Horizontal and vertical migration behavior of silver-phase Japanese eels in coastal, pelagic and spawning areas observed by pop-up satellite archival tags

Abstract

Pop-up satellite archival tags (PSAT) have been used to study the mysterious open ocean spawning migrations of several anguillid eel species including the Japanese eel, Anguilla japonica. To compare migration behaviors of Japanese eels in each general geographic region of their spawning migration, 7 silver eels (852–992 cm, 1095–1809 g) were tracked with PSATs and their swimming behaviors in regions of east of Japan, south of the Ogasawara Islands, and in their spawning area was analyzed. Three eels released from coastal Japan migrated east or southeastward, likely including some transport by the Kuroshio in the initial phase of the spawning migration. An eel released from the coast of Japan changed swimming depths as it crossed meanders and different water masses on different sides of the eastward flow of the Kuroshio Extension. An eel released west of the Ogasawara Islands moved mostly southward from the release site toward the spawning area where an eel released in the spawning area showed very stable diel vertical migrations (DVM). Five eels showed stable DVM for 9–47 days (60.0–72.1%) of the tracking periods (14–69 days), and 2 other eels showed abnormal behaviors. The daily rhythms of DVM behavior transitioned between shallower/warmer water at night (12.0–535.2 m/12.5–24.5 °C) and deeper/colder water during daytime (593.0–941.0 m/4.5–9.5 °C). The swimming depths of 5 eels had positive correlations with lunar age, moon altitude and sun altitude. Eels experienced almost the same daytime minimum water temperature (5.7 ± 0.98 °C) in the 3 areas regardless of depth, suggesting that they stop descending when they reach a temperature of approximately 5 °C. The ascending and descending occurred between around sunset and dusk, and dawn and sunrise, respectively. Three eels were ingested by predators near dawn (5:05–6:35) at depths of 119.5, 59, and 423.5 m when the eels start the decent phase of their DVM. Potential predators were estimated to be fish that have heat conservation ability such as tuna and swordfish based on rapid increases in recorded temperature. This study suggests that Japanese eels probably use similar DVM behaviors regardless of the location or environmental conditions during their spawning migration and that the swimming depths are determined by light and thermal environments. These behaviors are likely essential for predation avoidance.