Change in stable nitrogen isotope ratio in the muscle tissue of a migratory goby, <i>Rhinogobius</i> sp., in a natural setting

Abstract

Time lags associated with changes in stable isotope ratios are essential information for quantitatively analyzing shifts of food habits and habitats. To investigate time lags associated with fish growth in a natural setting, we monitored the change in δ 15 N in the muscle tissue of a fluvial–lacustrine amphidromous goby, Rhinogobius sp. (the orange form), in the Lake Biwa water system. Because δ 15 N is distinct between lacustrine and fluvial ecosystems, the δ 15 N of age-0+ fish drastically decreased after their upstream migration from the lake. About 80% of the change in δ 15 N was attributed to growth. Nitrogen was replaced at the rate of 5–20%·month –1 by metabolic turnover. The half-change period for δ 15 N was estimated as being longer than 1 month with the contribution of growth and metabolic turnover in the field. These results from the field show that growth is primarily responsible for isotopic changes in fish muscle and highlight the need to examine the role of metabolic turnover using slow-growing fish.