). In addition, significant decreases in δ 13 C signatures between time intervals ( p = 0.01), due to differences between the 2011–2020 decade and the 1971–1980 and 1981–1990 intervals, also indicated a change in prey. A significant decrease was found in the mid-dorsal body condition index (calculated using blubber thickness) between the 1981–1990 and 2001–2010 intervals ( p = 0.02). Our study provides evidence of a dietary shift in the endangered S. plumbea along South Africa’s KwaZulu-Natal coastline from 2001 to 2020, likely driven by declining prey availability resulting from overfishing and/or climate change. This highlights the species’ role as an indicator of environmental change in the coastal zone of the Indian Ocean and the importance of long-term data for assessing Ocean Health in the region.