Author(s): Patrick Holzapfel; Daniel Wildt; Lisa Schmalfuss; Gregory Pasternack; Christoph Hauer

Linked Author(s): Gregory Pasternack, Christoph Hauer

Keywords: Habitat connectivity; Larval dispersal; Path energy cost; Grayling; River hydraulics

Abstract: Fluvial ecosystems form dynamic mosaics of interconnected habitat patches . The spatial arrangement and temporal variability of these patches influence fish life cycles. For salmonid larvae, access to post -emergence rearing habitats is constrained not primarily by distance but by the energetic cost of moving through flowing water. We present Pathways, a Python algorithm quantifies microhabitat connectivity by identifying energetically feasible movement routes across two-dimensional, depth -averaged hydrodynamic fields and summarizing accessibility through an energy-based connectivity metric. Habitat patches are delineated using Habitat Suitability Modeling to identify source and target functional units. The algorithm computes multiple candidate paths across the flow field and evaluates their feasibility using species- and size-specific fatigue relationships across different implemented swimming modes, retaining only those routes consistent with sustained swimming capacity. For each feasible route, Pathways calculates the time-integrated swimming power required to traverse local velocity fields, treating this value as the energetic cost of movement. Connectivity from a source to one or multiple targets is summarized using the Habitat Connectivity Index (HCI), which weighting target Weighted Usable Area (WUA) by the median energetic cost across feasible routes and scales this value by a Probability of Discoverability term representing the likelihood that a fish can successfully locate and reach the target under its movement an d energetic constraints. Pathways outputs feasible pathways, path-specific energetic costs, and patch-level connectivity metrics, and operates directly on depth-averaged hydrodynamic grids. We applied the framework to grayling larvae in a riffle–pool reach of the Gail River (Austria) to address three questions: (i) How does discharge during emergence influence access to rearing habitats? (ii) How much do individual spawning locations differ in connectivity, and how does discharge modulate these differences? Results demonstrate strong discharge sensitivity and distinct variation in modeled connectivity among spawning sites, with mean HCI values declining from 69.8 m² mJ⁻¹ at low flow to 3.4 m² mJ⁻¹ at mean high flow. These findings indicate that increasing discharge substantially reduces accessible habitat by elevating movement costs and constraining the spatial extent of reachable habitat.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.342

Year: 2026