Effects of environmental factors within the spawning area and migration routes on the length of Anguilla japonica glass eels recruited to Taiwan

Abstract

Growth extent varies considerably among recruiting cohorts of Japanese eels Anguilla japonica. However, the effects of oceanic variation on their growth, particularly during the larval stage, remain unclear. We sampled glass eels in northeast Taiwan from 2010 to 2019 and investigated the effects of sea surface temperature (SST), salinity, chlorophyll a (chl a) concentration, and El Niño-Southern Oscillation (ENSO) events on the total length (TL) of these eels by developing a generalized additive model. The results revealed that mean SST between 21 and 24.5°C in the spawning area was associated with TL. Larger TL values were observed from 0.03-0.07 mg m-3 in the spawning area, and along the migration routes TL was greatest when chl a was > 13 mg m-3. Other variables, including mean SST and salinity along the migration route, influenced TL but contributed less than 4.5%. Larger and smaller A. japonica glass eels were observed during El Niño and La Niña years, respectively. Specifically, El Niño years exerted the greatest influence (67.1%) on TL, and this was followed by mean SST (12%) and accumulated chl a concentration (11.4%) within the spawning area. Our results indicated that environmental factors within the spawning area influenced the TL of A. japonica glass eels to a greater extent than did those along the migration routes, and climatic ENSO events exerted an additional important effect regarding changes in TL. Taken together, our results provide fundamental ecological information and a basis for fisheries to more effectively manage A. japonica.