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

We measured stable carbon and nitrogen isotopes and calculated trophic level and trophic niche of , 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 δC varied substantially between the two periods. The narrowed range and the smaller mean value of δC in 2018 suggested that their food source changed from upper-middle to lower-middle waters. The values of δN 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 in the past decade. The correlation between δC and preanal length was not significant, but a positive correlation between δN 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 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 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.