Shifts in the Assemblage of Summer Mesopelagic Fish Larvae in the Gaoping Waters of Southwestern Taiwan: A Comparison between El Niño Events and Regular Years

Abstract

We investigated changes in the assemblages of summer mesopelagic fish larvae between El Niño events and regular years in 2014–2018 and evaluated their relationships with the hydrographic conditions of the Gaoping waters off southwestern Taiwan. Seventy-five taxa or morphotypes were identified, with five types of Benthosema pterotum (31.2%), Diaphus slender type (19.9%), Cyclothone alba (7.2%), Diaphus stubby type (5.9%), and Vinciguerria nimbaria (4.4%) being most common during the study period. The hydrographic conditions of the Gaoping waters were likely influenced by large-scale climate change via oceanic physical processes. Apparently higher seawater temperature, mixed layer depth, and lower salinity in the upper 100 m were observed at the end of the strong El Niño events (summer 2016). In addition, the certain dominant taxa exhibited contrasting patterns between El Niño events and regular years. In this study, although the abundance and composition of mesopelagic fish larvae assemblage were not influenced directly by changes in large-scale climatic conditions, the occurrence of mesopelagic fish larvae was related to seawater temperature, salinity, and chlorophyll a concentration. We speculated that despite the abundant food availability and the more mesopelagic fish larvae onto the Gaoping waters transported by the increased inflow of the South China Sea Surface Current during El Niño events, the higher temperature and lower salinity at the inshore upper waters might be unsuitable for mesopelagic fish larvae, possibly resulting in low egg and larval survival.