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Environmental DNA (eDNA) analysis holds great promises as an efficient and noninvasive method to monitor not only the presence and biomass of organisms but also their spawning activity. In eDNA analysis-based monitoring of spawning activity, the detection of sperm-derived eDNA is a key point; however, its characteristics and dynamics are completely unknown. The present study focuses on the persistence and particle size distribution (PSD) of eDNA derived from the sperm of Japanese jack mackerel. First, we investigated the time-dependent degradation and the PSD of sperm-derived eDNA by artificially adding sperm to fish eDNA-free seawater. Next, we kept fish in tanks and examined the changes in DNA concentration and PSD before and after spawning. The data obtained from the two experiments showed that the degradation of sperm-derived eDNA proceeded rapidly, with PSD shifting to a smaller size, regardless of the DNA region (Cyt b or ITS1). Additionally, it was shown that the nuclei and mitochondria released from sperm through degradation had a size distribution that was not simply dependent on each organelle size. The results of this study will contribute to elucidating the characteristics and dynamics of eDNA specifically during the spawning season and to further developing eDNA analysis as a powerful tool for the monitoring of spawning activity.Abstract Figure

Environmental DNA (eDNA) has been utilized for the detection of target species, and its application in biomass estimation has great potential. However, there are several factors that may hinder this application, such as species-specific eDNA emission, interspecific interactions, and the effect of feeding. In this experiment, we quantified eDNA emission in the striped jack Pseudocaranx dentex and the jack mackerel Trachurus japonicus in single- and mixed-species tanks for 10 days over a 12-day experimental period. eDNA concentrations of both of these species peaked on the first day of the experiment and later stabilized. The linear mixed-effects model revealed that the temporal factor, being highest immediately after their introduction into experimental tanks, had significant effects on eDNA concentration, whereas fish species and feeding factors did not. Rearing conditions of mixed or single species were not significant either. Taken together with previous studies, eDNA emission may be equal in fishes that have similar morphology and behavior, and their interspecific interaction may be negligible for eDNA detection.

Environmental DNA (eDNA) analysis allows noninvasive and cost-effective monitoring of macroorganisms' distribution and composition in aquatic ecosystems. Benzalkonium chloride (BAC) is an inexpensive and simple preservative for eDNA in water samples and has been used in many eDNA studies for preventing its degradation during transportation. However, its preservation performance has limitedly been evaluated by species-specific assays, targeting short fragments of mitochondrial DNA in freshwater and brackish ecosystems. Here, we examined the performance of BAC in preserving eDNA in seawater samples, targeting different fragment lengths of mitochondrial and nuclear eDNA, and community information inferred by eDNA metabarcoding. We quantified the time series changes of Japanese jack mackerel (Trachurus japonicus) eDNA concentrations in experimental tanks and inshore seawater to compare the yields and decay rates of eDNA between BAC treatments. For both tank and field samples, BAC treatment substantially suppressed the degradation of all types of target eDNA and increased the eDNA yields at the start of the experiment. In addition, we performed eDNA metabarcoding targeting fish community to compare the species richness and composition in seawater samples between BAC treatments, showing that the number of fish species in field samples hardly varied throughout a day by BAC treatment. Our findings indicate high versatility of BAC in preserving both the quantitative (copy number) and the qualitative (species richness) information on various types of aqueous eDNA in various environmental conditions. BAC should therefore be used to minimize the false-negative detection of eDNA, regardless of target genetic regions, fragment sizes, environmental conditions, and detection strategies.

Various biodiversity indicators, such as species richness, total abundance and species diversity indices, have been developed to capture the state of ecological community over space and time. As biodiversity is a multifaceted concept, it is important to understand the dimension of biodiversity reflected by each indicator for successful biodiversity conservation and management. Here we present a method to characterize and classify biodiversity indicators according to their dynamical similarity and apply the methodology to the monitoring data of a marine fish community under the intermittent anthropogenic warm water discharge. Our analysis reveals that ten biodiversity indicators can be classified into three super-groups that possibly reflect the same dimension of biodiversity. Group I (species richness) with the most robust dynamics to the temperature changes; Group II (species diversity and total abundance) showing an abrupt change in the middle of the monitoring period, presumably due to a change in temperature; Group III (species evenness) with the highest sensitivity to temperature changes. The finding suggests that selecting indicators from different super-groups might help to fully assess biodiversity response to environmental change.

Here, we investigated the effect of dietary cysteic acid on the growth performance, sulfur amino acid content, and gene expression levels of taurine-synthesizing enzymes, growth hormone (GH), and insulin-like growth factor 1 (IGF-1) in Japanese flounder Paralichthys olivaceus. Juvenile flounder (0.9 g) were fed one of four diets for 30 days: with 0.25, 0.5, or 1.0% cysteic acid (C0.25, C0.5, C1.0) supplementation and without supplementation (control). Fish in the C0.25 and C0.5 groups showed significantly better growth than those in the control group (P 0.05), the expression level of IGF-1 in C1.0 fish was significantly higher than that in controls (P

Species richness, evenness, and taxonomic diversity have been proposed and used as indicators of biodiversity. However, the governing equations for the dynamics of these indicators are often unknown and an understanding of their mechanism is limited. By employing a nonlinear forecasting-based method to the time series of 10 biodiversity indicators of a marine fish community, we aimed to detect changes in their dynamics and classify the indicators according to the level and timing of dynamic changes. The 10 biodiversity indicators were classified into three super-groups: Group I (species richness and community center of distribution) with the most unchanged dynamics; Group II (species diversity and total abundance) with dynamics that had an abrupt change in the middle, presumably due to an increase in local temperature; and Group III (species evenness) with highly variable dynamics. Finally, a hypothesis regarding the mechanistic basis of the dynamic similarities between the biodiversity indicators was proposed.

Gradual transfer experiments were conducted to see whether Japanese dace Tribolodon hakonensis (freshwater species) and Japanese black porgy Acanthopagrus schlegelii (marine species) could acclimate to high and low temperatures in natal and non-natal osmotic environments. Two groups of each species, one acclimated to their natal osmotic environment and the other to a non-natal osmotic environment, were exposed to gradual temperature changes. Under high temperature conditions, the plasma osmolality (Posm) of both species approached the osmolality of ambient water, and high mortality was observed in the non-natal osmotic environments but not in the natal osmotic environments. In contrast, there was no clear evidence that the osmoregulatory capacity of either species had declined under low temperature conditions in the natal and non-natal osmotic environments. Limited adaptation of these fishes to non-natal osmotic environments at high temperature suggests that global warming may impair their entry to non-natal habitats.

Background Environmental DNA (eDNA) analysis has been recently applied to the surveillance of species distribution and composition in aquatic ecosystems. However, most eDNA studies have used mitochondrial DNA markers, and those using nuclear DNA markers are quite scarce. Moreover, although some studies reported the availability of nuclear DNA markers for eDNA analyses, the characteristics and dynamics of nuclear environmental DNA (nu‐eDNA) of macro‐organisms remain unknown. Herein, we re‐analyzed eDNA samples described in a previously published paper to investigate the shedding and decay rates of nu‐eDNA from Japanese Jack Mackerel (Trachurus japonicus) and compared them to those of mt‐eDNA (mitochondrial environmental DNA). Materials & Methods Tank experiments consisting of 12 combinations of four temperatures and three fish biomass levels were performed, and four tank replicates were prepared for each treatment level. Before and after removing the fish from the tanks, we sampled rearing water over time to quantify nu‐eDNA copy numbers. Results & Discussion Model fitting to eDNA decay curves demonstrated that nu‐eDNA decay rates increased in higher water temperature and with larger fish biomass. The estimated shedding rates of nu‐eDNA also increased with higher temperature and larger biomass. These results were generally consistent with those of mt‐eDNA. Moreover, the ratio of mt‐eDNA to nu‐eDNA shedding and concentration decreased with larger fish biomass levels, which implied that these values may be among the potential indices for estimating the age and body size of organisms from environmental samples. Our findings contribute to the understanding of eDNA characteristics and dynamics between different DNA markers and may help us to interpret future results of eDNA surveillance.

In artificially reared flatfish, especially the Japanese flounder Paralichthys olivaceus, pigmented skin (hypermelanosis) frequently appears on the fish’s blind side after normal metamorphosis. As no practical prevention method has yet been proposed, we examined the effectiveness of a loose net placed inside the rearing tank that covers the bottom and walls like a pouch. When juveniles (standard length [SL] 6 cm) were transferred to the net-lined tank (mesh size 4 mm) before the first appearance of hypermelanosis, the pigmented area after 2 months covered about 0.5% of the blind side; this is about 1/40th of the area covered by pigment in fish reared in an ordinary tank (20%). Although the initial appearance of pigmentation in the axilla area (the area covered by the pectoral fin) was not suppressed, utilization of a larger mesh size (12 mm) decreased the expansion of pigmentation in this area. Juveniles reared in the net-lined tank were about 5–15% smaller (SL) than those reared in the ordinary tank, but their body depth:SL ratio was closer to that of wild-caught juveniles. From the results of this study, we propose that net-lined rearing tanks with larger-sized mesh are a practical method of preventing hypermelanosis in Japanese flounder aquaculture systems.

Environmental DNA (eDNA) analyses have enabled a more efficient surveillance of species distribution and composition than conventional methods. However, the characteristics and dynamics of eDNA (e.g., origin, state, transport, and fate) remain unknown. This is especially limited for the eDNA derived from nuclei (nu-eDNA), which has recently been used in eDNA analyses. Here, we compared the particle size distribution (PSD) of nu-eDNA from Japanese Jack Mackerel (Trachurus japonicus) with that of mt-eDNA (eDNA derived from mitochondria) reported in previous studies. We repeatedly sampled rearing water from the tanks under multiple temperatures and fish biomass levels, and quantified the copy numbers of size-fractioned nu-eDNA. We found that the concentration of nu-eDNA was higher than that of mt-eDNA at 3-10 μm size fraction. Moreover, at the 0.8-3 μm and 0.4-0.8 μm size fractions, eDNA concentrations of both types increased with higher temperature and their degradation tended to be suppressed. These results imply that the production of eDNA from large to small size fractions could buffer the degradation of small-sized eDNA, which could improve its persistence in water. Our findings will contribute to refine the difference between nu- and mt-eDNA properties, and assist eDNA analyses as an efficient tool for the conservation of aquatic species.

Habitat preference is thought to be genetically programmed in fishes. However, fishes can choose habitat based on their personal experience of an environment. We investigated whether the environment in which fish are raised affects habitat preference in red sea bream Pagrus major juveniles, and tested if the formed preference lasts until later life stages. Juveniles were reared in tanks with a substrate of either sand or artificial seaweed for 40 days. Naive fish were raised without either type of substrate. In the preference test, individual fish were allowed to choose either a sand or artificial seaweed microhabitat. The tested fish were then kept in barren tanks, and similar tests conducted again on days 30 and 100. Sand and seaweed treatment fish preferred the corresponding habitat immediately after the rearing treatment, whereas naive fish did not exhibit any preference. These preferences were maintained when fish were tested on day 30, but not on day 100. The present study suggests that habitat preference is acquired through the rearing environment at the nursery stage, and that this preference lasts for at least 30 days. The formation of habitat preference should help juveniles to choose an optimal microhabitat in a fluctuating environment.

The study of changes in intraspecific habitat use when closely related species coexist aids our understanding of the relationships between intra- and interspecific interactions in fishes. However, evidence of this phenomenon is few and shown in a very limited number of taxa. In particular, the stream goby [Rhinogobius flumineus (Mizuno, 1960)] is known for intraspecific variation in habitat use. We used underwater visual surveys to investigate this species’ size-dependent habitat use, both in isolation and in the presence of a sympatric congener, Rhinogobius nagoyae Jordan & Seale, 1906. A generalized linear mixed model and multiple comparison tests revealed that in the absence of R. nagoyae, R. flumineus body length was positively correlated with river flow velocity. This correlation disappeared when R. flumineus coexisted with R. nagoyae. Additionally, R. nagoyae density increased with flow velocity. Observations of interspecific territorial behavior revealed that larger individuals dominated in male–male competition regardless of species combination. Of the two species, R. nagoyae is significantly larger, so it was likely to exclude even relatively big R. flumineus individuals from fast riffles, the habitat preferred by both species. This study demonstrated that interspecific competition dominates intraspecific competition under sympatric conditions with larger related species.

Angling gear avoidance learning is a possible factor that contributes to the vulnerability of caught-and-released fish to angling. Whereas past studies suggested angling gear avoidance learning, they were based on large-scale experiments on groups of fish and unable to verify learning accurately. Details of avoidance learning are also unclear. The present study investigated angling gear avoidance learning through a series of individual-based experiments using red sea bream (Pagrus major) juveniles. Fish avoided angling gear after only one or two catches while showing feeding motivation for pellets, representing avoidance learning for angling gear. Most of the experienced fish avoided krill attached to a fishing line, but not krill alone or pellets presented near the angling gear. Experienced fish were less vulnerable to angling than control fish. Approximately half of the experienced fish kept the memory of angling gear 2 months after learning. The learning effect through the catch-and-release procedure would reduce catchability and the value of fishery-dependent stock assessments.

Environmental DNA (eDNA) analysis with species-specific primer/probe sets is promising as a tool to quantify fish abundance and distribution. Nevertheless, several factors could reduce the accuracy of this method. Here, we aimed to analyze whether intraspecific variability and diel activity rhythm affect eDNA detection in Japanese eels (Anguilla japonica). For this purpose, we performed tank experiments focusing on two points. First, we assessed the effects of base pair sequences with probe region polymorphism on eDNA detection. Next, we evaluated the influences of diel rhythm, activity, and individual differences in eDNA release rate on eDNA concentration. We examined the base pair sequences of the probe regions of 20 individuals and found genetic mismatches in two of them. The eDNA concentration was estimated to be much lower in these variants than it was in the other individuals. We conducted a rearing experiment on four non-variant individuals to explore the influences of diel activity and inter-individual differences in eDNA detection. Nocturnal eel activity was reflected in the eDNA detection but the inter-individual differences remained large. The observed weak positive correlations between eDNA concentration and activity suggest that eDNA emission is highly dependent on basal metabolism. The present study suggests that consideration of polymorphic sites at the probe region and diel activity rhythms should improve the accuracy and precision of abundance estimation through eDNA. Such fine-tuning is applicable not only for eels but also for other fishes to be targeted by eDNA technology.

Memory retention based on appetitive and aversive learning was studied in juvenile red sea bream Chrysophrys major. The fish were individually trained via appetitive and aversive learning. In general, they retained appetitive memories for 30 days, but not for 60 days. Conversely, aversive memory endured for 1 day, but not for 3 days or longer. Analyses at the individual level revealed that some fish retained appetitive memories for 60 days, whereas others lost it within 3 days; this suggests considerable variability in memory retention capacity among individual fish. The memory duration for aversive learning was remarkably short, which should be considered when releasing trained fish into the wild for stock enhancement. Furthermore, the high inter-individual variability suggests that evaluating memory retention capacity through group experiments might lead to overestimation of fishes' ability.

Environmental DNA (eDNA) consists of DNA fragments shed from organisms into the environment, and can be used to identify species presence and abundance. This study aimed to reveal the dispersion and degradation processes of eDNA in the sea. Caged fish were set off the end of a pier in Maizuru Bay, the Sea of Japan, and their eDNA was traced at sampling stations located at the cage and 10, 30, 100, 300, 600 and 1000 m distances from the cage along two transect lines. Sea surface water was collected at each station at 0, 2, 4, 8, 24 and 48 h after setting the cage, and again after removing the cage. Quantitative PCR analyses using a species-specific primer and probe set revealed that the target DNA was detectable while the cage was present and for up to 1 h after removing the cage, but not at 2 h or later. Among the 57 amplified samples, 45 (79%) were collected within 30 m from the cage. These results suggest that eDNA can provide a snapshot of organisms present in a coastal marine environment.

Environmental DNA (eDNA) analysis has successfully detected organisms in various aquatic environments. However, there is little basic information on eDNA, including the eDNA shedding and degradation processes. This study focused on water temperature and fish biomass and showed that eDNA shedding, degradation, and size distribution varied depending on water temperature and fish biomass. The tank experiments consisted of four temperature levels and three fish biomass levels. The total eDNA and size‐fractioned eDNA from Japanese Jack Mackerels (Trachurus japonicus) were quantified before and after removing the fish. The results showed that the eDNA shedding rate increased at higher water temperature and larger fish biomass, and the eDNA decay rate also increased at higher temperature and fish biomass. In addition, the small‐sized eDNA fractions were proportionally larger at higher temperatures, and these proportions varied among fish biomass. After removing the fish from the tanks, the percentage of eDNA temporally decreased when the eDNA size fraction was >10 µm, while the smaller size fractions increased. These results have the potential to make the use of eDNA analysis more widespread in the future.

Hypermelanosis of the blind side of Japanese flounder frequently occurs in post-metamorphic juveniles reared in hatcheries. Due to the morphological similarity between darkened blind-side skin and normal ocular-side skin, hypermelanosis is regarded as a transition from normal blind-side skin to that of the ocular side. Hypermelanosis appears only in rearing tanks without sand on the bottom, and its incidence tends to increase under stressful conditions; however, the specific mechanisms responsible for its occurrence have not been identified to date. In teleosts, the hypothalamus-pituitary-interrenal axis (HPI axis) is activated under stress, which then accelerates cortisol secretion. Therefore, in this study, we orally administered cortisol to Japanese flounder to examine its effect on hypermelanosis. The results of our study indicated that cortisol promoted hypermelanosis in a dose-dependent manner. In addition, the effect of cortisol was canceled out in juveniles reared in tanks with bottom sand, suggesting that the well-documented inhibitory effect of bottom sand is stronger than the stimulatory effect of cortisol (i.e., stress) on hypermelanosis. However, administration of the cortisol antagonist RU486 failed to inhibit hypermelanosis, suggesting the contribution of other stimulatory factors to this process. Therefore, our findings strongly suggest that stress-induced cortisol release has the adverse effect of promoting hypermelanosis in juvenile flounders reared without bottom sand.

Body coloration in flatfish is one of the most distinctive asymmetries in the animal kingdom, although the fundamental molecular mechanism of the pigmentation is unclear. In the dorso-ventral coloration (countershading) of other teleost fishes, ventral-specific expression of agouti signaling protein 1 (ASIP1), an endogenous antagonist of melanocortin 1 receptor (MC1R), has been reported to play a pivotal role. Contribution of ASIP1 is also suggested in the asymmetrical pigmentation of flatfish. In order to confirm the contribution of ASIP1 and further examine receptor function in the body coloration of Japanese flounder, expression levels of asip1, mc1r, melanocortin 5 receptor (mc5r), and melanin-concentrating hormone receptor 2 (mchr2) were measured in the normally pigmented area of the left side, the normally non-pigmented area of the right side, and the abnormally pigmented (exhibiting hypermelanosis) area of the right side. Measurement was also carried out under conditions of hypermelanosis stimulated by cortisol and during the transition from non-pigmentation to pigmentation in areas of hypermelanosis. Contrary to our expectations, no difference was detected in asip1 expression between pigmented and non-pigmented areas. There was also no difference between normal and hormonally stimulated pigmented conditions in areas of hypermelanosis or during the transition process. Instead, the expression levels of mc1r, mc5r, and mchr2 were consistently higher in pigmented areas, and were especially increased under hormonally stimulated conditions. In addition, expressions of these receptor genes increased prior to pigmentation in areas of future hypermelanosis. Our results suggest that MC1Rand MC5R, but not necessarily ASIP1, contribute to pigmentation and hypermelanosis in Japanese flounder. We propose a yet unknown molecular mechanism for asymmetrical pigmentation in flatfish that is distinct from that of countershading in other vertebrates.

In finfish aquaculture, fish meal is heated during the manufacturing process, which affects the digestibility and protein absorption by fish. However, manufactured fishmeal that is not heated does not undergo thermal denaturation. Few studies have investigated the effects of non-heated animal protein sources on the growth performance of fish. We investigated the effects of heated and non-heated squid and krill meal as diets for red sea bream. Five test diets were formulated to contain heated squid meal, non-heated squid meal, heated krill meal, non-heated krill meal, and fish meal as a control. Fifty fish (initial mean weight = 3.5 g) were distributed in ten 100-l experimental tanks. Fish were fed one of the five diets 3 times daily until satiation for 5 weeks. Regarding growth performance, fish fed the krill meal diet exhibited better growth than those fed squid meal during the first week of the rearing period. However, the squid meal diet group showed better performance than the krill meal diet group during the third week. Moreover, differences in body weight among treatments were greater during the fifth week. Better weight gain and thermal growth coefficient were recorded in the non-heated diet groups than in the heated diet groups. Higher feed intake was observed in the non-heated diet groups than in the heated diet groups. These results suggest higher performance of non-heated squid and krill meal as the protein source of the red sea bream diet. Further, the suitability of the diet type (e.g., squid and krill) might depend on the feeding period and/or developmental stage of fish.

The ontogenetic change in turbiditaxis (i.e. attraction to turbid waters) was examined in the larvae and juveniles of Japanese anchovy Engraulis japonicus by testing three levels of turbidity (0, 20 and 100 ppm of kaolin). Larvae of 12, 20 and 30 mm standard length (LS ) exhibited turbiditaxis to both 20 and 100 ppm of kaolin, whereas 6 mm LS larvae and 45 mm LS juveniles did not exhibit any turbiditaxis. Turbiditaxis might explain the ontogenetic habitat shift from coastal to offshore waters reported for this species.

The Japanese sea cucumber Apostichopus japonicus plays important ecological and economic roles in the coastal areas of Japan. Here, we aim to reveal the relation between habitat characteristics and density of juvenile and adult sea cucumbers in Maizuru Bay. Density of adult sea cucumbers was estimated by underwater visual surveys and bottom trawls and that of newly settled individuals by seedling collectors distributed in the bay. The physical environment of the sea bottom was evaluated from median diameter, total organic carbon, total nitrogen, carbon-to-nitrogen ratio, carbon isotope ratio, nitrogen isotope ratio, depth, slope, and distance from the coastline. Adult sea cucumber density was higher in the east side than the west side of Maizuru Bay, the former having a no-take zone for sea cucumbers. Juvenile sea cucumbers were most abundantly collected in the central part of the bay. High organic matter from the sea, short distance from the coastline, and high seabed gradient (steepness) had a positive effect on the density of adult sea cucumbers, and depth had positive effects on their body size. We suggest that sustainable management of sea cucumbers can be achieved by a combination of the establishment of no-take zones and increased protection of nursery habitats.

Environmental DNA (eDNA) can be a powerful tool for detecting the distribution and abundance of target species. This study aimed to test the longevity of eDNA in marine sediment through a tank experiment and to use this information to reconstruct past faunal occurrence. In the tank experiment, juvenile jack mackerel (Trachurus japonicus) were kept in flow-through tanks with marine sediment for two weeks. Water and sediment samples from the tanks were collected after the removal of fish. In the field trial, sediment cores were collected in Moune Bay, northeast Japan, where unusual blooms of jellyfish (Aurelia sp.) occurred after a tsunami. The samples were analyzed by layers to detect the eDNA of jellyfish. The tank experiment revealed that after fish were removed, eDNA was not present in the water the next day, or subsequently, whereas eDNA was detectable in the sediment for 12 months. In the sediment core samples, jellyfish eDNA was detected at high concentrations above the layer with the highest content of polycyclic aromatic hydrocarbons, reflecting tsunami-induced oil spills. Thus, marine sediment eDNA preserves a record of target species for at least one year and can be used to reconstruct past faunal occurrence., 堆積物の環境DNAで探る過去の出来事 --津波直後のクラゲ大発生を検知--. 京都大学プレスリリース. 2021-08-23.

To better understand the habitat choices of Japanese sea cucumber Apostichopus japonicus juveniles, we investigated their responses to various stimuli. Specifically, their phototactic, thigmotactic, and geotactic responses were tested by placing sea cucumber juveniles in appropriately designed test tanks, while their responses to turbulence were tested in tanks in the presence or absence of wind-driven waves. Sea cucumber juveniles showed clear negative phototaxis, but there was no clear evidence of thigmotaxis or geotaxis. The juveniles frequently appeared at the surface of the water in a tank lacking turbulence, but much less frequently in a tank with turbulence. We conclude that sea cucumber juveniles choose their habitat via a combination of phototaxis and their limited ability to attach to hard substrates.

The advent of environmental DNA (eDNA) analysis methods has enabled rapid and wide-range ecological monitoring in aquatic ecosystems, but there is a dearth of information on eDNA degradation. The results of previous studies suggest that the decay rate of eDNA varies depending on the length of DNA fragments. To examine this hypothesis, we compared temporal change in copy number of long eDNA fragments (719 bp) with that of short eDNA fragments (127 bp). First, we isolated rearing water from a target fish species, Japanese Jack Mackerel (Trachurus japonicus), and then quantified the copy number of the long and short eDNA fragments in 1 L water samples after isolating the water from the fish. Long DNA fragments showed a higher decay rate than short fragments. Next, we measured the eDNA copy numbers of long and short DNA fragments using field samples, and compared them with fish biomass as measured by echo intensity. Although a previous study suggested that short eDNA fragments could be overestimated because of nontarget eDNA from a nearby fish market and carcasses, the eDNA concentrations of long fragments were correlated with echo intensity. This suggests that the concentration of longer eDNA fragments reflects fish biomass more accurately than the previous study by removing the effects of the fish market and carcasses. The length-related differences in eDNA have a substantial potential to improve estimation of species biomass.

In the aquaculture of the Japanese flounder Paralichthys olivaceus, hypermelanosis, a malpigmentation condition in which the scales of a significant area on the blind side express the characteristics of those on the ocular side, remains a major concern. Since introducing sand into the rearing tank effectively suppresses hypermelanosis, the inhibitory effects of various characteristics of the surface of the tank floor were investigated. Although an inhibitory effect was observed in both tanks with a sand-image floor and a sand-pasted floor, the strongest effect was found in tanks with a dimpled floor. In addition, covering the inner surface of the tank with net also inhibited hypermelanosis. Using a commercially available corrugated plate, the inhibition of hypermelanosis was confirmed with a noted increase in this effect when combined with light coloration. Juveniles tended to situate themselves in the valley portions of the corrugated plate, suggesting that floor contact with the blind side may contribute to hypermelanosis inhibition. Further observations on the floor contact area with various floor configurations suggested that hypermelanosis is locally suppressed in the areas with floor contact. Therefore, suppression by an undulated (both dimpled and corrugated) floor is likely due to an increase in floor contact area with the blind side.

Although many fish species are known to feed on jellyfish, only a few quantitative studies on this subject have been conducted in the field or laboratory. We combined field observations and feeding experiments to quantitatively evaluate the potential feeding capability of the black scraper Thamnaconus modestus on giant jellyfish Nemopilema nomurai in a jellyfish-abundant area. Underwater observations revealed that shoals of T. modestus fed voraciously on jellyfish when the jellyfish were close to the fish’s seaweed habitat. Gut contents of field-collected T. modestus almost exclusively consisted of jellyfish. In tank trials, feeding activity of black scraper was high when light intensity was between about 1.1 lx and 50 × 103 lx. The estimated feeding speed of T. modestus on the jellyfish in the Sea of Japan when the jellyfish bloom occurred was 10.0 ± 2.0 times fish body weight per day. The results support the prediction that T. modestus and probably other coastal medusivorous fishes have a substantial capability to eliminate jellyfish blooms. Considering that they are highly dependent on seaweed bed, protection of such habitats for these medusivores may be the most cost-efficient control method for jellyfish blooms.

Although jellyfish blooms are serious nuisances for fisheries and other industries, the utilization and nutritional value of by-catch jellyfish has drawn public attention. Here, we evaluated the efficacy of feeding tiger puffer Takifugu rubripes with moon jellyfish Aurelia sp. using 20-day rearing experiments. Feeding on jellyfish had no positive effect on growth performance, although it significantly reduced neutral lipids and increased proportions of polar lipids, n−3 and n−6 highly unsaturated fatty acids (HUFAs), especially arachidonic and docosahexaenoic acids, and taurine. Supplemental feeding on jellyfish plus pellets elevated activity and responsiveness in the fish, with no effect on burst swimming speed or prevention of aggressive biting by siblings. The present research suggests that feeding on jellyfish improves body composition and some behaviours in tiger puffers. As cultured fish tend to accumulate excessive amounts of lipids from lipid-rich pellets that can induce an unhealthy condition, and behavioural deficits of hatchery-reared fish reduce survival after release in the wild, we propose the use of jellyfish as a supplemental diet in the nursery production of tiger puffer. Moreover, feeding on jellyfish can overcome deficiencies in alternative soy-material feeds by supplying phospholipid, HUFAs, or taurine.