Assessing fish movement and physiological traits along a salinity gradient by measuring stable oxygen isotope values in fish blood water and muscle water

Abstract Variations in stable oxygen isotopes ( δ 18 O) in the body water of animals reflect changes in δ 18 O values in environmental water and the animals' physiological processes. Euryhaline/anadromous/catadromous fish species move across salinity gradients; changes in δ 18 O values in environmental water thus lead to changes in δ 18 O values in body water. Few studies have systematically evaluated the effect of salinity on δ 18 O values in fish body water. Accordingly, this study demonstrated a well‐established approach to investigating the relationship between salinity and δ 18 O values in fish body water (blood water and muscle water). This study comprised two experimental designs. First, we implemented continual changes in salinity levels to assess the corresponding variations in δ 18 O values in the blood water of Japanese eels under controlled laboratory conditions. Second, field (river and estuary) studies were conducted for multiple fish species on Japan's Amami Island. Samples of fish blood water and muscle water were analysed in accordance with a general headspace protocol to estimate δ 18 O values; this estimation was conducted using isotope ratio mass spectrometry, which involved an equilibration method for sample preparation. In the laboratory experiment involving changes in salinity, the difference in δ 18 O values in fish blood water and ambient water was less than 1‰ within 2–3 days after fish were moved to a new environment. However, the difference varied with salinity and metabolism. In the field, obvious differences in δ 18 O values between fish blood water and environmental water were observed in the estuary area, implying that fish frequently move across salinity gradients. In the laboratory experiment and field observations, δ 18 O values were higher in muscle water than in blood water. Analysing δ 18 O values in environmental water, blood water and muscle water is useful for monitoring the behaviours of regional migratory fish, such as temporary feeding in estuaries. Our analytical method is ideal for measuring δ 18 O values in fish body water because it enables analysing various organic tissues by using small amounts of body fluids, facilitating the examination of the ecology of aquatic animals.