When One Species’ Lifeline Endangers Another: Ninth Circuit Clarifies Injunction Standards Under the ESA

On December 3, 2025, the U.S. Court of Appeals for the Ninth Circuit issued an opinion in San Luis Obispo Coastkeeper et al. v. County of San Luis Obispo, addressing the limits of injunctive relief under the Endangered Species Act when court-ordered measures designed to protect one listed species may pose risks to others. The case arose from long-running disputes over the operation of the Lopez Dam and Reservoir in San Luis Obispo County, California, and the downstream effects of those operations on protected aquatic species.

Environmental organizations alleged that the County’s management of the dam unlawfully harmed threatened South-Central California Coast steelhead trout by altering the natural flow regime of Arroyo Grande Creek. The district court agreed and issued a mandatory preliminary injunction requiring the County to take affirmative steps to modify dam operations and pursue regulatory compliance. On appeal, the Ninth Circuit vacated that injunction, concluding that the district court had not applied the correct legal framework when multiple protected species were potentially affected.

Background

Arroyo Grande Creek supports several species listed under the Endangered Species Act, each with distinct habitat requirements. The steelhead trout is an anadromous fish that depends on seasonal high flows to trigger migration, maintain spawning habitat, and allow juveniles to reach the ocean. Federal agencies have designated the creek as critical habitat, recognizing its importance to regional steelhead recovery.

The creek and its associated lagoon are also home to the endangered tidewater goby and the threatened California red-legged frog. These species occupy different ecological niches and life stages that can be sensitive to rapid changes in flow. The County argued that flow releases intended to benefit steelhead could scour goby habitat or dislodge frog egg masses, potentially resulting in prohibited takes of those species.

This ecological overlap framed the central legal issue in the case: how courts should approach injunctive relief under the ESA when protecting one listed species may risk harm to another equally protected species.

At the U.S. District Court

The plaintiff organizations sought a mandatory preliminary injunction compelling the County to alter dam operations, implement specific flow release schedules, clear debris from culverts, study fish passage alternatives, and expedite preparation of a habitat conservation plan and incidental take permit application. They emphasized the urgency of the steelhead’s winter migration season and argued that immediate relief was necessary to prevent irreparable harm.

The district court granted much of the requested relief, finding a likelihood of success on the ESA claims and concluding that irreparable harm to steelhead was imminent. While the court summarized expert testimony regarding potential impacts on the tidewater goby and red-legged frog, it did not explicitly weigh those impacts in its equitable analysis. Instead, it directed the County to consult with federal wildlife agencies during implementation of the injunction.

The Ninth Circuit’s Decision

On appeal, the Ninth Circuit focused on the nature of mandatory preliminary injunctions under the ESA. Traditionally, courts applying the Supreme Court’s decision in Tennessee Valley Authority v. Hill have held that Congress displaced judicial balancing of equities and the public interest in ESA cases, prioritizing species protection regardless of cost. Under that framework, courts typically consider only likelihood of success and irreparable harm.

The panel held that this approach does not apply cleanly where injunctive relief for one listed species may jeopardize another. In such circumstances, the rationale of TVA v. Hill, which addressed a conflict between economic development and species preservation, does not resolve the competing interests at stake. Rigid application of that rule, the court explained, risks favoring one species while ignoring concrete risks to others that Congress also sought to protect.

Accordingly, the Ninth Circuit clarified that when a court considers a mandatory preliminary injunction under the ESA that may affect other listed species, it must weigh the competing equities and the public interest as they relate to those species. This holding does not reopen consideration of economic or developmental interests, but it does require courts to confront interspecies tradeoffs directly.

Application to the Lopez Dam Injunction

Applying this framework, the Ninth Circuit concluded that the district court’s analysis was incomplete. Although the lower court acknowledged evidence concerning the tidewater goby and red-legged frog, it did not resolve conflicting expert opinions or explain how potential harms to those species factored into the decision to grant mandatory relief. Reliance on future consultation with federal agencies was not an adequate substitute for judicial weighing of the equities at the preliminary injunction stage.

The panel also emphasized the heightened scrutiny applicable to mandatory preliminary injunctions, which compel affirmative action and alter the status quo before the merits are resolved. In complex ecological settings involving multiple protected species, that standard is particularly demanding. The court therefore vacated the injunction and remanded the case for further proceedings, instructing the district court to weigh the evidence regarding all affected species and determine whether some or all of the requested relief is warranted.

State Law Claims and Broader Context

The plaintiffs also brought claims under California Fish and Game Code section 5937, which requires dam owners to release sufficient water to maintain downstream fish in good condition. The Ninth Circuit agreed that, unlike the ESA, section 5937 does not displace the traditional four-factor test for preliminary injunctions. However, because the district court’s equitable analysis under state law mirrored its incomplete ESA analysis, the panel vacated the injunction on the state law claim as well.

Conclusions and Implications

The Ninth Circuit’s decision provides important guidance for courts and litigants confronting ESA disputes in ecosystems that support multiple protected species. While reaffirming the statute’s commitment to species protection, the court recognized that ecological reality often involves tradeoffs that require careful, explicit judicial analysis.

For courts, it emphasizes the obligation to weigh those competing risks when considering mandatory preliminary injunctions, rather than deferring difficult questions to future agency processes.

For public agencies operating dams and other infrastructure, the opinion highlights the importance of comprehensive planning tools such as habitat conservation plans and incidental take permits.

This article was originally published in the February 2026 edition of the California Water Law Law & Policy Reporter. It is posted here on our website with the permission of California Water Law & Policy Reporter, Argent Communications Group, Argent & Schuster, Inc., and the author, Darien Key. All rights remain with the original publisher.

Darien Key is a Land Use and Natural Resources Attorney based in Oakland. Darien’s practice focuses on Land Use entitlement (such as the Subdivision Map Act, Housing Accountability Act, and Builder’s Remedy), CEQA (California Environmental Quality Act), municipal law (such as the Brown Act, Public Records Act, Conflicts of Interest, etc.), water (SGMA compliance and groundwater adjudications), and related environmental laws both on the transactional side but also in litigation.