Exploring the potential of interview-based sighting history to estimate the abundance of a coastal dolphin

Estimating the population abundance of marine wildlife is fundamental for developing evidence-based management strategies. However, standard field-based methods for estimating marine animal abundance can be time-consuming and costly, and may not effectively monitor populations over long time frames or large spatial scales. In this study, we attempted to utilize local ecological knowledge (LEK) to estimate population abundance of a marine mammal by collecting interview-based sighting data from a large-scale fisher questionnaire survey in the mainland of China for the Indo-Pacific humpback dolphin (Sousa chinensis). To validate our findings, we compared this information with data obtained from standard field surveys published in the literature. Despite the inability to accurately predict absolute abundance, all four indices (encounter rate, encounter rate in the past decade, sighting frequency, and mean annual sightings in the past five years) derived from interview sighting history had a highly significant correlation with abundance rank from field survey. In addition, the severe population declines and identified causes (water pollution, habitat destruction, overfishing, and bycatch) reported by respondents also aligned closely with independent field survey data. This is the first time that interview data have been shown to provide accurate quantitative information on a marine species’ relative abundance. We therefore propose that interview-based surveys can serve as a valuable monitoring technique to assess the population status of cetaceans and other distinctive marine megafauna, particularly in systems where field survey programs are limited in scale and scope.