Disentangle the Ecological Impact of Vessel Activities on Delphinids Ranging Strategy: An Interaction Probability...

The ever-increasing maritime traffic has been imposing tremendous stress on the aquatic ecosystems. Marine conservation often conflicts with stakeholder interests, making it difficult to secure policy backing for the implementation and enforcement of conservation measures. Therefore, it is important to compartmentalize the vessel activities according to their types of impact and prioritize the significance of potential threatening factors, to maximize the conservation benefit with the minimal conservation resources. A resident delphinid population from northern Beibu Gulf, China, presented a shifted ranging pattern and demographic decline recently. We modeled the occurrence probabilities of dolphins using a maximum-entropy approach based on the field sightings reported before and after the population reduction. Then, the interaction probabilities (IPs) between dolphins and different vessel activities were computed by integrating the space-time occupancy rates of vessel activities and dolphins' occurrence probability. Our results showed that dolphins' IP with dolphin-watching vessels decreased across all temporal and spatial scales (and with the largest effect size), identifying the dolphin-watching activity as the most critical threat driving the spatial niche shift of dolphins. While the IPs with traveling, stationary, and fishing vessels, which either increased or fluctuated across the study area after the shift, cannot account for the observed change in dolphins' ranging pattern. Animals may mitigate these threats through trade-offs, prioritizing avoidance of primary disturbances. Therefore, the change in interaction probability may mirror the decision-making process of threatened species, which can be used as a unique tool to assess the relative importance of threats in a multi-use area. Our study provides a novel framework to prioritize conservation interventions in human-dominated seascapes.