## Facts

- NMFS issued a 2012 final rule under the Marine Mammal Protection Act (MMPA) authorizing the Navy’s incidental take of marine mammals from use of SURTASS Low Frequency Active (LFA) sonar during training, testing, and routine operations.
- LFA sonar emits powerful low-frequency signals that travel long distances; marine mammals depend on sound for navigation, communication, and foraging.
- The administrative record described risks from anthropogenic noise including injury, hearing loss, behavioral disruption affecting vital activities, and stress with potential population effects.
- The rule authorized operations over large ocean regions for five years, with certain exclusion zones, coastal buffers, operational limits, and seasonal/geographic restrictions.
- NMFS’s mitigation relied heavily on “biologically important areas” (BIAs), generally providing stronger spatial protections where NMFS had formally designated BIAs based on available science.
- Plaintiffs did not challenge NMFS’s “negligible impact” finding, but argued mitigation was unlawfully weak because NMFS failed to protect other scientifically supported important habitats that were not designated as BIAs and did not justify why broader restrictions were not practicable.
- The district court granted summary judgment to NMFS and the Navy; plaintiffs appealed.

## Issues

1. Whether NMFS’s 2012 mitigation measures for LFA sonar satisfied the MMPA requirement to impose measures achieving the “least practicable adverse impact” on marine mammals and their habitat.
2. Whether NMFS’s BIA-centered mitigation and its treatment of record evidence about other important habitats was arbitrary and capricious under the Administrative Procedure Act (APA).

## Decision

- The Ninth Circuit reversed the district court in relevant part and remanded.
- The court held NMFS failed to satisfy the MMPA’s “least practicable adverse impact” mitigation requirement.
- The court concluded NMFS inadequately explained why robust spatial protections were largely limited to formally designated BIAs despite record evidence of additional important habitat.
- The court held NMFS did not provide a reasoned justification that broader spatial or seasonal measures were impracticable in light of the record and the scope of authorized operations.

## Legal Principles

- The MMPA’s “least practicable adverse impact” requirement is a stringent, independent mitigation duty that applies even when NMFS finds the overall take will have a negligible impact.
- “Least practicable adverse impact” requires consideration of both (1) biological effectiveness in reducing harm to marine mammals and habitat and (2) practicability, including operational constraints, costs, and military readiness needs.
- An agency may choose among mitigation options, but it must connect its choices to the administrative record and give a reasoned explanation for rejecting measures that record evidence suggests would reduce harm.
- Under the APA, mitigation decisions are unlawful when the agency fails to address significant scientific evidence or fails to explain how it balanced expected biological benefits against claimed practicability limits.

## Conclusion

The Ninth Circuit required NMFS to reconsider LFA sonar mitigation because limiting meaningful spatial protections largely to formally designated BIAs, without adequate explanation in light of scientific evidence and practicability considerations, did not satisfy the MMPA’s “least practicable adverse impact” standard and was inadequately justified under the APA.


Nat. Res. Def. Council, Inc. v. Pritzker, 828 F.3d 1125 (9th Cir. 2016)