## Facts

- Federal fisheries officials listed the Central Valley steelhead distinct population segment (DPS) as a threatened species under the Endangered Species Act (ESA).
- Steelhead and resident rainbow trout are two life-history forms of the same taxonomic species (*Oncorhynchus mykiss*): steelhead migrate to the ocean and return to spawn, while rainbow trout remain in freshwater.
- The two forms can interbreed, and offspring may exhibit either life-history form.
- The listing covered only the anadromous steelhead component and excluded the non-anadromous rainbow trout.
- Central Valley irrigation districts and water agencies alleged the listing imposed operational constraints on water projects to protect steelhead.
- The plaintiffs challenged the listing under the ESA and the Administrative Procedure Act (APA), arguing the agency was required to list both interbreeding forms together or list neither.
- The district court upheld the listing decision, and the plaintiffs appealed.

## Issues

1. Whether the ESA permits listing a DPS consisting only of the anadromous steelhead component of *O. mykiss* while excluding interbreeding resident rainbow trout.
2. Whether the agency’s application of the DPS policy and its scientific determinations were arbitrary, capricious, or otherwise unlawful under the APA.

## Decision

- The Ninth Circuit affirmed the judgment upholding the threatened listing for the Central Valley steelhead DPS.
- The court held the agency reasonably interpreted the ESA to allow listing a subset of a vertebrate species as a DPS even if it can interbreed with conspecifics outside the DPS.
- The court held the decision to list only the anadromous component, based on ecological and conservation distinctions, was not arbitrary or capricious.

## Legal Principles

- The ESA’s inclusion of “distinct population segment” authorizes listing of subsets of a vertebrate species; the phrase “which interbreeds when mature” does not require the listed unit to include all interbreeding forms of the taxonomic species.
- Under the joint DPS policy, a DPS may be defined by discreteness and significance, even where some interbreeding occurs with populations outside the DPS.
- Courts owe substantial deference to agency scientific judgments in ESA listings; under APA review, the question is whether the agency considered relevant factors and provided a rational explanation supported by the record.
- An agency may change or refine prior approaches if it acknowledges the change and offers a reasoned explanation grounded in policy and science.

## Conclusion

The Ninth Circuit upheld the threatened listing of the Central Valley steelhead DPS, concluding the ESA permits listing an anadromous-only DPS within a species that also includes interbreeding resident forms, and finding the agency’s science-based application of the DPS policy reasonable under the APA.


Modesto Irrigation Dist. v. Gutierrez, 619 F.3d 1024 (9th Cir. 2010)