A treasure from the past: Former sperm whale distribution in Indonesian waters unveiled using distribution models and historical whaling data

Aim This study is the first in Indonesia to assess historical sperm whale (Physeter macrocephalus) seasonal distributions by combining historical whaling data with environmental factors associated with sperm whale habitat preferences. As current records of whale occurrence covering the whole of Indonesian waters are incomplete, we used historical whaling data summarized by Charles Haskins Townsend in 1935 to model its potential distribution for each season. Location Indonesian waters (92-143E, 9N-14S). Taxa Sperm whale (P. macrocephalus). Methods We used a presence-only habitat model - Maximum Entropy (Maxent) -, and a presence-pseudo-absence method - generalized additive model (GAM) - with nine submerged topographic variables to predict historical seasonal distributions. Results Both Maxent and GAM predict similar potential distribution which align closely with the whaling data. The results indicate that in four areas in the eastern part of Indonesia, no seasonal differences occurred in sperm whale distribution, while noticeable seasonal differences were indicated in other areas. The key parameters that characterize sperm whale habitat in both models were distance to coast, distance to -1,000 and -5,000 m isobaths, and submarine key features such as trough and trench. Main conclusions The historical catch data of this species can be used to describe the historical species distribution and provide a baseline to assess present distribution, prioritize current research and monitoring and recommend future data collection. Our models also predict distributions that are significantly larger than the ones occupied by sperm whales nowadays, another example of a shifting baseline. Our study demonstrates the benefits of incorporating historical whaling data into habitat models for ecological investigation and to inform conservation efforts for cetaceans.