Sperm whales return to the Sulawesi Sea after a century 

When marine ecosystems change, certain organisms are among the first to respond. These organisms are called indicator species: organisms whose presence, absence, or behavior reveals the indications about the current and past conditions of their environment. 

By paying attention to the presence and absence of these species, we can better understand not just what is happening beneath the surface, but what it means for the future of ocean health. 

Indicator species: nature’s early warning system 

Because indicator species are closely tied to specific habitats or food webs, changes in their populations can signal shifts in ecosystem balance, including prey availability, water quality, and human pressures like fishing and shipping. 

In marine environments, large predators such as sharks, tuna, and whales are especially useful indicators. Their survival depends on healthy, productive food webs. When they disappear, it often points to ecosystem stress. When they return, it can signal recovery. 

A rare signal from the deep 

During OceanX’s most recent mission in northern Indonesia, specifically in the Sulawesi Sea, scientists recorded an extraordinary observation: sperm whales in offshore waters where they had not been documented for more than a century. 

Historical records from the former Celebes Whaling Ground show that sperm whales were heavily hunted in this region in the late 19th and early 20th centuries. Since then, confirmed sightings in offshore Sulawesi waters have been exceedingly rare. That’s why documenting multiple encounters, including a pod of 20 individuals observed during helicopter surveys, represents a meaningful update to the scientific record, and will be included in a paper published alongside BRIN later this year. 

These sightings do not offer a single, simple explanation. Instead, they open a set of important scientific questions. 

What their return may be telling us 

While we don't know the exact reason, the presence of these sperm whales in these offshore waters could point to several possibilities. 

• Rebounding populations 

After centuries of exploitation, some sperm whale populations are showing signs of slow recovery. Their reappearance may indicate that regional populations are stabilizing or beginning to rebound.  

• Healthier prey communities 

Sperm whales depend on deep-sea squid and fish. Their return may signal improved prey availability, suggesting recovery in deeper food webs that were once heavily impacted by overfishing. 

• Shifts in migration routes 

Changing ocean conditions can reshape long-established migration corridors. The Sulawesi Sea may once again be serving as a critical passageway for these whales.  

• A refuge in a busy ocean 

Compared to other regions, these waters may offer relative protection: less ship traffic, suitable foraging grounds, and lower acoustic disturbance—conditions increasingly rare for large cetaceans. 

Taken together, these signals suggest that the Sulawesi Sea could be playing an important role in supporting large marine life, both as habitat and as a migration corridor. 

Why this matters beyond whales 

Sperm whales are ecosystem engineers. They are feeding at deeper depths and subsequently release nutrients that float to the surface. These nutrients fuel plankton that live on the ocean surface, allowing them to grow and drive the ocean’s carbon cycle. 

When whales thrive, ecosystems benefit. And when ecosystems are healthy, they are better able to support fisheries, coastal communities, and climate stability. 

“Seeing sperm whales return to the Sulawesi Sea is a powerful sign” remarked Kailani Acosta, an OceanX Scientist. “These animals sit at the top of the food web, so their presence tells us about the ecosystem beneath them may be strong enough to support life at every level. It’s an encouraging sign of resilience in a region that was once heavily exploited.” 

Findings like the return of sperm whales to the Sulawesi Sea highlight the importance of continued exploration. Achmad Sahri and Sekar Mira, two BRIN scientists, helped lead these marine mammal surveys that generated critical scientific insights, informing conservation efforts, shaping policy decisions, and fostering a deeper sense of stewardship. 

The ocean is resilient, but only if we give it the chance to thrive. By listening closely to what indicator species are telling us, we can help ensure that recovery stories like this one become more common across the ocean, and for generations to come.