We studied the effects of stocking density and multiple factors on the aggression and cannibalism of Pacific bluefin tuna, Thunnus orientalis (Temminck & Schlegel) larvae. If the appropriate food was abundant and the fish were uniform in size, the frequency of aggressive behaviour and cannibalism was unaffected by the increase in stocking density from low- (0.1 fish/l) to high-density (0.5 fish/l). However, high-density conditions significantly increased aggressive behaviour when food was scarce. Aggressive behaviour increased only slightly due to fish size differences, and there was no increase due to high-density conditions. When fish of different size were present under a restricted feeding regime, aggressive behaviour significantly increased compared to the case of restricted feeding alone and was independent of stocking density. Cannibalistic mortality was more likely for small-sized fish, which was significantly increased by high-density. In particular, when fish size differences and feeding shortages overlapped, cannibalism increased sharply as does aggressive behaviour. Even if the fish are reared at high-density, by preventing the shortage of appropriate food and the spread of fish size difference, aggressive behaviour and cannibalism is suppressed. As the result, rearing at least 0.5 fish/l could improve production efficiency.

The young-of-the-year (age 0) Japanese eels, Anguilla japonica, initially accumulate in upper tidal reaches and so those areas are important for conservation by providing initial settlement areas for the young eels recruiting to coastal and inland growth habitats. Habitat use of otolith-aged young-of-the-year eels was examined and compared to that of the older (age 1 or more) eels within the upper tidal limit of a small river. The young-of-the-year eels were locally distributed along the slow-current, shallow left shoreline, but older eels were distributed in deeper and various current habitats. The young-of-the-year eels were hidden within substrates of ~10 cm grain size that was finer than the stone refuge substrates used by the older eels. Our results show a narrower habitat use niche of the young-of-the-year eels at the upper tidal reaches and indicate the importance of conservation efforts targeting those areas.

We investigated the molecular weight (MW) range that stimulates gastric secretion by fractionating skipjack broth (SJT-Br), a gastric secretion stimulant, by MW. Pepsin-like protease (PLP) activity and secretion induced by fractions of over 14,000 Da, 6,000-14,000 Da, 3,500-6,000 Da, 2,000-3,500 Da, 1,000-2,000 Da, and 100-1,000 Da were determined by infusions into the ex vivo stomachs of black rockfish, and compared with artificial seawater (ASW, control) and SJT-Br. The results showed that the fraction over 14,000 Da did not stimulate the ex vivo stomach of black rockfish, and the activity and secretion of PLP were significantly lower than SJT-Br and comparable to ASW. In contrast, rapid stimulation of gastric secretion was observed with fractions of 2,000-3,500 Da and 3,500-6,000 Da. Thus, SJT-Br appears to contain bioactive peptides, which is consonant with numerous reports finding that fish protein hydrolysate (FPH) has bioactive properties. Future studies should further examine the gastric stimulant effects of these MW fractions in live fish, in order to evaluate their growth-promoting effects in vivo. Boiling broth and FPH discarded during fish processing may be valuable sources of bioactive molecules that could contribute to developing highly efficient aquaculture fish feeds.

In recent years the shortening of the fronds of cultivated nori Pyropia yezoensis has become a problem in the Seto Inland Sea off Hyogo Prefecture. In this study, we visually confirmed that feeding by black sea bream Acanthopagrus schlegelii is one of the causes of the shortening of cultivated nori fronds by using a time-lapse camera at a nori cultivation facility. Black sea bream were observed feeding on cultivated nori in a wide area throughout the prefecture, forming groups of several tens of fish that migrated to nori cultivation areas and fed on the nori for several minutes to several tens of minutes. In various nori farms the frequency of appearance tended to be higher in the afternoon than in the morning, suggesting that they may avoid the working hours of nori growers (mainly in the morning). Furthermore, they tended to appear more frequently on the shore side than on the offshore side of the cultivation facilities, which was consistent with the feeding damage situation.

Amphioctopus fangsiao is distributed in the coastal waters of Japan, the Korean Peninsula, and China. A. fangsiao is an important fishery stock in the Seto Inland Sea, Japan. Recently, the catch has greatly reduced, and stock enhancement and aquaculture are desired for the stable production. To obtain information on the fecundity, egg size and spawning period, I individually reared eleven brooding females ranging from 11.0 to 150.7 g in body weight (BW) from March 5 to July 2, 2021. In this experiment, the start and end of egg-laying was observed on 25 March and 30 May, respectively. The egg-laying period of individuals ranged from 7 to 61 days, with a mean of 34.8 days and during the egg-laying period each of the octopus individuals laid between a total of 42 to 262 eggs. The relationship between the number of eggs laid (TE) and BW was expressed as follows: TE = 5.22 BW^0.804. Egg length ranged from 6.2 to 10.0 mm with a mean of 8.1 mm (n = 1479). Generalized linear models suggested that the body weight and water temperature had positive and negative effects on egg length, respectively.

We investigated the effects of ultra-fine bubbles (UFBs) on the water quality in a closed recirculating system for larval tilapia, Oreochromis niloticus, and the UFBs’ effects on the water microbiome. The levels of inorganic nitrogen and phosphorus and chemical oxygen demand in the UFB group were maintained at lower level than those in the control group. A metagenomic analysis revealed that the ratio of Bacteroidetes increased, and that of Fusobacteria decreased in the rearing water in the UFB group compared to the control group. The study results thus indicate that the UFBs improved the water quality and changed the microbiome in the rearing water.