Three false killer whale populations occur in the waters of Hawaiʻi. There are two island-associated populations—one around the main Hawaiian islands and another around the Northwestern Hawaiian Islands—and a broadly ranging pelagic population. The small main Hawaiian Islands insular distinct population segment of false killer whales was listed as endangered in 2012 and faces multiple ongoing threats. Evidence suggests—based on high rates of dorsal fin and mouthline injuries—that fisheries interactions represent one of the most significant threats to this population. This threat may also act synergistically with other threats, such as exposure to pollutants and reduced genetic diversity, on the insular population. Scientists at the Pacific Islands Fisheries Science Center led the development of a new method to more accurately estimate the size of the main Hawaiian Islands insular false killer whale population by incorporating animal movement.
We analyzed photos of dorsal fins of 202 main Hawaiian Islands insular false killer whales encountered over 20 years (1999–2022) and movements from 53 satellite tags deployed from 2007–2022. We found that the population numbered 139 whales in 2022 and declined by 3.5 percent per year during the last 10 years of the study. Our findings provide the first reliable trend estimate for the population since it was listed as endangered and suggest the population will number fewer than 100 individuals within 10 years.
How do we estimate false killer whale population size?
Individual false killer whales are identified by unique nicks and notches on their dorsal fins using photo identification. By encountering these individuals over time during research surveys, we can estimate their population size using an analytical method known as mark-recapture. To collect photos of false killer whale dorsal fins from boat surveys around the main Hawaiian Islands, we partner with nonprofit organizations such as:
- Cascadia Research Collective
- Pacific Whale Foundation
- Wild Dolphin Foundation
In a previous analysis, we used this photo dataset and a conventional mark-recapture approach to estimate the size of the main Hawaiian Island insular false killer whale population from 2000–2015. However, we learned that we could not tell the full story about the size of the population using conventional methods.
Why the Usual Methods Might Not Work
False killer whales in the main Hawaiian Islands form long-term social groups that range widely between different island areas. However, we can only cover so much of the area by boat, and we can only find and photograph false killer whales on the leeward sides of the islands with relatively calm seas. This uneven survey coverage leads to bias when using the photo dataset to estimate population size, since we can only survey a portion of the population in any given year. The resulting estimate may not reflect the full population size in each year making it difficult to determine whether the population is growing or declining.
A New Method: Adding Animal Movement Data to Improve Estimates
To overcome the bias from partial sampling, we developed a new method of estimating the size of the insular false killer whale population using another data stream: satellite tag data. Cascadia Research Collective and NOAA Fisheries have been deploying satellite tags on this population since 2007. These tags are a low-impact and valuable tool for understanding the movement and habitat use of insular false killer whales around the main Hawaiian Islands.
Using the tag data, we compared where the animals move to the tracks of our research vessels. We developed a measure of how well our surveys overlapped with the movement of insular false killer whale social groups in a given year. We then included that overlap metric in a mark-recapture model. The resulting estimates of population size now account for the partial survey coverage in that year. These estimates are more accurate than those from previous approaches and allow us to determine whether the population is growing or declining. Going forward, each new satellite tag deployed and dorsal fin photographed will give us a clearer picture of where insular false killer whales roam. This information can be used in future assessments.
Supporting Science-Based Management
While this new methodology provides resource managers valuable information on abundance and trends, continued work on understanding threats to the insular main Hawaiian Islands false killer whale population and strategies to mitigate those is necessary.
NOAA Fisheries is working with other federal and state agencies, the scientific community, the public, and other groups. Our goal is to better understand what’s going on with this population and how we can collectively support its recovery.
Since 2012, NOAA Fisheries has implemented a range of management actions under the Endangered Species Act and the Marine Mammal Protection Act to better protect this population. These include:
- Designating critical habitat to protect the essential areas insular false killer whales rely on for feeding, breeding, and movement between the main Hawaiian Islands
- Publishing a comprehensive recovery plan and implementation strategy, providing a clear roadmap for recovery actions necessary to address threats and promote the long-term survival of the species
- Incorporating reasonable and prudent measures into federal fishery operations through Endangered Species Act section 7 consultations to reduce impacts of these actions
- Providing funding for the cetacean stranding program and specialized research efforts at the University of Hawai'i Manoa, as well as state-led research and threat reduction efforts via NOAA Fisheries’ Species Recovery Grants to States Program
- Establishing a False Killer Whale Take Reduction Plan that includes protective measures, such as prohibiting longline fishing in designated areas around the main Hawaiian Islands
Collectively, these efforts aim to address key threats such as fishery interactions, habitat degradation, and critical data gaps concerning the biology and recovery needs of insular false killer whales.
How NOAA recovers threatened and endangered species
Overall, the results of this study provide strong evidence supporting the endangered status of the main Hawaiian Islands insular false killer whale population. Research efforts like this are crucial to informing management efforts and recovery progress.
This feature story is based on the recent peer-reviewed journal article, "Accounting for sampling bias reveals a decline in abundance of endangered false killer whales in the main Hawaiian Islands" published in Endangered Species Research.
