Trophic position estimates of formalin‐fixed samples with nitrogen isotopic compositions of amino acids: an application to gobiid fish (Isaza) in Lake Biwa, Japan

Abstract

AbstractTo evaluate the influence of formalin‐fixation on the nitrogen isotopic composition of individual amino acids, we conducted a formalin‐fixation experiment over 62 weeks using muscle tissues of largemouth bass (Micropterus salmoides) and freshwater minnow (Zacco platypus) collected in Lake Biwa. The results indicated that the formalin‐fixation does not affect the nitrogen isotopic composition of amino acids including glutamic acid and phenylalanine from muscle protein of the fish. We applied this technique to formalin‐fixed gobiid fish “Isaza” (Gymnogobius isaza, Tanaka) specimens that had been collected from Lake Biwa through the twentieth century. The pattern of the δ15N values for these amino acids remains relatively constant, even though each amino acid exhibits a gradual increase by ~3 ‰ from 1916 to 1992. With a formula proposed by Chikaraishi et al. (Limnol Oceanogr Method 7:740–750, 2009), we estimated the temporal variations in the trophic position of Isaza in Lake Biwa. The trophic position of Isaza remained quite constant (3.2–3.3) across a major eutrophication period in 1960–1980. The estimated trophic position of Isaza fish is consistent with the stomach content analysis in the previous reports. The constant trophic position suggests that the eutrophication did not apparently affect the trophic position of Isaza fish, although Isaza's dietary preference changed from zooplankton to gammarids around 1970.