Migration, residency and habitat utilisation by wild and cultured Japanese eels (Anguilla japonica) in a shallow brackish lagoon and inflowing rivers using acoustic telemetry

Abstract

This study monitored post‐release movements of 20 wild Japanese eels (Anguilla japonica) [mean ± S.D. 520.8 ± 92.3 mm total length (TL), 217.9 ± 146.3 g body mass (BM)] in a brackish water lagoon in northeastern Japan using acoustic telemetry to elucidate how wild Japanese eels use different river, estuary and marine environments. In addition, 12 cultured Japanese eels (TL = 578.9 ± 18.0 mm, BM = 344.9 ± 25.5 g) were released to understand the comparative behaviours of wild and cultured eels. Both types of eels were simultaneously released in the southern inner part of the lagoon in September 2016 where there are freshwater influences from a river. Following release, eight of the wild eels (40%) were largely sedentary near the released point (river mouth) and stayed at the site for overwinter. Nonetheless, several individuals showed behavioural plasticity of habitat use: three wild eels moved towards the northern part of the lagoon with stronger influence from the sea during May–July 2017. Two wild eels showed clear repeated movements from the lagoon to a river at night and returned to the lagoon by dawn for more than a week every day, and one wild eel migrated upstream for overwintering. Signals from 55% of the wild eels could be detected for more than 6 months, whereas those from all of the cultured eels were lost by December 2016, indicating a short resident time of large cultured eels (BM > 200 g) released in a brackish water area. One wild silver eel migrated to the outer sea during the ebb tide at night in November 2016, probably triggered by the decrease in water temperature (from c. 20°C to c. 13°C), and seven cultured eels similarly moved to the outer sea during October–November 2016. The results revealed the similarities (e.g., nocturnal movements) and differences (e.g., stay period and seasonal movements) in the behavioural characteristics of wild and cultured eels and indicated that habitat connectivity among river, estuary and coastal waters is crucial for enabling eels to efficiently utilise these productive habitats through their behavioural plasticity.

Factors influencing migration of short‐finned eels (Anguilla australis) over 3 years from a wetland system, Lake Condah, south‐east Australia, downstream to the sea

“…Variation in migratory life histories among individuals may relate to a complex array of factors, such as condition, energetic status, and sex (Senner et al, 2020), and may have implications for the resilience of eel populations (Moore et al, 2014; Schindler et al, 2010). For instance, extended estuarine residence may allow eels to capitalize on increased food availability (Noda et al, 2021), potentially leading to increased fitness prior to their seaward migration. In contrast, eels remaining in the estuary for an extended period may need to adjust to a constantly changing environment (e.g., salinity) and be subject to greater pressure from predators (e.g., large predatory estuarine fish such as mulloway Argyrosomus japonicus ) or commercial harvesting compared to eels residing in freshwater reaches.…”

Water temperature and precipitation stimulate small-sized Japanese eels to climb a low-height vertical weir

“…Similarly, in previous studies, the behavioral activity of eels increased at night, with two peaks just after sunset and before sunrise in Japanese eels as well as other eel species [ 23 , 34 , 35 ]. Noda et al [ 24 ] reported the diel movement of yellow Japanese eels; eels move to a river after sunset and return to a brackish lagoon just before sunrise, possibly for foraging purposes [ 38 ]. However, the two peaks of eel climbing are unlikely to be explained only by foraging behavior because eel climbing is likely to be an energy- and time-consuming activity.…”

“…We recorded continuously for 24 hours every day from February 13 to May 31, 2021 at a resolution of 1280 × 720 pixels and 30 fps. As anguillid eels are nocturnal [ 23 , 24 ], two infrared lights per camera were lit at night to observe the climbing behavior of eels. To estimate the TL of eels from the video, we painted a white line (AquaCort, HERMETIC, Tokyo, Japan) on top of the weir ( Fig 1C and 1D ).…”

“…We hypothesized that eels climb weirs at night when water temperature exceeds 13°C. This is because anguillid eels, including Japanese eels, are nocturnal [ 23 , 24 ], and the activity of Japanese eels decreases below 13°C [ 23 ].…”

The drivers of anguillid eel movement in lentic water bodies: a systematic map

“…month, date and season) was also a key driver of movement, with significance reported in 79% of the 14 articles that addressed it. Seaward migration of adults from lentic systems generally peaked during autumn in European eel (Verhelst et al Verhelst, Reubens, et al, 2018), American eel (Winn et al 1975) and Japanese eel (Noda et al 2019(Noda et al , 2021, and late summer in shortfin and New Zealand longfin eel (Jellyman and Unwin 2017;Stuart et al 2019). A seasonal pattern for non-migratory movements is also seen but this may be more variable between location and species.…”