For over two centuries since the Industrial Revolution, increasing anthropogenic activities, particularly shipping, have significantly altered underwater soundscapes in marine and freshwater ecosystems, profoundly impacting aquatic fauna. Among these, cetaceans, which rely heavily on sound for navigation and feeding, are among the most affected groups. 

Existing research has demonstrated that daytime-dominated human activities disrupt diel rhythms in terrestrial mammals, driving a widespread shift toward nocturnality, which represents a critical challenge for global biodiversity conservation. However, for small cetaceans in aquatic environments, while most studied species are described as 'nocturnal,' the understanding remains limited regarding whether their behavioral rhythms are influenced by the frequent and asymmetric day-night distribution of shipping activities. 

Recently, a research group led by Prof. Zheng Jinsong from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences has revealed for the first time that shipping activities can cause a complete diel inversion of diel feeding rhythms in the Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis), a small toothed whale species. This behavioral shift further leads to increased energy expenditure for basic survival needs and elevates mortality risks. Besides, a comprehensive review indicates that the global prevalence of nocturnality among small toothed whales may be closely linked to increasing shipping activities worldwide. This study was published in National Science Review. 

In this study, researchers collected three consecutive months of monitoring data from three locations in the Yangtze River mainstream, Poyang Lake, and their confluence, spanning the period before the COVID-19 pandemic to its initial phase. The results showed that before the pandemic when shipping activities were frequent and predominantly during daytime, Yangtze finless porpoises mainly preyed at night, but shifted to daytime during the initial pandemic period when shipping was restricted. This suggests that the observed nocturnal feeding tendency of wild Yangtze finless porpoises may be due to pressure from daytime-dominant shipping activities. 

Subsequently, researchers designed controlled experiments to verify this hypothesis at an ex situ conservation area. In quiet environments without boats (control), the feeding buzz signals of Yangtze finless porpoises were significantly higher during daytime than at night, confirming they are naturally diurnal predators. However, the presence and noise of boats during daytime caused them to switch to nocturnal feeding. Moreover, the porpoises quickly reverted to daytime feeding after boat disturbances ceased. The study also found that when switching to nocturnal feeding, the porpoises significantly increased their nighttime non-feeding echolocation signals. Analysis suggests this phenomenon may occur because reduced light at night diminishes visual assistance, requiring the porpoises to compensate by more frequent acoustic detection, thereby consuming additional energy to ensure successful predation. This rhythm shift may lead to increased energy demands and higher survival risks.

The research team also conducted a review study on the diel rhythms of small toothed whales globally, finding that over two-thirds of reported species were described as nocturnal, with these study areas showing high overlap with busy shipping routes. These results indicate that the prevalent nocturnality observed in small toothed whales worldwide may be closely related to increasing shipping activities, a phenomenon that demands urgent attention. 

These findings provide new insights for global cetacean conservation, shifting focus from the "visible" behavioral impacts of human activities to the previously overlooked "invisible" diel rhythm disruptions. Furthermore, the research confirms that such rhythm disturbances can quickly reverse when shipping interference is controlled. The study recommends implementing shipping restrictions (speed and volume limits) in Yangtze finless porpoise key habitats to restore natural rhythms, and calls for attention to potential rhythm changes and risks for small toothed whales under increasing global shipping pressure.

(Editor: MA Yun)