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The past, present, and a future for native charr in Japan
Get PDFCharrs (Salvelinus) reach their southernmost distribution in Japan, and are uniquely adapted to the short, steep streams of this island archipelago. Southern Asian Dolly Varden (Salvelinus curilus) occur only in Hokkaido Island, whereas white-spotted charr (Salvelinus leucomaenis) range to southern Honshu. Both species diverged from an ancestral lineage during the late Pliocene/early Pleistocene, when lowered sea levels created semi-enclosed water bodies in the seas of Japan and Okhotsk. Genetic analyses showed S. curilus represents the most ancient divergence from the Dolly Varden (Salvelinus malma) - Arctic charr (Salvelinus alpinus) group, and revealed five lineages of S. leucomaenis which align differently than traditional subspecies. Japanese charr display diverse and flexible life histories including anadromous fish with partial migration, and fluvial, adfluvial, and resident forms. In Hokkaido, Dolly Varden are distributed upstream and white-spotted charr downstream. They coexist in narrow sympatric zones through adaptive shifts by Dolly Varden in behavior and morphology that facilitate benthic foraging. Both species hybridize with native and nonnative salmonids, and are displaced from microhabitats and decline in abundance when rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) invade. Japan streams contain over 95,000 erosion control dams which create short stream fragments (medians ~200 m). This has increased extirpation of charr populations via lower genetic diversity and stochastic and demographic factors. Tributaries provide complex rearing habitats, afford refuges from floods, and supply recruits that sustain populations in mainstem fragments and create metapopulations in connected riverscapes. Charr play central roles in linked stream-riparian food webs, and cause direct and indirect effects that cascade to streambed algae and riparian predators when linkages are disrupted by anthropogenic effects or altered by native parasites. Many charr populations are threatened by habitat fragmentation and introgression or invasion by nonnative forms, but efforts to conserve charr are growing. These include restoring connectivity among pure populations above barriers that prevent invasions, protecting tributary nurseries, and instituting angling regulations to protect headwater populations. Key steps include inventorying pure populations, identifying conservation units, selecting appropriate management based on connectivity and biotic interactions, and engaging stakeholders and youth to engender an ethic for conserving irreplaceable charr lineages.journal articl
Diets comprising hen egg yolk and milk proteins as potential alternatives to shark egg-based diets for larvae of the Japanese eel Anguilla japonica
Get PDFThe only diet that has been capable of rearing eel larvae to the glass eel stage in captivity is the shark egg (SE)-based diet (SE). This study investigated the potential of alternative dietary components, namely hen egg yolk (HEY), milk proteins, and fish protein hydrolysate (FPH), on the growth and survival of eel larvae. In the first experiment, a diet containing HEY and skimmed milk powder (HS) was compared to SE. There were no significant differences in growth and survival rate between the two diets except in the early part of the experiment period. In the second experiment, HS was modified by the addition of FPH and casein (FC), and larval performance was evaluated for three dietary regimens (SE, HS, and FC). The performance of larvae fed HS and FC was found to be comparable or superior to those fed SE. It was observed that larvae fed the alternative diets to SE were able to progress to glass eels. However, more skeletal abnormalities were observed in HS in experiment 1. The results of this study indicate that a combination of HEY and milk proteins is suitable as a larval eel diet, and that it has the potential to replace SE.journal articl
A method to enhance kelp production by suspending floats above the barren seabed off Sanriku coast, northeast Japan
Get PDFWe attempted to create a simple apparatus to sustainably grow and provide seaweed to herbivorous benthos on a barren seabed. The apparatus was designed in such a matter that suspension culture of the seaweed reduced grazing pressure by sea urchins during the early growth, while fully grown seaweed could be provided to them as feed. The apparatus consisted of a float and sandbag connected with a 1-2m rope. We deployed 50 units of this apparatus in the study area (20×20m2, 6.7-8.4m deep), located off the Sanriku coast (north-eastern Japan) in July 2017. Kelp species such as Saccharina japonica and Undaria pinnatifida grew densely on these floats and ropes. The maximum wet weight of the seaweed attached to the apparatus was 16.0kg/unit and 5.2kg/unit in June 2018 and July 2019, respectively. However, only Desmarestia viridis was observed on the seabed in both years. Despite the dense growth of edible seaweed, the gonad index of the sea urchins Mesocentrotus nudus was not enhanced in the experimental area. The productively of kelp and the amount of kelp accessible to the benthic herbivores need to be improved by adjusting the components of the apparatus.ウニ類などの植食動物が多く磯焼け状態となっている地点で，植食動物のアクセスを低減させるために簡易な装置を中層に浮かべ，海藻類を生育させる実験を行った。2017年7月に海底（水深6.7～8.4m）から高さ1～2mにフロートを浮かべたところ，フロートやロープにマコンブやワカメなどが着生し，海藻類の湿重量は1基当たり最大で16.0kg（2018年 ）， 5.2kg（2019年）となった。一方，海底には両年ともケウルシグサのみが生育した。海藻類を生育させることはできたが，その周囲のキタムラサキウニの身入り改善には至らなかったため，今後はフロートの浮力や海底からの高さを調整し，海藻類の生産量やベントスへの餌料供給量をさらに増加させることが望まれる。ファイル名の末尾が”_abs_j”のファイルは和文要旨departmental bulletin pape
A seasonal forecast for set net fishing of chub mackerel Scomber jaronicus using coastal water temperature in the Pacific coast of Tohoku
Get PDFThe relationship between coastal water temperature and the amount of chub mackerel Scomber japonicus landed through set nets in the Pacific coast of Tohoku between 1995 and 2020 was investigated. Based on the seasonal fluctuations in the coastal water temperature, periods of rising and falling water temperatures between April-August and SeptemberMarch, respectively, were classified. The majority of the landing for the year occurred during the period of rising water temperature. It was suggested that the fishing season started earlier and the temperature at the start of the fishing season decreased in 2014-2017. A seasonal forecasting Web system for set net fishing using coastal water temperature in the Pacific coast of Tohoku was initiated in December 2020. A previous 3-year average of the mean 10-day coastal water temperature was used as the temperature index to forecast the start of fishing season. Report of the 10-day catch amount is displayed along with the forecast in the Web site.1995-2020年の三陸常磐海域におけるマサバ定置網水揚量と沿岸水温との関係を調べた。沿岸水温の季節変動から，4-8月の水温上昇期と9-翌3月の水温下降期に区分した。水揚量が主体となっていた水温上昇期において2014-2017年に漁期開始時期の早期化とその時の水温の低下が認められた。直近3年間の漁期開始水温の平均値を指標水温とするマサバ定置網の漁期開始時期予報のWeb配信を2020年12月に開始し，旬別水揚量の速報値とともに表示させた。ファイル名の末尾が”_abs_j”のファイルは和文要旨departmental bulletin pape
Coexistence of dominant marine phytoplankton species sustained by nutrient specialization
Get PDFProchlorococcus and Synechococcus are the two dominant picocyanobacteria in the low-nutrient surface waters of the subtropical ocean, but the basis for their coexistence has not been quantitatively demonstrated. Here, we combine in situ microcosm experiments and an ecological model to show that this coexistence can be sustained by specialization in the uptake of distinct nitrogen (N) substrates at low-level concentrations that prevail in subtropical environments. In field incubations, the response of both Prochlorococcus and Synechococcus to nanomolar N amendments demonstrates N limitation of growth in both populations. However, Prochlorococcus showed a higher affinity to ammonium, whereas Synechococcus was more adapted to nitrate uptake. A simple ecological model demonstrates that the differential nutrient preference inferred from field experiments with these genera may sustain their coexistence. It also predicts that as the supply of NO3− decreases, as expected under climate warming, the dominant genera should undergo a nonlinear shift from Synechococcus to Prochlorococcus, a pattern that is supported by subtropical field observations. Our study suggests that the evolution of differential nutrient affinities is an important mechanism for sustaining the coexistence of genera and that climate change is likely to shift the relative abundance of the dominant plankton genera in the largest biomes in the ocean.journal articl