Fishery catches of Japanese eels Anguilla japonica have declined over the long term, and stocking programs have been implemented to enhance their stocks. Predation is considered a major cause of mortality in fish stocking, yet knowledge of predation on stocked eels remains limited. We conducted a field survey to identify the predatory fish of stocked juvenile eels by collecting potential predators in the surrounding area immediately after releasing eels into the upper estuaries of two rivers in Japan. To examine how predation risk and predator species vary with developmental stage, we simultaneously released two groups: the underdeveloped glass eel group (n = 6641) and the newly transformed small yellow eel group (n = 3326). Across three intensive field studies conducted over three consecutive years, a total of 368 individuals (13 species) of potential predatory fish were collected. Thirteen glass eels were found in the stomachs of five predatory fish species, whereas only two yellow eels were found in the stomachs of larger Japanese eels. Our findings suggest a potential vulnerability of released underdeveloped juvenile eels to a wider range of predatory fish in the released area, offering valuable insights into the ecology and conservation of anguillid eels.
Eukaryotic microalgae fundamentally contribute to marine primary production throughout the year. However, a comprehensive understanding of their year-round fluctuations in coastal ecosystems remains limited. In this study, we identified the major microalgae and their fluctuation patterns in Osaka Bay by analyzing the 18S rRNA gene amplicon sequencing of seawater samples collected monthly for 18 months. The eukaryotic community structure, comprising over 3000 amplicon sequence variants (ASVs), exhibited clear seasonal dynamics. In particular, Picochlorum geojense (Trebouxiophyceae) was the major microalga in Osaka Bay, reaching a relative abundance of over 60
The red seaweed (Porphyra haitanensis) is considered an excellent alternative source of protein. This study investigated the effect of the pH-shifting method combined with ultrasound at varying amplitudes (20–80
To sustainably use neon flying squid resources in the high seas, understanding the fishing operation locations and status in the various countries involved is essential. We analyzed automatic identification system (AIS) data installed on the fishing vessels to describe the operational characteristics of Japanese and Chinese neon flying squid fishing vessels in the central North Pacific (in the range of 35°–45° N and 165° E–165° W). From May to July in 2016–2019, fishing vessels of these two countries were in operation with the west side for Chinese and east side for the Japanese vessels in most of the cases. In addition, Japanese vessels were on average located 3° farther north than Chinese vessels. Trend analysis showed that Chinese fishing efforts had gradually increased to the east, recently overlapping the fishing grounds between the two countries in the central North Pacific in study period. Our analysis of ocean environmental data indicated that vessels from both countries mainly operated around 15 °C during May–June each year. In contrast, in July, Chinese vessels operated 2–5℃ higher than Japanese vessels. These differences in distribution and environments in fishing location suggest that the two countries vessels may have exploited different cohorts.
Information on population genetic structure, movement, and distribution is fundamental for effective management of marine species. In the western North Pacific, the common minke whale faces both bycatch and commercial whaling, the latter having resumed in Japanese waters in 2019. In this study, we refined the population genetic structure and spring–summer spatial distribution of this species using 6196 genetic samples collected in feeding grounds and migratory corridors during 1994–2024. Genetic analyses of the mitochondrial DNA control region and 16 microsatellite loci confirmed the previously recognized Sea of Japan–Yellow Sea–East China Sea (J) and Sea of Okhotsk–West Pacific (O) stocks, which overlapped along the Pacific and Sea of Okhotsk coasts of Japan. Spatial analyses identified fine-scale differences in distribution by stock, sex, and sexual maturity class. In both stocks, mature whales were found farther offshore than immature whales. In the J stock, both sexes were concentrated in coastal waters, whereas the O stock exhibited a broader distribution that varied by sex and maturity class, with mature males being found more frequently offshore. These findings have important implications for stock assessment and management and may inform future management strategies for western North Pacific common minke whales.
