Climate-related changes in seasonal habitat pattern of Sthenoteuthis oualaniensis in the South China Sea

Abstract

Sthenoteuthis oualaniensis is an economically important species widely distributed in the South China Sea. In this study, a habitat suitability index model (HSI) was established to examine seasonal habitat pattern of S. oualaniensis in the South China Sea from spring to autumn and assessed the impacts of the El Niño event on habitat variations using three critical environmental factors including sea surface temperature (SST), Chlorophyll-a (Chla) and sea surface height (SSH). Environmental conditions and habitat patterns showed significant seasonal and spatial variations. Spatial pattern of favorable environmental conditions played an important role in regulating the ranges and longitudinal and latitudinal distributions of suitable habitats (areas with HSI ≥ 0.6) of S. oualaniensis across seasons. Significant positive correlation was found between SST and HSI, whereas negative relationship was revealed between HSI and Chla as well as SSH on the main fishing ground of S. oualaniensis by correlation analysis. Comparing to normal climate condition in spring 2014, the El Niño events in spring 2015 and 2016 yielded relatively lower SST and higher Chla and SSH, which were unfavorable for the formation of high-quality habitats for S. oualaniensis. Favorable ranges of environmental conditions showed decreasing trend from spring 2014 to 2016. Consequently, the suitable habitats in spring 2015 and 2016 largely contracted and became discontinuity. Our findings indicated that seasonal habitat patterns of S. oualaniensis in the South China Sea were closely associated with local environmental conditions. Importantly, the El Niño events strongly affected spatio-temporal patterns of S. oualaniensis habitat in the South China Sea.

Migration Route of Sthenoteuthis oualaniensis in the South China Sea Based on Statolith Trace Element Information

“…In addition, the diurnal vertical movements of individuals and changes and transitions in feeding behaviors also affect the composition of trace elements in the population [ 3 , 26 , 27 , 29 ]. Under recent global climate change, annual seasonal changes in marine environmental conditions may also affect the absorption of trace elements by populations, and the seawater temperature affects the pH value of the blood and endolymphatic fluid system [ 37 , 38 , 39 ].…”

“…Therefore, the Mg:Ca, Sr:Ca, and Ba:Ca ratios can be used as indicators to characterize habitat changes during the life cycle of S. oualaniensis in the South China Sea. The differences in element ratios among stocks may be related to differences in habitat, and the relative contributions of biotic and abiotic factors to variations in element ratios may also differ among life history stages [ 3 , 22 , 26 , 29 ].…”

“…As gonads mature, the active swimming ability increases and the population migrates over long distances for feeding to meet their physiological needs [ 3 , 4 ]. It is the largest semi-enclosed deep-water basin in the western Pacific, a typical monsoon region, and has significant local air–sea interactions [ 29 ]. The surface and upper-level circulation of the South China Sea is significantly affected by monsoons [ 45 , 46 ].…”

“…Generally, early larvae exhibit active feeding, with a focus on crustaceans, and their feeding habits change from zooplankton to nocturnal free-swimming fish [ 20 , 22 , 29 ]. According to the MRT in this study, the ratios of the effective elements in each stage of growth differed among stocks.…”

“…Seawater temperature affects biological characteristics such as individual growth, reproduction, and migration [ 3 ]. The temperatures at different water layers affect population distribution and habitats [ 3 , 29 ]. In this study, the temperature was evaluated at different water layers, including depths of 5 m (Temp_5), 25 m (Temp_25), 55 m (Temp_55), 75 m (Temp_75), and 105 m (Temp_105).…”