Abstract

Swimming performance and energetics of the Japanese eel (Anguilla japonica) across growth and early migratory phases remain poorly quantified. We used swim-tunnel respirometry to compare selected environmental and developmental contrasts in yellow and early silver eels from the Tone River estuary. Critical swimming speed (Ucrit), optimal swimming speed (Uopt), minimum cost of transport (minCOT), oxygen consumption at Uopt, and low-flow MO2 measured at 0.1 m s−1 were evaluated in sequential freshwater trials at 25 and 18 °C, an exploratory Y2 freshwater–seawater comparison at 18 °C, and a Y2–S1 comparison in seawater at 18 °C. Ucrit and low-flow MO2 were numerically higher under FW25 than FW18, but this contrast was confounded with fixed trial order and fasting history. In Y2 eels, SW18 was associated with higher low-flow MO2 than FW18, whereas active-speed-derived variables were limited by sample size. Under SW18, no clear stage-associated differences were detected between Y2 and S1 eels. Laboratory-derived Uopt and Ucrit exceeded field-estimated active horizontal swimming speeds, but this comparison should be interpreted cautiously.

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