Abstract
Long-term (1967–2008) glass eel catches were used to investigate climatic effects on the annual recruitment of Japanese eel to Taiwan. Specifically, three prevailing hypotheses that potentially explain the annual recruitment were evaluated. Hypothesis 1: high precipitation shifts the salinity front northward, resulting in favorable spawning locations. Hypothesis 2: a southward shift of the position of the North Equatorial Current (NEC) bifurcation provides a favorable larval transport route. Hypothesis 3: ocean conditions (eddy activities and productivity) along the larval migration route influence larval survival. Results of time series regression and wavelet analyses suggest that Hypothesis 1 is not supported, as the glass eel catches exhibited a negative relationship with precipitation. Hypothesis 2 is plausible. However, the catches are correlated with the NEC bifurcation with a one-year lag. Considering the time needed for larval transport (only four to six months), the one-year lag correlation does not support the direct transport hypothesis. Hypothesis 3 is supported indirectly by the results. Significant correlations were found between catches and climate indices that affect ocean productivity and eddy activities, such as the Quasi Biennial Oscillation (QBO), North Pacific Gyre Oscillation (NPGO), Pacific Decadal Oscillation (PDO), and Western Pacific Oscillation (WPO). Wavelet analysis reveals three periodicities of eel catches: 2.7, 5.4, and 10.3 years. The interannual coherence with QBO and the Niño 3.4 region suggests that the shorter-term climate variability is modulated zonally by equatorial dynamics. The low-frequency coherence with WPO, PDO, and NPGO demonstrates the decadal modulation of meridional teleconnection via ocean–atmosphere interactions. Furthermore, WPO and QBO are linked to solar activities. These results imply that the Japanese eel recruitment may be influenced by multi-timescale climate variability. Our findings call for investigation of extra-tropical ocean dynamics that affect survival of eels during transport, in addition to the existing efforts to study the equatorial system.
Highlights
Climatic effects on fluctuations of fish populations and fisheries have long been recognized [1] and continue to be critical: understanding these effects is an essential step toward conserving and managing marine resources [2,3,4]
For Hypothesis 3, we investigated various climate indices that have been shown as likely to affect ocean productivity and/or eddy activities, such as the Southern Oscillation Index (SOI), Quasi Biennial Oscillation (QBO), North Pacific Gyre Oscillation (NPGO), North Pacific Index (NPI), Pacific Decadal Oscillation (PDO), and Western Pacific Oscillation (WPO)
While particle-tracking simulation results suggested that latitudinal shifts in North Equatorial Current (NEC) bifurcation critically affect the strength of the Japanese eel larval transport [34,37], our time series analysis only partially supported this direct transport hypothesis
Summary
Climatic effects on fluctuations of fish populations and fisheries have long been recognized [1] and continue to be critical: understanding these effects is an essential step toward conserving and managing marine resources [2,3,4]. The fluctuation of the Japanese eel, Anguilla japonica, has gained particular attention [15], due to its high economic value [16], complex life history [17], and its declining recruitment since the 1970s [18,19]. The reason for the declines in recruitment of these temperate Anguilla eels is not clear, but is possibly caused by overfishing, habitat degradation, pollutions, parasites, virus, and global climate change [19,21,22,23,24,25,26]. In addition to the trend for a long-term decline in Japanese eel, fluctuations at interannul and decadal scales are observed [19,21,24], which warrant further investigation
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