Acoustic evaluation of the vertical distribution of dwarf ayu Plecoglossus altivelis altivelis in Lake Biwa

SUMMARY: The vertical distribution of dwarf ayu Plecoglossus altivelis altivelis in the pelagic waters of Lake Biwa was evaluated from June to September in 1995–97 from eight acoustic surveys using a quantitative echosounder. In each survey, echoes from a depth range of 3 m to the sea bottom were collected at a station every 2.7 s for 24 h together with measurements of vertical profiles of water temperature and chlorophyll-a. The ayu's echoes were identified using an underwater video camera. The ayu were observed near the maximum chlorophyll-a depth and above the thermocline. Their density was highest at depths of 4–11 m with 2–4 individuals/m ³ and was almost zero below 20 m. Echo signs were recorded as having a frequent duration of more than 1 h at night, whereas were of a shorter duration in the day. The fish stay in the epilimnion during the day without any clear vertical migration, but are distributed more uniformly at night. The advantages of remaining in the epilimnion are discussed in terms of food availability and predator avoidance.     

碧流河水库陆封型香鱼生物学特性的演变

1982年碧流河水库建成后，被陆封在水库中的香鱼（Plecoglossus altivelis)经10余年适应后，其生物学特性发生了很大变化，其中一部分个体春季水温上升后暂不进入河道，而留在水库中生活，至7月份才进入河道，与刚陆封时不同，在其植物食性阶段，消化道内有大量浮游植物出现，繁殖期也由9－10月提前到8－9月，性腺发育速度明显加快，个体进一步变小。     

Histopathological studies of bacterial haemorrhagic ascites of ayu, Plecoglossus altivelis (Temminck & Schlegel)

A histopathological study was carried out on ayu, Plecoglossus altivelis, with bacterial haemorrhagic ascites. The fish were obtained from culture ponds in Wakayama Prefecture in 2003. The causative agent was identified as Pseudomonas plecoglossicida by a slide agglutination test using anti-P. plecoglossicida FPC941 serum. Histopathological studies revealed lesions in spleen, kidney, liver, intestine, heart and gills. Lesions in the spleen and haematopoietic tissue were prominent and invaded by P. plecoglossicida. Necrotic lesions accompanied by haemorrhage, fibrin deposition and oedema occurred in the splenic pulp and sheathed tissue, and in the kidney. The liver also had necrotic lesions and abscess formations. However, the intestine, heart and gills were only slightly invaded by P. plecoglossicida. No lesions or bacteria were observed in the brain.     

Diet shift of larval and juvenile landlocked Ryukyu-ayu <Emphasis Type="Italic">Plecoglossus altivelis ryukyuensis</Emphasis> in the Fukuji Reservoir,Okinawa Island,Japan

ABSTRACT:   The diet shift of larval and juvenile Ryukyu-ayu Plecoglossus altivelis ryukyuensis was investigated for the landlocked population stocked in the Fukuji Reservoir, Okinawa Island. Gut contents of 274 specimens (5.1–31.4 mm in body length) including 31 yolk-sac larvae (5.7–8.0 mm) were examined. The diet of larval and juvenile landlocked P. a. ryukyuensis was mainly composed of zooplankton such as dinoflagellates, rotifers, copepods, and cladocerans. Feeding incidence was 48% in total and it gradually increased with growth. Diet analysis clearly demonstrated a diet shift during the larval and juvenile periods. For yolk-sac larvae and 5.0–19.9 mm fish, the diet was dominated by dinoflagellates Gymnodinium sp., and Peridinium sp., both numerically and by frequency, followed by rotifers that mainly consisted of Polyarthra spp. Diet breadth (Levin's standardized index B_A) diversified with growth, and feeding on copepods and cladocerans gradually increased in 10.0–14.9 mm fish. Diet overlap (Morisita's index: C) showed that diet drastically changed in 20.0–24.9 mm fish with a specialization on cladocerans Bosminopsis deitersi and Diaphanosoma brachyurum . This diet shift probably corresponded with an increase of feeding ability resulting from an enlargement of the mouth, increased swimming ability, and the development of sense organs.     

Habitat shift of ayu Plecoglossus altivelis altivelis in early stages from waters adjacent to the bank to the center of flow in the Shimanto Estuary

ABSTRACT: To clarify the habitats of the ayu in the early stages of development in the Shimanto Estuary, the size and hatching date of the larval and juvenile ayu were examined. Sizes of larvae and juveniles were larger in the center of the flow area than in the waters adjacent to the estuary's banks, which suggests that larvae shift habitat to the center of flow from the waters along the estuary's banks. This shift began at approximately 20 mm body length. However, the resident term in the waters adjacent to the estuary's banks changed with the hatching dates; that is, the early and late-hatched larvae became short and long-term residents, respectively. A similar phenomenon was found also in the juveniles collected in the freshwater section that were migrating upstream. The growth rates of larval ayu in the waters adjacent to the banks tended to decrease with hatching dates. The fluctuations in growth rate with hatching date appear to be a factor leading to the variation of resident term.     

Food habits of great cormorant Phalacrocorax carbo hanedae at Lake Biwa, Japan, with special reference to ayu Plecoglossus altivelis altivelis

ABSTRACT:   The recent increase in numbers of individuals of a piscivorous bird, the great cormorant Phalacrocorax carbo hanedae , around Lake Biwa has affected the commercial catch of ayu Plecoglossus altivelis altivelis . The food habits of the bird were studied based on 66 stomachs in 1998 and 2001–2002, and a seasonal variation in food contents was observed. The proportion of ayu in stomachs increased after April, and became most dominant during July and August (0.62 in proportion of weight, 0.85 in frequency of occurrence). In contrast, ayu was not found in the stomachs after late October. The increase of ayu in individuals and density in the surface layer of the pelagic zone of the lake during spring was considered as a trigger for the change of the food habits of the birds. The feeding impact of great cormorant on ayu in Lake Biwa was estimated to be approximately 46 t during April to June, and 310 t in July and August. During the main period for the commercial ayu catch (November to the following June), the estimated mass consumed by the bird is much smaller in comparison with the commercial catch.     

Comparison of habitat use during early life stage between ayu Plecoglossus altivelis and ice goby Leucopsarion petersi along the sanriku coast of Japan, as determined from otolith microchemistry

ABSTRACT:   The habitat use and migration of ayu Plecoglossus altivelis was compared to that of ice goby Leucopsarion petersi using otolith microchemistry analysis. Both species were collected along the Sanriku Coast. Otolith Sr : Ca ratios of ayu fluctuated strongly along the life history transect in accordance with the migration (habitat) pattern from fresh water to sea water. The Sr : Ca ratios of the center region averaged 3.2 × 10⁻³; thereafter, the ratios increased sharply, averaging 9.2 × 10⁻³, and were maintained at the higher levels until the outermost regions. By contrast, the Sr : Ca ratios of ice goby showed consistently high values along the life history transect of the otolith, ranging 9.0 × 10⁻³ in the center to 9.2 × 10⁻³ in regions outside the center, with further increases around the otolith edge. These findings indicate that ayu shows a typical amphidromous migratory pattern, while ice goby does not show the anadromous migratory pattern previously reported. The use of a freshwater environment during the early developmental stage in ice goby along the Sanriku Coast was less prominent than that of ayu in the same region.     

Effects of dietary protein and lipid level on growth and body composition of juvenile ayu (Plecoglossus altivelis) reared in seawater

A 3 (protein levels, 380, 460 and 520 g kg^–1 diet) × 2 (lipid levels, 65 and 140 g kg^–1 diet) factorial experiment with three replicates was conducted. Weight gain, feed efficiency and daily feed intake were not significantly affected by dietary protein level, but were by dietary lipid level. Weight gains of fish fed 65 g lipid kg^–1 diet were significantly, or slightly, higher than for 140 g lipid kg^–1 diet at all protein levels. Daily protein intake was significantly affected by both dietary protein and lipid levels ( P  < 0.002). Daily lipid intake was not significantly affected by dietary protein level, but was by dietary lipid level ( P  < 0.001). Protein efficiency ratio was significantly affected by dietary protein level ( P  < 0.02), but not by dietary lipid level. Protein efficiency ratio tended to improve with the decrease of dietary protein level at the same lipid level. Moisture, protein and lipid contents of whole fish were significantly affected by dietary lipid level ( P  < 0.01). Increased dietary lipid did not improve growth or feed efficiency, but increased body fat deposition. It was concluded that the optimum dietary protein and lipid level for growth of juvenile ayu may be 380 and 65 g kg^–1 diet, respectively, when fish were fed to satiety three times daily in seawater.     

Annual changes in the hatching period of the dominant cohort of larval and juvenile ayu Plecoglossus altivelis altivelis in the Shimanto Estuary and adjacent coastal waters during 1986–2001

ABSTRACT:   Larval and juvenile ayu from seven year-groups (1986, 1987, 1992, 1995, 1996, 1999 and 2000) were collected in the Shimanto Estuary and adjacent coastal waters. The present study focuses on the variation among year-groups in the hatching period estimated by age determination using the otolith. The hatching period of the dominant cohorts in the 1986, 1987 and 1992 year-groups occurred from late October to mid-November. A delay of the hatching period of the dominant cohort was observed from the 1995 year-group, which occurred in late November to early December, and the dominant periods in the 1996, 1999 and 2000 year-groups were observed from early to late December. It appears that the delay of the hatching period of dominant cohorts was not due to a delay of spawning, but a high mortality of early hatched larvae. The water temperature in autumn in the coastal waters adjacent to the Shimanto River has tended to rise over the past 20 years and this trend was especially notable in the late 1990s. The delay in the hatching period of the dominant cohort observed from the 1995 year-group was likely to be related to the rise in seawater temperatures in autumn.     

Changes in drug resistance of Vibrio anguillarum in cultured ayu, Plecoglossus altivelis Temminck and Schlegel, in Japan

Abstract. Two-hundred-and-fifty-nine strains of Vibrio anguillarum isolated from diseased cultured ayu, Plecoglossus altivelis in various districts of Japan from 1974 to 1977 were studied for their sensitivity to 12 different chemotherapeutic agents: chloramphenicol (CM), tetracycline (TC), streptomycin (SM), kanamycin (KM), aminobenzyl penicillin (ABP), colistin (CL), nalidixic acid (NA), oxolinic acid (OA), piromidic acid (PA), furazolidone (NF), sulpharaonomethoxine (SA) and trimethoprim (TMP). One-hundred-and-thirty-four of the strains were additionally tested for sensitivity to O/129(2,4-diamino-6,7-diisopropyl pteridine) and ormethoprim (OMP). All strains were susceptible to SM, KM and ABP and also highly sensitive to CL. A correlation of resistance was found among the chemically-related drugs NA, OA and PA, and TMP, OMP and O/129, Nine strains were sensitive to all the drugs tested. The remaining 250 strains were resistant to various combinations of six drugs (CM, TC, NA, NF, SA and TMP). In particular, strains resistant to NA and NF (20.1%), to CM, TC, SA, NA and NF (29.7%) and to all six drugs (28.2%) were frequently detected. NA- or NF-resistant strains have increased since 1974 and TMP-resistant strains have increased since 1976. Transferable R plasmids were detected in 165 out of 250 resistant strains. The most common type of R plasmid determined resistance to CM, TC and SA. These multiple drug-resistant strains with transferable R plasmids were isolated from many different districts.     

Effects of bicarbonate ions and pH on acquisition and maintenance of potential for motility in ayu, Plecoglossus altivelis Temminck et Schlegel (Osmeridae), spermatozoa

The effects of extracellular ions on the acquisition and maintenance of the potential for motility in the spermatozoa of the ayu, Plecoglossus altivelis Temminck et Schlegel (Osmeridae), were examined. Testicular spermatozoa and milt spermatozoa were obtained from fully matured males and diluted with buffered solution (BS, 20 mM HEPES–NaOH, pH 7.5). Testicular spermatozoa showed a significantly low rate of motility (0.8 ± 0.4%), whereas milt spermatozoa showed a high rate (89.4 ± 2.1%). The spermatozoa were incubated with various isotonic media for 2 h, diluted with BS, and changes in the rates of motility were then compared. When incubated for 2 h with artificial seminal plasma (ASP), corresponding in terms of ionic constituents to seminal plasma buffered at pH 8.0, both spermatozoa showed a high rate of motility. Testicular spermatozoa acquired and milt spermatozoa maintained the potential for motility in response to the HCO₃^– ion concentrations (between 0 and 20 mM) in the ASP. The differences in the pH of the ASP had a significant effect on the acquisition and maintenance of the potential for motility, and spermatozoa showed the highest rate of motility with the ASP at pH 8.0 and 8.5. These results suggest that the quality of milt in the ayu can be regulated by controlling the concentration of bicarbonate and the pH of the incubating media.     

Development of new self-feeding system for mass rearing of ayu <Emphasis Type="Italic">Plecoglossus altivelis altivelis</Emphasis> under artificial and natural light-dark cycles

Self-feeding of grouped ayu Plecoglossus altivelis altivelis was examined under an artificial light-dark (LD) cycle and natural day length using a trigger with a photosensitive sensor. In Experiment 1, approximately 15 juvenile fish (mean body weight, 0.6 g) were kept in 60-L glass tanks (four tanks) under LD 12∶12 (lighting period 06∶00–18∶00 hours) and self-feeding and locomotor activities were simultaneously recorded. Self-feeding was detected within one day and feeding activities were strongly synchronized to the LD cycle, with almost strictly diurnal feeding (98.2%). Although locomotor activities in the upper part of the tank were also greatest during the light phase (76.8%), the activity profiles were different from those of self-feeding, suggesting the validity of the photosensitive sensor. In Experiment 2,25 fish (mean body weight, 15 g) were kept in outdoor 1000-L tanks (four tanks) under natural day length and water temperatures in May, and self-feeding was recorded from mid-August. Stable self-feeding was observed from early September in all the tanks. Nearly all food demands were diurnal, and usually crepuscular. Somatic growth was seen in all tanks. These results demonstrate that the self-feeding system using a photosensitive sensor is applicable for grouped juvenile ayu as small as 0.6 g and adult fish reared under natural conditions.     

Spatial and seasonal variations of radiocesium concentrations in an algae-grazing annual fish,ayu Plecoglossus altivelis collected from Fukushima Prefecture in 2014

Fisheries Science - To elucidate spatial and seasonal variations of radiocesium (134Cs and 137Cs) concentrations in ayu Plecoglossus altivelis, amphidromous fish samples were collected both from...