Upper Extent of Fish

We detected coastal cutthroat trout eDNA above the electrofishing last-fish boundary in over half of the streams, extending the upstream-leading edge of fish by 50–250 m from the electrofishing boundary (Penaluna et al. 2021). The success of eDNA relative to electrofishing in determining the geographic boundary of fish makes a significant contribution to fisheries science by detecting fish in low abundances, which has direct implications for species conservation and forest management. We propose that eDNA merits inclusion among the sampling approaches considered to identify the upper extent of fish. Multiple approaches could be used for reliability and to account for methodological shortcomings. Ongoing work with LiDAR imagery suggests that we can improve our understanding of fish distributions using geophysical metrics.

Photo Credit

USDA Forest Service photo

A small waterfall forms a natural physical barrier that limits the upper extent of fish in Muletail Creek in the Nestucca River basin in the Oregon Coast Range.

In a second study, we developed a predictive model that allows land managers the ability to identify the upper extent of fish from a two-stage model that uses a logistic regression algorithm calibrated to observations of coastal cutthroat trout occurrence and variables representing hydro-topographic aspects of the landscape, such as stream size, slope, and elevation (Penaluna et al. 2022). The model, UPRLIMET, maps both the probability of fish and the upper limit of fish along stream reaches throughout a watershed and includes a stopping rule to more finely identify a discrete upper limit point above which all stream reaches are classified as fishless. It outperforms and has smaller error than all other models considered, evidence that it better captures the upper extent of fish in headwater streams. Although there is no simple explanation for the upper distribution limit of fish identified in UPRLIMET, the intersection of stream size, slope, and elevation together locate the upper limit of fish and had the highest importance. UPRLIMET predicted more fish on private lands than on land managed by the state, U.S. Department of Agriculture Forest Service, or U.S. Department of the Interior Bureau of Land Management, highlighting the importance of using transparent, spatially explicit maps across a region and working across ownerships when developing management plans for fish and forests. The availability and use of common models, data, and maps across landownerships will streamline policy and management planning and activities.

In a third study, we detected the hidden aspect of biodiversity in watersheds using multigene eDNA metabarcoding, revealing a shift in stream assemblages, detection of sparsely distributed organisms, and newly discovered cryptic lineages of sculpins (Penaluna et al. 2023). We also detected salmonids further upstream with eDNA than expected, similar to Penaluna et al. (2021). These findings unify stream concepts owing to the marked species shift at the upper extent of fish, elevating the importance of this boundary. The detection of salmonids further upstream suggests a distribution extension. The sculpin detections reveal a cryptic species complex with ramifications for fish conservation, potential pockets of endemism, and possible identification of new species. We show extensive diversity in watersheds, some of which was previously hidden from view, transforming our understanding of species presence and distributions and affecting our ability to track, protect, and manage them across watersheds.

Photo Credit

USDA Forest Service photo

Hiking upstream in Panther Creek in the Oregon Coast Range in search of last fish, the uppermost extent of fish on that stream.

To detect uppermost fish to inform forest management, land managers can use eDNA in addition to traditional tools. We suggest that as the discussion of eDNA as a management tool continues, it is important to distinguish between the science of eDNA (e.g., methodological sensitivities, limitations) and the implications derived from its information (e.g., fish presence). As managers start to incorporate eDNA surveys to detect the uppermost fish, they may want to use more than one criterion to define a positive eDNA detection as part of a decision-making framework. For example, a threshold of a positive eDNA detection could be set for a given number of replicates to separate a consistent series of strong detections from a few weak detections as well as incorporating information about potential barriers to fish movement and other habitat characteristics (e.g., wetlands, habitat complexity).

Photo Credit

Ivan Arismendi, Oregon State University

A coastal cutthroat trout (Oncorhynchus clarkii clarkii) captured from Mack Creek at the HJ Andrews Experimental Forest in the western Cascades of Oregon. The form of cutthroat found in Mack Creek is more golden in base color than many cutthroat populations.

Collaborators

• Ivan Arismendi, Jennifer Allen, Tiffany Garcia, Taal Levi, and Kevin Weitemier (Oregon State University)

• Kitty Griswold and Brett Holycross (Pacific States Marine Fisheries Commission)

• Jason Walter (Weyerhaeuser Company)

• Bureau of Land Management

• Oregon Department of Forestry

• Weyerhaeuser Company

• Hancock Forest Management

• Port Blakely