Changes in the trophic niche of Trichiurus japonicus in the Beibu Gulf in different periods.

We measured stable carbon and nitrogen isotopes and calculated trophic level and trophic niche of Trichiurusjaponicus, with samples collected from the Beibu Gulf from 2008 to 2009 and 2018. The differences between two periods were compared and analyzed to explore the changes of its ecological adaptation. The results showed that value of δ13C varied substantially between the two periods. The narrowed range and the smaller mean value of δ13C in 2018 suggested that their food source changed from upper-middle to lower-middle waters. The values of δ15N were stable, and the range and mean values of trophic level (3.38-3.43) did not change significantly, which indicated a stable trophic level of T.japonicas in the past decade. The correlation between δ13C and preanal length was not significant, but a positive correlation between δ15N and preanal length. In terms of trophic niche, the indicators had decreased in different degrees in 2018 with 1.1%-32.1%. The value of total area and standard ellipse area decreased from 20.20 and 4.68 to 14.20 and 3.18, respectively, indicating that the niches of T.japonicas in the Beibu Gulf had varied obviously and that their ability to use resources and adapt to the environment had declined. It was speculated that in the past decade, the mean trophic level of T.japonicas in the Beibu Gulf had not changed significantly. Due to the change of food sources, however, the diversity of trophic sources had decreased, and the trophic niche had become smaller.