Possible influence of typhoons, volcanic ash, and ocean productivity on early larval survival and Japanese eel annual cohort recruitment

Abstract

Northern hemisphere anguillid eel populations declined after continental habitat losses and ocean-atmosphere regime shifts occurred, but the exact causes of declines or recruitment fluctuations remain uncertain. Recruitment variations have been hypothesized to result from variable early larval survival, which could be related to phytoplankton blooms caused by typhoons, because leptocephali feed on productivity-related marine snow. We investigated typhoon occurrences in the Japanese eel early larval growth zone in the western North Pacific relative to Taiwan and Japan glass eel recruitment, sea surface temperature (SST), and Chl-a (2003–2017). Recruitment to the 2 areas was significantly correlated, but was not correlated with general typhoon parameters (wind speed, duration, storm radius) or the number of typhoons each year. The highest (2005, 13 typhoons) and lowest (2012, 14 typhoons) recruitment years had very different storm tracks, and SST and Chl-a anomalies were higher in 2005. Larval transport modelling showed regions where larvae were transported in 2005 in relation to typhoon tracks, and 2005 was unique for having 6 April–August typhoons/storms that affected areas with larvae compared to all other study years (194 total typhoons). Storm-induced phytoplankton blooms and species composition shifts (e.g., diatom increases) are temporary and likely need to occur just before new moon to affect the most vulnerable first-feeding larvae (spawning only occurs near new moons). Typhoon Nesat passed near the spawning area ∼6 days before new moon in June 2005, just before the first-ever caught Japanese eel preleptocephali were collected, and there were 2 well-timed-placed August typhoons. The largest-ever eruption of the Anatahan Volcano north of Guam (eruptions 2003–2008) appeared to deposit potentially iron-rich ash (can cause blooms) into the region before new moon in April 2005, which might have increased April-spawned larval survival. Increased early larval survival could have occurred in 3-of-5 new moon spawning periods in 2005, and ash could also have increased productivity, which might have increased recruitment. Further research is needed to examine if marine snow composition and early larval survival may influence anguillid eel recruitment.