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.
Highlights
Freshwater eels of the genus Anguilla are catadromous fishes that spawn over deep water at tropical latitudes and use the ocean for their larval development before entering estuarine and freshwater growth habitats [1,2]
Anguillid eel populations including those of the Japanese eel, Anguilla japonica, have declined worldwide in recent decades [9,10], but the exact causes of the declines are difficult to determine partly because their reproductive ecology is hidden by the vast open ocean
The spawning areas of the Atlantic eels, the European eel, Anguilla anguilla, and the American eel, Anguilla rostrata, were discovered early in the last century [11] and were later found to be associated with temperature fronts in the Sargasso Sea based on the distribution of small larvae [12,13], with spawning occurring across a wide latitudinal area [14,15]
Summary
Freshwater eels of the genus Anguilla are catadromous fishes that spawn over deep water at tropical latitudes and use the ocean for their larval development before entering estuarine and freshwater growth habitats [1,2]. All three northern temperate species of anguillid eels consist of single panmictic populations [3,4,5,6,7], with all of their reproductively maturing individuals migrating long distances offshore to spawn in a single spawning area for each species [2,8]. Anguillid eel populations including those of the Japanese eel, Anguilla japonica, have declined worldwide in recent decades [9,10], but the exact causes of the declines are difficult to determine partly because their reproductive ecology is hidden by the vast open ocean. The spawning area of the Japanese eel in the western North Pacific (Fig. 1) was discovered in 1991 [16] and has been intensively studied in the last few decades [17,18,19]. The surveys that succeeded to collect newly hatched larvae [19,20,21], eggs and spawning-condition adults [19,21,22]
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