The intestinal tract of fish is colonized by diverse taxa of bacteria, forming intestinal microbiota, with densities of 10⁹-10¹¹ cells/g. However, the intestinal microbiota is not only affected by the fish species, developmental stages, and temperature and salinity of the rearing water, but is also characterized by large individual differences and daily fluctuations. Some of these intestinal bacteria produce vitamins to supply the host, produce enzymes that break down organic matter to aid in host digestion, and produce antimicrobial substances to prevent the growth of invading pathogens. Thus, it is suggested that there is a close symbiotic relationship between the intestinal microbiota and the host fish. On the other hand, many bacterial species belonging to the family Vibrionaceae are known as opportunistic bacteria in marine fish. These bacteria multiply in the intestinal tracts of fish and are released with feces into rearing water, often causing opportunistic infections. Because of the economic impact of infections, it is necessary to regularly monitor the density of Vibrionaceae in rearing water to reduce the risk of developing infections.

To clarify the early life history of the tiger puffer Takifugu rubripes in the Seto Inland Sea, trawls using a small-mesh seine net were conducted in the littoral zone of the estuaries of the Yoshii and Takahashi rivers, from June to August in 2010-2012. A total of 206 juvenile Takifugu (9.8-67.9 mm total length: TL) were collected and subsequently identified as T. xanthopterus (60.2%), T. rubripes (36.4%), T. alboplumbeus (1.9%), and T. poecilonotus (1.5%) by mitochondrial DNA analysis. The juvenile T. rubripes with 10 mm TL occurred in mid-June and grew to 60 mm TL in early August. Most of the T. rubripes individuals (90%) were collected in habitats with low salinity (sample range 5.1-22.9), high turbidity (range 33.3-73.4 formazin turbidity unit), and muddy bottoms (range 20.5-47.4% mud content), suggesting that the muddy tidal flats of estuaries are a suitable nursery ground during the early life stage of T. rubripes. In contrast, several large-sized individuals over 35 mm TL were collected in areas with high salinity and sandy bottoms. The findings suggest that T. rubripes juveniles may begin to move away from the nursery ground in the river mouth at sizes of > 35 mm TL, as they expand the area they inhabit in the region.

Genetic monitoring was conducted on a yellowfin tuna Thunnus albacares (YFT) broodstock population held in a land-based facility at the Inter-American Tropical Tuna Commission (IATTC) Achotines Laboratory. The number of females contributing to a single spawning event were identified and their spawning quantity and periodicity were monitored by the analysis of the mitochondrial DNA (mtDNA) D-loop region. There were 50 broodstock YFT in the tank during the study period of 2011-2014. By comparing broodstock mtDNA sequences against 555 offspring eggs it was possible to identify at least 11 broodstock females that spawned during the study period and identify the most fertile individuals. Multiple females participated in single spawning events. An individual female spawned every month for a period of 1.5 years, supporting the concept that YFT females in captivity are capable of spawning for extended periods of time. Another individual female spawned intermittently for about 26 months. The results demonstrated that the D-loop region of YFT displays enough variation to discriminate among individuals and highlights the value of molecular biological approaches in elucidation of the spawning ecology of wild YFT populations and in the management and selection of tuna broodstock for aquaculture purposes.

To investigate the hatch date and larval growth patterns of Ayu Plecoglossus altivelis, otolith microincrements of larvae and juveniles collected in the coastal waters off the eastern and western parts of Tottori Prefecture from 2016 to 2019 were examined. Of all collection area and year-class, the hatch date ranged from the latter half of September to the first half of January, and the peak hatch date was observed between the latter half of October and the first half of November. The standard length of 30 days after hatching (SL₃₀) for each individual was estimated from the radius of the 30th otolith daily ring. The SL₃₀ of individuals with earlier hatch dates was larger, and the water temperature which each individual experienced was positively correlated with SL₃₀. Variations in early growth could be strongly influenced by the hatching period rather than the year-class strength or collection area. These results suggest that early hatching could be advantageous for early growth.

Catches of two flounder species, namely the marbled flounder Pseudopleuronectes yokohamae and the stone flounder Platichthys bicoloratus, in western Hiuchi-Nada, central Seto Inland Sea have decreased to less than one-fifth during 18 years from 1999 to 2017. Here to assess the possible decline in fisheries catches during this period, we investigated the occurrence of juveniles at shallow bottoms of the coast from 2002 to 2006 (early period) before the decrease in fisheries catches, and from 2014 to 2017 (late period) after its decrease. Population densities of both species were high at water temperatures between 9°C and 14°C, while fewer juveniles were collected from water temperatures exceeding 20°C. The population density of juvenile marbled flounders was similar during both survey periods, whereas that of stone flounders increased to approximately two times during the late period. No apparent link was noted between the trends of juvenile occurrence and fluctuations in adult catches of both species during the study period. Therefore the recent decline in the annual catches of both flounder species can be attributed to increased mortality after the emigration of juveniles from nursery grounds.

Plant-based starter diets (PD1) with supplemental zinc (Zn) at 35, 60 and 85 mg/kg (Zn35, Zn60, Zn85) were fed to rainbow trout Oncorhynchus mykiss fry for 3 weeks, and subsequent utilization of a plant-based grower diet (PD2, 60 mg/kg supplemental Zn) in the juveniles was examined in a 9-weeks feeding trial. Fishmeal-based starter (FD1) and grower (FD2, applied only to FD1 experienced fish) diets were used as references. Although growth was not improved, whole body Zn levels of fry increased as dietary Zn level increased, and the Zn85 group attained a level similar to the level of fry fed FD1. In juveniles fed PD2, fish having experienced Zn35 and Zn85 showed advanced growth, and all PD1-experienced fish showed higher feed efficiency ratio, relative to fish without experience of PD1. Whole body Zn levels of juveniles fed PD2 were lower than fish fed FD2. Within fish fed PD2, the whole body Zn level of fish having experienced PD1 tended to be lower than fish without experience of PD1. These results suggest that supplementation of Zn to a plant-based grower diet at 60 mg/kg is insufficient to fully enhance the effect of early feeding a plant-based starter diet.

It was possible to replace total fish meal in fish diet with protein source and additives, however, inclusion of plant protein sources often causes several disadvantages. Insect meals have shown their possibility as a prospective ingredient. One of the most popular insect meals, yellow mealworm (Tenebrio molitor) has been utilized for various fish species but few study was tested for red sea bream. Three iso-nitrogenous (45%) and iso-lipidic (15%) experimental diets were prepared. Control diet was formulated with fish meal (FM) and plant protein sources were used for formulating a non-fish meal diet (PP). 15% of mealworm meal was included in PP diet as a protein source and replaced the plant protein sources (MW). Only PP employed 1% bonito peptide to enhance the palatability. Duplicate groups of juvenile red sea bream (18.3 g) were fed the experimental diets for 60 days to near satiation. There were no significant differences among all dietary groups in the growth result, fish body proximate composition and digestibility. According to the result, it was possible to utilize 15% of mealworm meal in the non-fish meal diet for red sea bream.

We investigated the impact of livelihood on happiness through a comparative analysis of three communities engaged in rice farming, fishing, and fish aquaculture in the Mekong River Basin, Cambodia. Social psychology and economic experiments, the dictator and ultimatum games, determined their daily well-being. Consequently, global trends evaluated this region with modest selfishness and advanced empathy overall, regardless of their livelihood, with a negative correlation between those outputs. Furthermore, each community was distinguishable by limited stress, hyper time-discounting, and lower equity. In conclusion, each livelihood has its own characteristics that are acceptable for daily life while enhancing happiness.

A culture experiment was conducted to investigate the growth performance and flesh quality of the Pacific oyster Crassostrea gigas using hatchery early-produced spats. The spats were settled onto the collectors, and subsequently transferred to farming sites and reared with the suspended line culture system from May 2013 to February 2014, in Toba, Mie Prefecture. The early spats showed rapid shell growth during May-November 2013, from 5 to 76-85 mm average shell height, indicating that they had exceeded a marketable size. Flesh quality such as condition index and nutrient contents in soft tissue of early spats were evaluated as commercially good. Moreover, early spats had a remarkably lower ratio of individual deaths (7%), compared to natural spats (27.6-34.2%) introduced from Miyagi Prefecture, until ready to be harvested for market. This is probably due to maintaining good nutritional and physiological conditions in the early spats during summer. Thus, efficient cultivation of about six months may be possible for the Pacific oyster by using early-produced spats.