New accounting method reveals a sharper decline for false killer whales living around the islands

One population of false killer whales living in the waters around the main Hawaiian Islands faces ongoing threats, with the most significant being interactions with fisheries.

False killer whales are predatory dolphins that resemble killer whales; however, the population is facing more of a decline than previously understood due to a new method of estimating population size.

With a population of less than 200 individuals and a decline continuing at least until the early 2000s, the main Hawaiian Islands’ insular false killer whale distinct population segment is listed as endangered under the Endangered Species Act and as depleted under the Marine Mammal Protection Act.

According to the National Oceanic and Atmospheric Administration’s National Marine Fisheries Service (NOAA Fisheries), false killer whales are known to depredate, taking fish and bait off of fishing lines, which can lead to hooking or entanglement. This is particularly concerning for false killer whales that interact with the Hawaiʻi longline fishery.

Scientists at the Pacific Islands Fisheries Science Center developed a new method to more accurately estimate the size of the main Hawaiian Islands’ insular false killer whale population by incorporating animal movement.

Individual false killer whales are identified by unique nicks and notches on their dorsal fins using photo identification, allowing NOAA to estimate the population size using an analytical method known as mark-recapture. Nonprofit organizations such as Cascadia Research Collective, Pacific Whale Foundation, and Wild Dolphin Foundation help collect photos while conducting their surveys.

Estimating the population size includes analyzing photos of dorsal fins and movements from 53 satellite tags, which have been deployed by Cascadia Research Collective and NOAA Fisheries. The 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.

The estimates derived from the tag data and dorsal photos are more accurate than previous approaches in determining whether the population is growing or declining.

The researchers found that the population numbered 139 whales in 2022 and declined by 3.5% per year during the last 10 years of the study. It is likely to number fewer than 100 individuals within 10 years.

While this new methodology provides resource managers with 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 threats is necessary.

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 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 Hawaiʻi Mānoa, 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.

The study can be found in a 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.