Feeding and migration of Todarodes pacificus in the East China Sea based on the stable isotopic information of gladius

Todarodes pacificus, a significant warm oceanic cephalopod species, is relatively well understood in terms of population structure, age, growth and reproduction. However, the species' migration patterns and feeding habits remain elusive due to limitations in artificial breeding techniques and the shortcomings of stomach contents analysis. Further research is essential to elucidate the migration path and feeding habits of T. pacificus for effective resource management. This study focused on the the proostracum of gladius of T. pacificus caught collected from the East China Sea in 2018 and 2019. The proostracum was sequentially sectioned, and the carbon and nitrogen stable isotope ratios of fragments were measured. Using trophic niche and stable isotope sequence, we examined the migration and feeding behaviors of T. pacificus across its life history over two years and preliminarily explored the reasons for interannual variations in these behaviors. The trophic niche analysis revealed distinct and non-overlapping ranges for T. pacificus in 2018, while in 2019, the ranges were similar and overlapping 2018: total area (TA) 3.11‰², standard ellipse area (SEAc) 0.81‰²; 2019: TA 3.51‰², SEAc 0.77‰². The stable isotope sequence indicated significant changes in the values for T. pacificus when the gladius proostracum reached 120 mm from the distal end in both years. On the population level, the average δ¹³C and δ¹⁵N values of T. pacificus from each spawning ground population in 2018 were positively correlated with gladius growth, while in 2019, only the average δ13C values showed such a correlation. Statistical tests revealed significant differences in the average δ¹³C values between 2018 and 2019 (t-test, F=1.104, P<0.01), but no significant differences in average δ¹⁵N values (t-test, F=3.903, P>0.05). The analysis indicated that T. pacificus individuals in 2018 originated from various spawning grounds, and all the individuals of T. pacificus in 2019 came from the same spawning grounds, whereas in 2019, they all came from the same grounds. In both years, the T. pacificus began migrating to low latitudes and coastal waters once the gladius proostracum reached 120 mm, with prior movements confined to the vicinity of spawning ground. The broader migration range in 2019 may be attributed to the El Niño event. As T. pacificus individuals grew and migrated, their trophic level increased in 2018, whereas in 2019, it remained relatively stable. This study demonstrated the utility of continuous gladius sampling for analyzing the feeding and migration of T. pacificus, providing a scientific foundation for future research on T. pacificus.