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arxiv: 2602.06606 · v2 · submitted 2026-02-06 · 🧬 q-bio.PE · math.DS

Multiple timescales in collective motion: daily and intraday upstream fish migration focusing on Feller condition

Pith reviewed 2026-05-16 06:50 UTC · model grok-4.3

classification 🧬 q-bio.PE math.DS
keywords fish migrationdiffusion bridgesFeller conditionstochastic differential equationscollective motionAyutimescalesintermittency
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The pith

Daily fish migration is less randomized and intermittent than intraday migration according to the Feller condition on diffusion bridges.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper models upstream fish migration using diffusion bridges, nonlinear stochastic differential equations pinned at both initial and terminal conditions. Drift and diffusion coefficients come from time-dependent average and variance curves fitted directly to fish count data, which also guarantees unique solutions exist. Sample paths of these bridges exhibit qualitatively different behaviors depending on the Feller condition, the ratio of diffusion size to drift size. When applied to juvenile Ayu upstream migration counts in Japan, the daily data and intraday data yield distinct Feller indices, with daily paths appearing less randomized and intermittent. The work concludes that the Feller condition offers a practical way to compare and evaluate migration phenomena across multiple timescales.

Core claim

Application to juvenile upstream migration of Plecoglossus altivelis altivelis shows that daily and intraday fish count data correspond to distinctive Feller indices, showing that the former is qualitatively less randomized and intermittent. Diffusion bridges are built from nonlinear SDEs whose drift and diffusion coefficients are fixed by the fitted time-dependent parameterized average and variance curves, with unique existence of solutions rigorously guaranteed; sample paths then differ in qualitative properties according to the ratio between the sizes of diffusion and drift.

What carries the argument

Diffusion bridges from nonlinear SDEs with pinned ends, whose drift and diffusion coefficients are set by time-dependent parameterized average and variance curves fitted to fish count data; the Feller condition (ratio of diffusion size to drift size) controls qualitative path properties such as intermittency.

If this is right

  • Daily-scale migration paths are less intermittent and less likely to exhibit extreme fluctuations than intraday paths under the fitted models.
  • The Feller condition serves as an effective quantitative tool for evaluating and comparing fish migration across daily and intraday timescales.
  • Unique existence of solutions is guaranteed for the constructed diffusion bridges at both timescales.
  • Similarities and differences between daily and intraday Ayu migration can be clarified by comparing their respective Feller indices.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same diffusion-bridge construction could be tested on other migratory species to check whether distinct Feller indices appear consistently across timescales.
  • If Feller indices reliably separate short-term and long-term collective motion, the approach might help forecast how environmental changes affect migration at each scale.
  • Conservation planning for Ayu could incorporate separate models for daily versus intraday movement to improve timing of habitat interventions.
  • Extending the fitting procedure to include environmental covariates would test whether the Feller indices remain stable or shift under varying river conditions.

Load-bearing premise

The time-dependent parameterized average and variance curves fitted to fish count data correctly capture the true drift and diffusion coefficients of the underlying migration process.

What would settle it

New fish count data that produce similar Feller indices for both daily and intraday scales, or numerical simulations of the fitted bridges that fail to show the predicted difference in path intermittency and randomization.

Figures

Figures reproduced from arXiv: 2602.06606 by Hidekazu Yoshioka.

Figure 1
Figure 1. Figure 1: Functional shapes of , (1 ) m n t m n f c t t =− ( , , , 0 m n cmn  ) with mn, c chosen so that 01 max 1 t t f  = . Values of (mn, ) are Black: (0.5,0.5) , Red: (5,10) , Green: (10,5) , and Blue: (10,10) . For the models using real data, see Section 3 [PITH_FULL_IMAGE:figures/full_fig_p008_1.png] view at source ↗
Figure 8
Figure 8. Figure 8: Comparison between empirical (circles) and fitted (curves) autocorrelation functions (ACF): (a) daily case (b) intraday case. Colors represent the results for t = 0.01 (red), t = 0.36 (green), and t = 0.72 (blue) [PITH_FULL_IMAGE:figures/full_fig_p019_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Comparison of the Feller indices F between daily (red) and intraday (blue) cases on an ordinary logarithmic scale [PITH_FULL_IMAGE:figures/full_fig_p019_9.png] view at source ↗
read the original abstract

Fish migration is a collective phenomenon that has multiple timescales, ranging from daily to intraday (hourly or even finer). We propose a unified mathematical approach using diffusion bridges, nonlinear stochastic differential equations with pinned initial and terminal conditions, to model both daily and intraday fish migration phenomena. Drift and diffusion coefficients of these bridges are determined based on time-dependent parameterized average and variance curves fitted against fish count data, with which the unique existence of their solutions is rigorously guaranteed. We show that sample paths of the diffusion bridges have qualitatively distinctive properties depending on the Feller condition, namely, the ratio between the sizes of diffusion and drift. Our application study about the juvenile upstream migration of Plecoglossus altivelis altivelis (Ayu) in Japan clarifies similarities and differences between daily and intraday migration phenomena. Particularly, we discuss that the daily and intraday fish count data correspond to distinctive Feller indices, showing that the former is qualitatively less randomized and intermittent. The results obtained in this study suggest that the Feller condition potentially serves as an effective tool for evaluating fish migration phenomena of Ayu across different timescales.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes a unified framework using diffusion bridges constructed from nonlinear SDEs whose drift and diffusion coefficients are obtained from time-dependent parameterized average and variance curves fitted to fish-count data. It asserts that the Feller condition (ratio of diffusion to drift) governs qualitative properties of the sample paths, including degree of randomization and intermittency, and applies the method to daily versus intraday upstream migration counts of Ayu (Plecoglossus altivelis altivelis), concluding that the daily regime exhibits a distinctive Feller index indicating qualitatively less randomization and intermittency than the intraday regime.

Significance. If the central distinction survives robustness checks, the work supplies a mathematically grounded method for comparing collective motion across timescales via the Feller index of diffusion bridges. The explicit guarantee of unique existence once coefficients are fixed is a clear technical strength that could be leveraged in other ecological time-series settings.

major comments (2)
  1. [Application study on Ayu migration] Application study: the claim that daily and intraday data correspond to distinctive Feller indices (and therefore qualitatively different intermittency) rests on post-fit drift and diffusion coefficients without reported uncertainty quantification, error bars on the fitted curves, or sensitivity tests to alternative parameterizations of the average and variance functions. Because the Feller index is computed directly from these curves, any instability in the fitting step directly affects the load-bearing distinction.
  2. [Diffusion bridge construction] Diffusion-bridge section: the manuscript states that unique existence is guaranteed once coefficients are fixed, yet does not verify that the standard Feller-condition link to boundary behavior and path intermittency continues to hold for the time-inhomogeneous bridges with pinned endpoints used here; explicit simulation or analytic confirmation of this link is required for the qualitative interpretation.
minor comments (2)
  1. The specific functional forms chosen for the time-dependent average and variance curves should be stated explicitly, together with the optimization procedure used to fit them to the count data.
  2. [Mathematical framework] Cross-reference the precise theorem or proposition that establishes unique existence of the bridges once the coefficients are fixed.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments that highlight important aspects of robustness and verification. We address each major comment below and describe the revisions we will incorporate.

read point-by-point responses
  1. Referee: [Application study on Ayu migration] Application study: the claim that daily and intraday data correspond to distinctive Feller indices (and therefore qualitatively different intermittency) rests on post-fit drift and diffusion coefficients without reported uncertainty quantification, error bars on the fitted curves, or sensitivity tests to alternative parameterizations of the average and variance functions. Because the Feller index is computed directly from these curves, any instability in the fitting step directly affects the load-bearing distinction.

    Authors: We agree that uncertainty quantification is essential to support the distinction in Feller indices. In the revised manuscript we will add bootstrap-based standard errors and 95% confidence bands on the fitted average and variance curves for both the daily and intraday regimes. We will also perform sensitivity analyses by refitting with alternative parameterizations (higher-degree polynomials and cubic splines with varied knot placements) and report the resulting range of Feller indices, confirming that the qualitative ordering (daily less intermittent than intraday) is preserved across these choices. revision: yes

  2. Referee: [Diffusion bridge construction] Diffusion-bridge section: the manuscript states that unique existence is guaranteed once coefficients are fixed, yet does not verify that the standard Feller-condition link to boundary behavior and path intermittency continues to hold for the time-inhomogeneous bridges with pinned endpoints used here; explicit simulation or analytic confirmation of this link is required for the qualitative interpretation.

    Authors: We acknowledge that the link between the Feller index and path intermittency, while standard for time-homogeneous diffusions, requires explicit confirmation in the time-inhomogeneous pinned-bridge setting. In the revision we will add a dedicated subsection containing Monte Carlo simulations of sample paths generated from the fitted time-inhomogeneous bridges. These simulations will demonstrate that the daily regime (higher Feller index) produces visibly smoother, less intermittent trajectories than the intraday regime, thereby validating the qualitative interpretation for our specific bridges. revision: yes

Circularity Check

1 steps flagged

Feller indices and intermittency claims reduce directly to fitted mean/variance curves on the same data

specific steps
  1. fitted input called prediction [Abstract]
    "Drift and diffusion coefficients of these bridges are determined based on time-dependent parameterized average and variance curves fitted against fish count data, with which the unique existence of their solutions is rigorously guaranteed. We show that sample paths of the diffusion bridges have qualitatively distinctive properties depending on the Feller condition, namely, the ratio between the sizes of diffusion and drift. ... Particularly, we discuss that the daily and intraday fish count data correspond to distinctive Feller indices, showing that the former is qualitatively less randomized."

    Feller index is the ratio of diffusion to drift coefficients; both coefficients are set directly from the fitted average and variance curves to the fish count data. The reported distinction between daily and intraday regimes (and the associated claim of different randomization/intermittency) is therefore the immediate numerical outcome of the chosen parameterization and fit, not a separate prediction.

full rationale

The derivation fits time-dependent parameterized average and variance curves to fish count data, defines drift and diffusion coefficients from those curves, computes the Feller ratio (diffusion/drift) from the same coefficients, and then states that the data exhibit distinctive indices and qualitative intermittency properties. This chain makes the central distinction a direct output of the fitting parameterization rather than an independent prediction or derivation from first principles. The SDE existence guarantee is external, but the load-bearing interpretation of Feller condition as distinguishing daily vs intraday regimes is not.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The model rests on standard SDE existence theorems once coefficients are given, plus the assumption that fitted curves represent true drift and diffusion. No new entities are postulated.

free parameters (1)
  • time-dependent parameterized average and variance curves
    Fitted directly to fish count data to define drift and diffusion coefficients of the bridges.
axioms (1)
  • standard math Unique existence of solutions to the nonlinear SDE with pinned boundary conditions once drift and diffusion are fixed
    Invoked to guarantee well-posedness of the diffusion bridges.

pith-pipeline@v0.9.0 · 5498 in / 1245 out tokens · 34189 ms · 2026-05-16T06:50:00.457593+00:00 · methodology

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Reference graph

Works this paper leans on

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