Dissolved-oxygen gradients and fish passage in large-river tailraces: a KITS framework

Restoring upstream fish passage at large hydropower facilities depends not only on fishway provision but also on whether migrating fish can locate small entrances within hydraulically complex tailraces. This paper develops a literature-aided theoretical framework for using dissolved-oxygen (DO) gradients as supplementary scalar cue fields for migration-period entrance discovery, rather than as substitutes for proper entrance siting or hydraulic attraction. Synthesising evidence from fish physiology, behavioural ecology and environmental fluid mechanics, we link oxygen sensing, aerobic scope, tailrace DO transport and entrance-search behaviour. We propose the Key Induction Threshold System (KITS), comprising a maintenance threshold (DOₘaint), a minimum inducing gradient (Gₘin), a coupled temperature–oxygen avoidance surface ( (T, DO) ₐvoid) and a bio-hydrodynamic optimum window (Ωₒpt). KITS provides a probabilistic, species- and context-dependent framework for identifying when entrance-oriented DO contrasts may be detectable and metabolically favourable. Implementation follows a three-layer hierarchy: avoiding physiological exclusion; creating detectable DO contrasts through source-water accounting, selective withdrawal, discharge routing or local oxygenation; and integrating scalar cues with entrance siting, attraction flow and CFD–DO–individual-based modelling. Oxygen-aware entrance optimisation is therefore a testable extension of conventional fishway hydraulics requiring before–after field validation. Notation: Throughout this review, dissolved oxygen is denoted as DO; oxygen partial pressure as PO2; standard metabolic rate as SMR; maximum metabolic rate as MMR; aerobic scope as AS; cost of transport as COT; the maintenance threshold as DOₘaint; the minimum inducing gradient as Gmin; the combined temperature–oxygen avoidance threshold as (T, DO) ₐvoid; and the bio-hydrodynamic optimum window as Ωₒpt. When gradients are evaluated along a candidate corridor, the along-path coordinate is denoted by s, the effective coherence length by Lₑff, the DO contrast between the corridor core and background water by ΔDOcore, and the short-term standard deviation of background DO fluctuations by σDO