Spawning Sites of the Japanese Eel in Relation to Oceanographic Structure and the West Mariana Ridge

Abstract

The Japanese eel, Anguilla japonica, spawns within the North Equatorial Current that bifurcates into both northward and southward flows in its westward region, so its spawning location and larval transport dynamics seem important for understanding fluctuations in its recruitment to East Asia. Intensive research efforts determined that Japanese eels spawn along the western side of the West Mariana Ridge during new moon periods, where all oceanic life history stages have been collected, including eggs and spawning adults. However, how the eels decide where to form spawning aggregations is unknown because spawning appears to have occurred at various latitudes. A salinity front formed from tropical rainfall was hypothesized to determine the latitude of its spawning locations, but an exact spawning site was only found once by collecting eggs in May 2009. This study reports on the collections of Japanese eel eggs and preleptocephali during three new moon periods in June 2011 and May and June 2012 at locations indicating that the distribution of lower salinity surface water or salinity fronts influence the latitude of spawning sites along the ridge. A distinct salinity front may concentrate spawning south of the front on the western side of the seamount ridge. It was also suggested that eels may spawn at various latitudes within low-salinity water when the salinity fronts appeared unclear. Eel eggs were distributed within the 150–180 m layer near the top of the thermocline, indicating shallow spawning depths. Using these landmarks for latitude (salinity front), longitude (seamount ridge), and depth (top of the thermocline) to guide the formation of spawning aggregations could facilitate finding mates and help synchronize their spawning.

The ecology of oceanic dispersal and survival of anguillid leptocephali

“…Collections of leptocephali resumed after Schmidt's studies in the Sargasso Sea spawning area (Schoth and Tesch 1982;Kleckner and McCleave 1988;Munk et al 2010;Hanel et al 2014) and continued in the western North Pacific Aoyama et al 2014) to clarify where spawning occurs in relation to hydrographic features or to examine how larger leptocephali disperse from their spawning area (McCleave and Kleckner 1987). Collections were also made closer to the recruitment areas of both A. anguilla and A. japonica (see Shinoda et al 2011;Miller et al 2015a).…”

“…5, but additional preleptocephali were collected after 2007 in similar areas near the West Marina Ridge Aoyama et al 2014). The regional current patterns indicate that A. japonica larvae are transported westward by the NEC until they reach the area where the current bifurcates into northward and southward flows along the eastern side of the Philippines .…”

“…For example, the spawning areas of the Atlantic eels discovered by Schmidt (1922) were further studied by surveys starting in 1979 as overviewed recently (Miller et al 2015a). In the western North Pacific, after the spawning area was discovered in 1991 (Tsukamoto 1992), sampling surveys for eggs, prefeeding stage preleptocephali, and adults have confirmed that A. japonica spawns in a narrow area along the southern tip of the West Mariana Ridge in the few days before new moon periods Aoyama et al 2014). The ages at metamorphosis and estimates of total age have shown that the larval durations of anguillid eels can vary widely in relation to how far the larvae must travel to reach recruitment areas (Kuroki et al 2014).…”

“…Their larvae then move to each side of the North Atlantic subtropical gyre where they either recruit to North America (A. rostrata) or Europe and North Africa (A. anguilla) (Tesch 2003). The leptocephali of the Japanese eel (Anguilla japonica) are also widely distributed to the west of their spawning area , which is located along the West Mariana Ridge Aoyama et al 2014) in the western North Pacific, by the North Equatorial Current (NEC) and the Kuroshio Current (Fig. 1).…”

Occurrence and biological characteristics of glass eels of the Japanese eel Anguilla japonica at the Cagayan River of Luzon Island, Philippines in 2009

“…Global atmospheric changes that have caused frequent El Niño events reduced precipitation in the tropical region (Chou et al 2009). The salinity front that is a landmark to determine the spawning site of Japanese eels in the North Equatorial Current (Aoyama et al 2014) then moved southward due to a reduction of the low-salinity water supply by rainfall to the ocean surface. More information from both oceanographic and ecological approaches is required to understand the fluctuations in the recruitment success in the western North Pacific, which directly influences the stocks of the important eel fishery resources.…”