Fig 1.
Conceptual models adapted from Morris [29] demonstrating the relationship between the fitness-density curve and isodar plots under density-dependent habitat selection.
(A) Illustrates relative differences in fitness between the core and periphery and (C) the resulting isodar curve when habitat quality is not significantly different between habitats (slope = 1) under ideal free distributions (IFD). Panels (B) and (D) demonstrate relative differences in fitness between core and periphery and the resulting isodar curve when habitat quality is significantly different between habitats (slope ≠1) under IFD.
Fig 2.
Map of the Chena River basin and its location in Alaska, USA (inset).
The Core (C1 and C2) in the top panel represents the concentrated spawning area identified by aerial surveys and state biologists during carcass surveys, with the periphery (P1 and P2) located northeast of the core. The known distribution of spawning Chinook salmon in the Chena watershed is depicted by the Anadromous Waters Catalog (https://www.adfg.alaska.gov/sf/SARR/AWC/). The bottom panel shows predicted spawning habitat quality (range 0–1; darker is higher quality) based on resource selection function analysis for each study reach. The Middle Fork confluence is identified by MFC.
Table 1.
Model selection results for candidate models predicting Chinook salmon peak spawner counts in the Chena River, Alaska.
Table 2.
Model validation from assessment metrics.
Fig 3.
Predicted peak spawner counts from model 2 (y-axis; Table 1) as a function of annual Chinook salmon escapement estimates (1986–2013; x-axis) for the Chena River, Alaska.
Observed (symbols) and predicted counts (lines and confidence ribbons) through time for C1 and P1 study reaches (top panel) and C2 and P2 study reaches (bottom panel) are shown. Dark and light grey ribbons represent 90% CIs for core and periphery habitats, respectively. Open symbols are observed counts used to fit the model and solid symbols are out-of-sample counts used for model validation. Predicted counts for each study reach are adjusted for the offset (study reach specific stream length).
Fig 4.
Isodars for predicted counts between core and periphery Chinook salmon spawning areas in the Chena River, Alaska based on model 2 (see Table 1 for model list) count predictions.
Individual gray lines represent the 10,000 isodars developed from bootstrapped counts from both a core and periphery habitat. The solid black line represents the mean isodar from the 10,000 simulated isodars. Isodars were constructed for the core and periphery, based on count predictions from each habitat of similar length (making the offset for each habitat the mean length of all study reaches).
Fig 5.
Frequency distribution of slope and intercept estimates from the isodar model comparing counts in the core and periphery.
Each vertical bar indicates the frequency of the coefficient estimate (slope or intercept) from the 10,000 isodar curves. Dark vertical lines indicate the critical value in which isodar coefficients (habitat quality different = slope, habitat quantity = intercept) would favor either the core or periphery. dark gray and light gray bands below vertical bars indicate the 90% and 95% confidence intervals, respectively.