BIOTELEMETRY STUDIES ON POST-RELEASE BEHAVIORS OF CULTURED AND WILD JAPANESE EELS Anguilla japonica IN SHALLOW BRACKISH WATERS IN JAPAN

Japanese eel (Anguilla japonica) populations have  been rapidly decreasing due to overfishing and habitat loss since  the 1970s in Japan. U nderstanding the  detailed  ecological aspects of eels is essential to establish effective conservation measures . To augment depleted stocks, cultured eels have been widely released by fisheries cooperatives . However,  studies on  the ecology and behavior of cultured eels after release are limited , and  the effectiveness of stocking remains unclear. The aim of this study  is to  examine behavioral characteristics of wild and cultured eels released in  the wild  to consider appropriate management policies for  eel populations in Japan.

Studies were conducted in Matsukawa-ura lagoon in  Fukushima (northern Japan) and the Tonda River in Wakayama  (central Japan) using ultrasonic telemetry, which estimates the position of each eel by ultrasonic signals. Twenty-eight  and 13 receivers  were set in  the  Matsukawa-ura  and Tonda River, respectively. Twenty wild and 12 cultured eels  were implanted with acoustic transmitters and  released in Matsuka wa-ura in September 2016,  and 20 wild and five cultured eels  were similarly outfitted and released in the Tonda River in June 2018 . Temporal and spatial changes of received signal data of each eel were analyzed.

Some eels migrated to the sea, and some moved between the lagoon/ river  and the sea several times after release .  Migration from the  Matsukawa-ura  lagoon to  the open sea occurred at night during ebb tide in autumn (October to November ; Table 1 ).  Out of the eight eels that  migrated to the sea, seven individuals were cultured. However, it is unlikely that the seven cultured eels  (2 years old)  were mature  enough for a spawning migration. This migration behavior is thought to be abnormal  for cultured eels that  were raised under constant high temperature.  In the Tonda River, three cultured and one wild eel migrated to the  open  sea in September and October. Seasonal patterns of behavioral activity were found: high in summer and autumn, and low in winter for both eel groups. Both eel groups had nocturnal feeding activity ;  in particular wild eels were most active around dusk. Diel rhythms of cultured eels were  less pronounced  compared to wild ones. The results of the present study indicate that  the  behavior of cultured eels differs from that of wild eels and  that  this difference most likely is  caused by  the artificial rearing conditions implemented in the  aquaculture ponds  0.5 - 2 years before release. Although some cultured eels migrated to the sea, it is difficult to ass ess  the contribution of these eels to  overall  reproduction in the wild. Previous studies indicated that  the  release of cultured eels may negatively impact ecosystems. Therefore,  the  release of cultured eels may disrupt the ecosystem; and unknown effects on reproduction should be reduced as much as possible. Instead, establishing rules to prevent overfishing of glass eels and restoring habitats  in rivers and coastal areas  to foster the establishment of  wild  yellow eels as future spawning stock are important.