Changes in the gonadosomatic index, nutritional quality and taste of elongate ilisha, Ilisha elongata, were examined monthly in the year 2018 at Harima-Nada, Japan. To determine nutritional quality, proximate composition and free amino acids were measured in the dorsal muscle tissue of fish. The gonadosomatic index showed that the elongate ilisha spawned from June to August. The proportions of lipid and total free amino acids in fish muscle tissue showed significant seasonal variation, with maximum lipid content (1.3%) occurring in January and maximum free amino acids (376.2 mg/100 g) occurring in November. An electric taste-sensing system revealed that umami and umami richness peaked in October and April, respectively. The results for the umami taste were consistent between the instrumental taste evaluation and a sensory evaluation. A principal component analysis revealed that the umami taste in elongate ilisha was strongest during spring and autumn and may be influenced by free amino acids, whereas the dominant taste during winter may be influenced by lipids. During the summer spawning period, lipids and free amino acids were reduced, which may result in a lighter taste.
Rainbow trout swim-up fry were first fed a fishmeal-based diet (FMD) or a plant-based (5% fishmeal) diet (LFMD) for 3 weeks and their growth performances and body compositions were compared. Two lines of fry were used: a control group that had not experienced low fishmeal diets in previous generations and a group that had been selected for growth with 5% fishmeal diets over 2 generations. Docosahexaenoic acid (DHA, as tuna oil) and phospholipid (soybean lecithin) as well as methionine and lysine were fortified to LFMD. Survival rates were similar between treatments at 99%. Large differences in growth were not observed between the fish lines, however, the growth of fish fed LFMD was consistently inferior to fish fed FMD. Whole body ash, calcium, phosphorus and zinc concentrations, and the proportions of eicosapentaenoic acid (EPA) and DHA in the whole body polar lipid, of fish fed LFMD were lower than those of fish fed FMD. In fish fed LFMD, whole body taurine and free histidine concentrations were also lower while the anserine and carnosine concentrations were higher, compared to fish fed FMD. These results suggest that plant-based diets could be used as starter feeds in rainbow trout with further supplementation of certain nutrients.
The production of kuruma prawn seedlings uses mated females, but this requires breeding and raising virus-free seedlings to adults, as wild adult populations are not easily available for broodstock. Managing virus-free aquaculture and genetic diversity will require careful study of the breeding pattern, a task still incomplete. But detailed breeding behavior and breeding contributions in the kuruma prawn are still unclear. In this study, we focused on the exuviation interval of mating pairs, and reproductive ecology. Multiple males and females were reared in aquaria, and individual identification was via microsatellite DNA polymorphism analysis using the exuvia and, in males, the spermatophore and exuvia. Rearing tests conducted in four tanks revealed exuviation intervals from 20-40 days, with 57%-77% of females mated. Sexes did not differ in exuviation interval. Additionally, female exuviation interval was not affected by presence or absence of the spermatophore. It is clear this is a multiple mating pattern, with males and females mating with different individuals as well as the same individuals.
The honeycomb grouper, which is a lunar-synchronized spawner, is an important fishery target species in Okinawa. In this study, to clarify the spawning characteristics of this species, we observed mass spawning (4 or 6 males and 20 or 22 females in 3-t tank) and single-pair spawning (one male and one female in a 200 l tank) using rearing experiments. The mass spawning results indicated spawning started 3 days after the full moon and continued for several days. However, the single-pair spawning results showed the starting date of spawning varied between individuals. The spawned eggs in 1 female were confirmed for 4 consecutive days (20,000–70,000 per day). The egg buoyancy, fertilization, and hatching rates were high in single-pair spawning until day 3. However, the number of spawned eggs, buoyancy, fertilization, and hatching rates at day 4 were extremely low, suggesting that actual spawning occurred for 3 days.
Characteristics of the population dynamics of bluegill Lepomis macrochirus were studied in the south basin of Lake Biwa in 2009-2018. The bluegill CPUE in set nets during April to July fluctuated in the range of 1.9-30.2 kg/set net/day. According to multiple regression analysis, the largemouth bass (Micropterus salmoides) CPUE was detected as a negative factor, and the previous year’s water transparency reflecting the state of submerged macrophyte biomass was detected as a positive factor in relation to changes in bluegill CPUE. It was considered that the abundance of submerged macrophyte, which serve as hideouts for small bluegill, and the abundance of largemouth bass as predators were related to bluegill abundance. In addition, the size composition, and the nutritional condition of age-0 bluegill in the fall fluctuated annually, so it is necessary to clarify the relationship between the size and condition of the fish and overwinter mortality.
Payment of fishing license fees by anglers who targeted five species, namely ayu Plecoglossus altivelis altivelis, white-spotted charr Salvelinus leucomaenis, masu salmon Oncorhynchus masou，rainbow trout O. mykiss, and Japanese smelt Hypomesus nipponensis, in rivers and lakes was investigated using online survey data with the aim of improving the administration of inland fishery cooperatives in Japan. Totals of 89.5% and 65.0% of the male and female anglers recognized the payment of the fee as obligatory. Totals of 74.8% and 52.5% of the male and female anglers paid the fee. The ratio of recognition and the ratio of payment were both found to be lower among female anglers than among male anglers. For male anglers, the ratio of recognition and the ratio of payment were both found to be lower among younger anglers than among older anglers. For male anglers, no significant differences were observed both in the ratio of recognition and the ratio of payment among the anglers targeting these species. The main reasons for nonpayment by male anglers were that they assumed that they were not required to pay the fee, and they were unaware of the rule that fishing in rivers and lakes attracted a fee.
Several stock characteristic values required to assess the population dynamics of red tilefish offshore the Shimane Peninsula were investigated from July 2003 to March 2019. Between July 2015 and November 2018, 663 individuals were collected as samples from Koizu Fishing Port. For all samples, the sex was identified; total length, body weight and gonad weight was measured; and age was estimated using otolith. The spawning season was estimated to be mid-May to end of September, and peaked in late September. Ages assessed were 2-14 years old for females, 2-16 for males, and the estimated growth curve equation was as follows; Lt=346 (1-exp-0.542(t+0.045)) for females and Lt=472 (1-exp-0.328(t+0.134)) for males. The total length composition by sex from April 2006 to March 2019 was estimated from catch records by size brand using these results. Each piece of information is useful for performing VPA (Virtual Population Analysis) and evaluation of the seed release effects.
We examined the dietary composition and prey size of larval yellow goosefish Lophius litulon, based on 77 preflexion, 1 flexion and 6 postflexion larvae. Preflexion larvae fed on rotifer Synchaeta sp. and copepod eggs. The flexion larva fed on copepod copepodites and chaetognath Sagitta elegans. The postflexion larvae fed on S. elegans. The range of prey size ratio (second largest length/mouth depth) was stable through the larval stage (10-35%) and 97.8% of prey items were included in this range. Drastic increase in prey size occurred with the rapid increase of gape size between preflexion and postflexion stages.
The response of early development of bamboo sole to gradual salinity drop (34, 24, 14 psu) with different exposure times (1, 3, 6 h) to mimic natural conditions during spawning season were investigated in a series of small-scale laboratory experiments. An experimental set-up containing six replicates per treatment using plastic microplates were each stocked with 78 gastrula stage eggs and exposed to gradual salinity drop conditions following recovery after certain periods of exposure time. Onset hatching time was significantly prolonged by 6 h exposure, irrespective of salinity level. Hatching success rate, percentage of morphological abnormality and survival rate at 4 days post-hatch (dph) showed no individual main effects and interaction between salinity and exposure time. Although the total length of newly hatched larvae (NHL) and yolk sac volume showed significant interaction, NHL pre-anal length and final total length at 4 dph showed no interaction but individual main effects of salinity and exposure time.
To assess the commercial catch and fishing ground of rabbitfish Siganus fuscescens in Harima-nada, eastern Seto Inland Sea, Japan, we investigated 11 years of recent data from seven fishery cooperatives in the Okayama, Kagawa and Tokushima Prefectures. The annual catch variations in the three prefectures were similar from 2004-2014, and catches were high in 2011-2014, especially in 2011. The fish were caught from May to December. The center of the fishing ground with the most abundant fish shifted from western Harima-nada (Okayama Prefecture) to eastern Harima-nada (Tokushima Prefecture) near the Kii Channel with the warm water from September to November 2011. The fish most likely could not survive over the winter in Harima-nada due to the low water temperature (approximately 8°C). This suggested that the fish might move from Harima-nada to the Kii Channel from November to December for wintering.
The frequency of fish with a deformity in the inter-nostril epidermis, the permanency of this abnormality, and its effects on survival and growth of the artificially produced blacktip grouper Epinephelus fasciatus were examined to clarify its potential as an external marking in stocking effectiveness surveys. The possibility of producing a percentage of ca. 90% of deformed inter-nostril epidermis under larviculture was inferred by the use of condensed freshwater Chlorella vulgaris containing n-3 HUFA for supply into larval-rearing water and supplement of nutritional enrichment of rotifer. No significant differences were observed in growth and survival rates of fish with deformed and normal inter-nostril epidermis during two years. The inter-nostril epidermis usually forms completely by a total length of about 12 mm. Fish with incomplete inter-nostril epidermis at this size did not show any subsequent improvement and could still be clearly identified by this deformity even after 2 years. These results represent that this marking method is a potential marking for stocking effectiveness surveys with blacktip grouper
To improve the precision of chum salmon stock assessments, this study investigated the status of adult chum salmon capture for hatchery broodstock in rivers on the Japan Sea side of northern Honshu, Japan. Hatchery weir, which exhibits relatively low annual shifts in fishing effort, and other fishing techniques such as casting net and hook fishing, which exhibit relatively large annual variations in fishing effort, were applied on 65% and 35% of the 66 rivers, respectively. The proportions of chum salmon caught in rivers using other fishing techniques from 2014-2016 were low (12.4%-15.7%) but varied considerably across the prefectures (0%-30.4%). The number of users of other fishing techniques decreased considerably in the studied rivers. The results imply that the stock levels estimated based on the catch in the rivers where other fishing techniques were used could be underestimated.