No 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.

No abstract

Abstract:The moonsnail Laguncula pulchella (Naticidae, formerly known as Euspira fortunei) is an invasive species that has had a negative impact on the Manila clam (Ruditapes philippinarum) population in northern Japan. In southern Japan, few records of L. pulchella exist; furthermore, the knowledge concerning the ecology of this organism is currently limited. Unexpectedly, we found many L. pulchella individuals in the Misuji River estuary, located in Hiroshima in southern Japan, during May 2015. We conducted a field survey on the density and body size of L. pulchella at a tidal flat in the Misuji River estuary (Site H) and compared those to L. pulchella in Matsukawaura Lagoon, located in Fukushima in northern Japan (Site F). The adult population of L. pulchella in a clam fishing ground (40,500 m 2 ) at Site H was estimated to be 2,048 individuals in 2015. Densities of both surfaced individuals and egg collars of L. pulchella were lower at Site H than at Site F. Sizes of L. pulchella individuals and their egg collars as well as the somatic weight of the individuals at Site H were significantly smaller than those at Site F. These results suggest that the biological characteristics of L. pulchella largely differ between the two sites.

The naticid snail Laguncula pulchella is an invasive species that preys on clams in tidal flats and has serious impacts on clam fisheries in Japan. Laguncula pulchella burrow in sand, but often crawl on sediment surfaces during low tide. We investigated seasonal changes in the abundance and sex ratio of crawling L. pulchella during the daytime at Matsukawaura Lagoon, Japan, from March to October from 2015 to 2019. The density of crawling individuals peaked in July. The sex ratio of crawling individuals varied with months and years but was significantly biased towards males during the main copulation period (July–August); males accounted for 77–98% of the mature crawling individuals (≥ 25 mm shell height). The somatic condition of mature males declined from June to August, whereas that of females was constant during this period. These results indicate that mature males actively come to the sand surface during low tide to search for females for copulation from July to August. Fishermen make efforts to remove crawling individuals in summer, but the male-biased sex ratio must also be considered for effective population control of this species.