Diel vertical migration of myctophid fishes (Family Myctophidae) in the transitional waters of the western North Pacific

The diel vertical migration patterns of adult myctophid fishes were determined in the transitional waters of the western North Pacific off Japan, using day–night sampling from 20 to 700 m depths with a commercial otter trawl in the summer of 1995. A total of 12 species belonging to 9 genera were collected. Four patterns were recognized in the diel vertical migration of 11 of the 12 species. (1) Migrants showing clear day–night habitat separation with peak abundance above 200 m at night: Symbolophorus californiensis, Tarletonbeania taylori, Notoscopelus japonicus, Diaphus theta, Ceratoscopelus warmingi, and Diaphus gigas. (2) Semi-migrants, in which part of the population often remains in the daytime habitat at night. The distribution depths of migratory and nonmigratory individuals do not overlap: Stenobrachius leucopsarus . (3) Passive-migrants, in which there is no separation of day–night habitats, but the upper limit of daytime distribution depth shifts to a shallower layer at night, probably as the fish follow migratory prey: Lampanyctus jordani . (4) Nonmigrants: Stenobrachius nannochir, Lampanyctus regalis (> 140 mm SL), and Protomyctophum thompsoni . The day–night habitat temperature ranges are also given for the 11 species. No remarkable east–west differences were seen in the vertical migration patterns compared with previous knowledge of eight of these species in the eastern Pacific. The diel migration patterns are newly described for three other species endemic to the western Pacific. The standing stock of myctophids in the study area was conservatively estimated at 18.5 ± 4.7 g m⁻² (avg. ± SD).     

Ontogeny of anti-predator behavior in hatchery-reared jack mackerel Trachurus japonicus larvae and juveniles: patchiness formation, swimming capability, and interaction with jellyfish

ABSTRACT:   Laboratory rearing and behavioral observations of larval and juvenile jack mackerel Trachurus japonicus were conducted to elucidate their life-history traits with emphasis on the interaction with the moon jellyfish Aurelia aurita . Jack mackerel were raised from naturally spawned fertilized eggs and they attained 10.3 ± 0.7 (mean ± standard deviation) mm in body length (BL) by 30 days post hatching (dph) and 26.6 ± 1.8 mm BL at 48 dph raised at 19.3–25.0°C. Patchiness (dense aggregation) was confirmed from 5–16 dph (2.9–5.9 mm BL) during the day and from 9–48 dph (4.0–27 mm BL) at night. Cruise and burst swimming speeds were 1.5–1.9 BL/s and 16–24 BL/s, respectively, during larval and juvenile stages, in experimental tanks. Although escape performance from jellyfish was not different between the larvae of jack mackerel and chub mackerel Scomber japonicus , both species showed better survival compared to red sea bream Pagrus major larvae of a similar size. Although jack mackerel were slow in growth, their slow cruise swimming may reduce the chance of encountering predators and fast burst swimming may help escape from gelatinous predators.     

Spatial behaviour of young-of-the-year northern pike (Esox lucius L.) in a temporarily flooded nursery area

Abstract –  Unlike adult northern pike ( Esox lucius ), small-bodied young-of-the-year (YOY) individuals cannot be tracked by conventional telemetry, and virtually no data exist on their spatial behaviour. Here, we monitored 192 individuals released into a nursery area (0.47 ha, subdivided into 253 cells of 4 m × 5 m) that dried out progressively using a portable passive integrated transponder detector. Among them, 66 spent more than 5 days in the nursery area and were localised at least twice, allowing spatial behaviour analyses. The mean radial distance moved averaged 14.3 m (±8.4 SD, range 2.2–41.0) and the average daily movement was 8.0 m·day⁻¹ (±5.3 SD, range 0.6–31.1). Pike abundance in a cell significantly correlated with the abundance observed in adjacent cells within 32 m, confirming a patchy distribution. A selective use of the deepest area was measured while no effect of vegetation cover was observed at the cell scale. The existence of YOY pike aggregations and the positive correlation between water depth and their spatial distribution might, however, facilitate predation and/or cannibalism during this critical period. This also suggests that the choice of release habitats requires particular attention during restocking procedures.     

Distribution patterns of larval myctophid fish assemblages in the subtropical–tropical waters of the western North Pacific

We examined larval myctophid fish assemblages and their distribution patterns, based on discrete depth sampling. Samples were collected at 19 stations along a transect that crossed the subtropical–tropical waters of the western North Pacific. In total, we collected 27 189 larvae of 40 myctophid species or types, belonging to 15 genera. Three assemblages were recognized, based on their species composition: Kuroshio Axis (KuA), Kuroshio Countercurrent (KCC), and Subtropical Countercurrent–North Equatorial Current (SCC–NEC) assemblages. The distributions of these assemblages were well defined by the positions of the KuA and the Subtropical Convergence. Each species had a specific distribution depth, and none showed diel vertical migration. Larvae of the subfamily Lampanyctinae were distributed in shallower waters (0–50 m) than larvae of the subfamily Myctophinae (50–150 m). Larvae of the same species were distributed at lower depths in the SCC–NEC area than in the KCC area. This corresponded to the abundance of chlorophyll a, which would reflect abundance of prey organisms such as copepod nauplii and copepodites.     

Spatial distribution and feeding habits of Pacific cod (Gadus macrocephalus) larvae in Mutsu Bay, Japan

The spatial distributions and feeding habits of Pacific cod ( Gadus macrocephalus ) larvae, and the spatial distributions of copepod nauplii and copepodites, their main prey, were examined in Mutsu Bay from February to March during 1989–92. Yolk-sac larvae were caught at 30–45 m depth at the bay mouth. Larvae without yolk were collected at 8–45 m depth at the bay mouth and the inner part of the bay, and large larvae were chiefly found in the bay. This geographical pattern in larval size may have been because of transport to the inner part of Mutsu Bay by the Tsugaru Warm Current. The dominant taxa of copepod nauplii and copepodites in the diet and the environment changed each year. Larvae fed mainly on abundant taxa in the environment, suggesting that larvae are opportunistic feeders. Nauplii and copepodites were abundant in the bay, especially in 1992. Copepodites were slightly more abundant in the diet of cod larvae in 1992 than in 1991, but this difference was smaller than in the environment. In addition, larvae with empty digestive tracts were scarce in 1991 and 1992. Prey concentrations in the bay in 1991 and 1992 seem to have been high enough to sustain most Pacific cod larvae.     

Spawning habitat of the Atlantic menhaden in Onslow Bay, North Carolina

The Continuous, Underway Fish Egg Sampler (CUFES) was used to sample pelagic eggs of the Atlantic menhaden ( Brevoortia tyrannus ) from 3-m depth off North Carolina in winter 1993–94 and 1994–95. Simultaneous measurements were made of temperature, salinity, and the concentration of chlorophyll a . The maximal concentration of eggs was 346 eggs m⁻³. Eggs were highly aggregated in patches which occurred between the Gulf Stream and mid-shelf fronts (17–23°C, 36.0–36.4‰). Unexpectedly, eggs were found almost exclusively in water of 20–60 m (mode 20 m) bottom depth. Thus, spawning appears related to bathymetry as well as hydrography. Variograms for egg concentration indicated a mean (± SE) patch scale of 3.6 ± 1.7 km and a high degree of spatial variance explained by CUFES sampling. Lagrangian modelling of particles moving in response to tides, winds, and a prescribed flow from the north indicated that the region of observed, maximal occurrence of eggs is favourable for the retention of eggs and larvae on the shelf adjacent to inlets used to enter nursery areas.     

Fish larvae distribution off Mejillones Peninsula (northern Chile) during a coastal upwelling event in Spring 1999: interactions with the cold upwelling plume

We examined the interaction between vertical and horizontal distribution of fish larvae off Mejillones Peninsula (23°S), northern Chile, under conditions of active coastal upwelling. An oceanographic survey covered spatial variability in temperature, chlorophyll- a (chl a ), dissolved oxygen, salinity and water density. Fish larvae were sampled during daytime and nighttime periods through two consecutive days in four stations: two inside and two outside of a well-developed upwelling plume, and at three depth strata: 0–20, 20–80 and 80–200 m. Eighteen taxa were analysed, of which the Myctophidae Diogenychthys atlanticus , Diogenichthys laternatus , and the anchovy Engraulis ringens , were most abundant. Our data showed little evidence for diel vertical migration and larvae were more abundant at depth (>80 m) under low temperature (∼12°C) and low chl a (∼2 mg m^–3), below the highly advective upper layer. The exploratory K -means analysis allowed the separation of data into two distinct habitats: upwelling and nonupwelling types. Most taxa were allocated in nonupwelling waters, i.e. outside the cold plume. However, short-term variations (<24 h) in the position of the upwelling plume influenced both horizontal and vertical occurrence as well as abundance of taxa, and caused variability in temperature, oxygen and chl a . These changes in oceanographic conditions, caused by upwelling circulation and the dynamics of the cold plume, may sharply modify the habitat of fish larvae and have an important role in survivorship and recruitment success.     

Horizontal and vertical distribution patterns of larval myctophid fishes in the Kuroshio Current region

The horizontal and vertical distribution of myctophid fish larvae is described based on discrete depth sampling from the surface down to 200 m at seven stations on a transect crossing the Kuroshio Current on 6–9 July 1977. A total of 7819 larvae was collected. Myctophid larvae accounted for 72% of the total fish larvae and included 18 species or types belonging to 12 genera. Based on horizontal distribution patterns these species (or types) were categorized into three groups, i.e. Kuroshio-axis group ( Myctophum asperum , M. orientale , Lampadena luminosa and Symbolophorus evermanni ), Kuroshio east group ( Hygophum reinhardtii , Lampanyctus alatus , Diogenichthys atlanticus , Lobianchia gemellarii , M. nitidulum , Benthosema suborbitale , Lampadena sp. and Ceratoscopelus warmingii ) and Pan- Kuroshio group [ Diaphus stubby type (mostly D. garmani ) and D. kuroshio ]. Larvae of the subfamily Lampanyctinae were distributed in shallower (0–30 m) waters than those of the subfamily Myctophinae (50–150 m), while this relationship was opposite of the night-time depth distribution of adults of the two subfamilies.     

Growth and ontogenetic migration of greeneye Chlorophthalmus albatrossis in Tosa Bay, Pacific Coast of South-Western Japan

ABSTRACT:   Growth and ontogenetic migration of greeneye Chlorophthalmus albatrossis were studied in the area from 100–400 m depths in Tosa Bay, from April 1996 to March 1997, using samples collected by an otter trawl net with a 1.3 mm mesh cod end. In the monthly changes of the standard length (SL) composition, four year classes were recognized, although the third- and fourth-year classes were not clearly separated in all the monthly samples. Individuals of 40–50 mm SL settled on the bottom of the 150–200 m depth zone from August to February. By May and June, they grew to 75–80 mm SL modal size, and migrated to 200–300 m depth. In one year after settlement, they grew to 125–130 mm SL, and migrated to 300–350 m depth. They grew to a maximum of 160 mm SL and were not sampled in the winter three years after settlement, suggesting either migration or mortality. Seasonal changes in the gonadosomatic index for combined samples of males and females showed that greeneye did not mature in Tosa Bay. It is suggested that the planktonic larvae are transported from the spawning grounds, the location of which remains unknown.     

Natural spawning, early development and first feeding of the semicircle angelfish [Pomacanthus semicirculatus (Cuvier, 1831)] in captivity

Successful natural spawning of Pomacanthus semicirculatus in captivity from 11 September to 18 October, 2006 is described for the first time. Each female laid an average of 230 000 eggs during the spawning period. Fertilized eggs were spherical, transparent and buoyant and had a mean diameter of 0.61 ± 0.03 mm (mean ± SD). Embryonic development lasted 18–21 h at 28.5 °C. Newly hatched larvae were 1.35 ± 0.02 mm in total length (TL) with 27 (12+15) myomeres and had an oil globule in the ventroposterior area of the yolk sac. Larvae completed yolk absorption within 3 days post hatching at 2.37 ± 0.05 mm TL. Larvae were fed either 100% microalgae ( Nannochloropsis sp.), 100% s-type rotifers ( Brachionus rotundiformis ), 100% dinoflagellates ( Gonyaulax sp.) or different combinations of the three (50%:50%:0%, 30%:35%:35%) to determine the effect of live feed on the survival rate. The survival was significantly ( P <0.001) better in larvae fed a combination of diets (30%: 35%: 35%) than others. These results indicate that P. semicirculatus is a potential species for captive-breeding programmes and the use of a combination of diets (microalgae plus s-type rotifers and dinoflagellates) may be a suitable first food for fish larvae.     

Night-time vertical distribution and abundance of small epipelagic and mesopelagic fishes in the upper 100 m layer of the Kuroshio-Oyashio Transition Zone in Spring

ABSTRACT:   In order to simultaneously compare vertical distributions of small pelagic fishes and mesopelagic fishes during the night in the upper 100 m, one oblique and 11 horizontal tows of large-sized (opening area 480 m²) trawl operations were conducted at different depths in the Kuroshio–Oyashio Transition Zone during 20–22 May 1995. While Japanese sardine Sardinops melanostictus and Japanese anchovy Engraulis japonicus were concentrated in the upper 20 m, mackerels Scomber spp. were collected uniformly in the upper 80 m layer. Myctophid fishes Ceratoscopelus warmingii, Diaphus kuroshio, Diogenichthys atlanticus , Myctophum asperum , Notoscopelus spp. mostly N. resplendens , and Symbolophorus californiensis , a gempylid Nealotus tripes , a microstomatid Lipolagus ochotensis and a phosichthyid Vinciguerria nimbaria were dominant components of mesopelagic fishes that ascended to the upper 100 m layer at night, where these mesopelagic fishes concentrated at depths of 20–80 m. In the upper 20 m layer, Japanese anchovy comprised 60% of the total wet weight of all fishes followed by D. atlanticus (3%), C. warmingii (2%) and S. californiensis (2%). Juvenile myctophids were dominant in the upper 20 m layer compared to deeper layers.     

Distribution and abundance of two species of codlet (Teleostei, Bregmacerotidae) larvae from the south-eastern Brazilian Bight

Two species of codlet (Bregmacerotidae) larvae were collected during nine survey cruises conducted in the south-eastern Brazilian Bight between 23^oS and 30^oS in 1985-91. Of 4846 codlet larvae collected during nine cruises, 99.2% were identified as Bregmaceros canton and only 0.8% were B. athanticus. The two species were more abundant during winter in the neritic region (60–135 m depth). Vertical distribution of codlet larvae was studied using the BIOMOC net, and vertical profiles of temperature/salinity and chlorophyll a concentration were recorded by CTD and fluorescent sonde. Only 9.0% of larvae were collected in the upper 30 m and 91.0% were found deeper (> 40 m). The depths of highest density of larvae bore a close relationship to the subsurface chlorophyll maximum layer. Some B. canton larvae were found in the upper 30 m layer at night, but no larva was found during the day. Water temperatures where B. canton larvae occurred ranged from 16 to 23^oC, but most larvae were found at 18–20^oC and in salinities of 35.5–36.5 psu.     

Larval fish abundance and distribution during the late winter bloom off Gran Canaria Island, Canary Islands

The species composition, distribution and abundance of fish larvae off Gran Canaria (Canary Islands), an oceanic island in the NE Atlantic, were studied from January to August 2005. Weekly samplings were carried out at six stations in the eastern and southern flanks of the island, at the edge of the island shelf, in order to analyze the fluctuations of the planktonic community. The highest chlorophyll a values were recorded in March, coinciding with the lowest values of temperature in the mixed layer, which gave rise to the 'late winter bloom'. Mesozooplankton biomass peaked with only a week-lag to phytoplankton, and its average value (13.5 ± 10.9 SD mg dry weight m⁻³) was typical for the area. A clear windward-leeward distribution pattern was found for small mesozooplankton and neritic larvae, showing higher values in two stagnation points, upstream and downstream of the island. A total of 128 taxa of fish larvae were identified. Neritic and oceanic larvae appeared in quite similar proportions. Only three families accounted for half of the total larval fish collected: Myctophidae (24.9%), Sparidae (12.7%) and Clupeidae (11.9%). Sardinella aurita (8% of total larvae collected) was the most abundant species, appearing during the whole period of study and at each of the six sampled stations. Sardina pilchardus larvae were rarely captured but were always encountered with the arrival of NW African upwelling filaments to the island coast, suggesting that these larvae were transported in those mesoscale structures.     

Larval feeding behaviour of the endangered fish golden bubblebee goodeid, Allotoca dugesi, implications for conservation of an endangered species

Abstract To improve survival of cultured juvenile fishes, it is important to understand prey preferences and predation behaviour of fish larvae. In this study, the feeding behaviour and prey preference of Allotoca dugesi (Bean), a small fish species endemic to Mexico and currently threatened with extinction are presented. Three zooplanktonic prey, viz. Brachionus calyciflorus (Pallas) , Moina macrocopa (Goulden) and Daphnia pulex (Leydig) were tested. The mean size (standard length ± SE, mm) of the larvae ranged from 9.0 ± 0.1 mm at week 1 to 18 ± 2 mm at the end of the experimental period (8 weeks). The gape size (mean ± SE) increased from 0.125 ± 0.002 mm at the first week to 2.300 ± 0.361 mm by the eighth week. Capture success (capture/attack) ranged from 0.80 to 0.98 with Brachionus , 0.72–0.94 with Moina and 0.17–0.46 with Daphnia . Prey preference experiments were conducted using B. calyciflorus, M. macrocopa and D. pulex at a fixed ratio of 5 : 2 : 2 ind. mL⁻¹, respectively, and revealed a positive selection for rotifers and Moina , but avoidance of Daphnia . The results are discussed with reference to conservation efforts for Allotoca dugesi .     

Performance of a new artificial abalone hatchery culture system in terms of settlement of larvae and growth and survival of postlarval Haliotis discus discus (Reeve)

ABSTRACT:   A new abalone postlarval culture system, Stott's abalone postlarval production system (SAPPS), is being developed as an alternative method for settling larvae and on-growing postlarval abalone. SAPPS was tested in trial 1 using two commercially available artificial diets, Adam and Amos (Adam; supplied by Adam and Amos, Mt Barker, SA, Australia) and Cosmo (Cos; supplied by Cosmo, Matsuyama, Japan), against the diatom biofilm method (Diatom). In trial 2, SAPPS was tested against the previous production system (PPS, used by Stott et al. 2002). Larvae were induced to metamorphose and the resulting postlarvae were on-grown in 10-L flow-through tanks for 28 days in their respective treatments. The final survival of postlarvae was significantly higher ( P  < 0.05) in the SAPPS-Cos group (56.7 ± 11.15%) than in the Diatom (9.4 ± 2.7%) and SAPPS-Adam groups (8.5 ± 1.1%). The final length of postlarvae in SAPPS-Cos (1065 ± 73 µm) was significantly higher ( P  < 0.05) than those in the other two treatments (average of 841–883 µm). In trial 2 the metamorphosis rate was improved by using Spirulina platensis. The final length of postlarvae in SAPPS (1449–1471 µm) was significantly higher ( P  < 0.05) than PPS (1065–1075 µm) and Diatom (993 µm). There is potential for SAPPS to be used as an alternative to the current diatom method in the culture of postlarval abalone.     

The feeding ecology of larval Pacific hake (Merluccius productus) in the California Current region: an updated approach using a combined OPC/MOCNESS to estimate prey biovolume

A combined optical particle counter (OPC) and multiple opening and closing net and environmental sensing system (MOCNESS) was used to obtain simultaneous measurements of the fine‐scale distribution of ‘prey‐sized’ particles and the vertical distribution of larvae of Pacific hake (Merluccius productus). Physical properties were also measured. The data were used to describe the feeding ecology of Pacific hake larvae, and to explore the effects of prey abundance, time of day, temperature and depth on feeding. Pacific hake larvae consumed a wide variety of prey including copepod eggs, nauplii, copepodites, and euphausiid metanauplii. Calanoid copepodites comprised > 75% of the ingested prey volume. First‐feeding larvae were 2.5–3.0 mm SL. These larvae consumed prey 40–200 μm wide. Larvae 3.0–6.5 mm long ingested prey 40–400 μm wide and larvae > 6.5 mm long ingested prey 400–700 μm wide. There were clear diel patterns in feeding incidence and prey volume ingested. Feeding commenced between 06:01 and 08:00 hours PST and continued until ～ 16:00 hours. Ingested prey items remained in the gut until complete gut evacuation occurred near dawn. The volume of food ingested was estimated using two indices, the cube root of the prey volume (CRPV) and the cube root of the stomach volume (CRSV). Standard length, log elapsed time since gut evacuation, depth and particle biovolume contributed significantly to variation in both indices. Temperature did not contribute to variability in either CRPV or CRSV.     

Larval Development and Rearing of Longtooth Grouper Epinephelus bruneus in Jeju Island, Korea

Eggs and sperm were obtained from a female (6.3 kg/BW) and a male (8.4 kg/BW) longtooth grouper Epinephelus bruneus following HCG injection in July 2003. The eggs were fertilized artMcially with the sperm and incubated in one of two 50-m³ tanks after washing the fertilized eggs. The fertilized eggs were 830–950 pn (average 900 ± 2 μm) in diameter and the respective fertilization and hatching rates were 97.7 ± 0.6% and 96.8 ± 0.5% at a water temperature of 25.0 ± 0.5 C. With this regime, the survival rate by day 93 was 7.5% in the 50-m³ tank. The elapsed time from hatching to opening the mouth was 3 d at 25 C. The initial mouth size (z) of the larvae was 0.22–0.23 mm. The newly hatched larvae were 2.02 ± 0.02 mm TL; this increased to 4.12 ± 0.09 mm TL by day 11. By day 54, the larvae had metamorphosed into juveniles and reached 41.12 ± 1.20 mm TL, and by day 93 the juveniles reached 93.78 ± 1.98 mm TL. In all, 49.5% of the larvae were malformed and the type of malformation was diverse.     

Larval molting and growth of the Japanese spiny lobster Panulirus japonicus under laboratory conditions

ABSTRACT:   Ten newly hatched phyllosoma of Panulirus japonicus were cultured individually to monitor body length ( BL ) and intermolt period, and 2000 were cultured in groups to sample specimens for measurement of body weight. Phyllosoma were fed with Artemia and mussel gonad; the culture seawater temperature was 24–26°C. The individually cultured phyllosoma showed an increment in body length by the first molt of approximately 0.5 mm, and the molt increment increased to approximately 1 mm at 5 mm BL ; it was constant to 15 mm BL . Thereafter, the molt increment increased exponentially. The duration of the first instar was 6–7 days. Instar duration increased with development up to approximately 2 weeks at the 20th instar (∼16 mm BL ) and then became constant. Of the 10 larvae reared individually, five metamorphosed to the puerulus stage. The entire phyllosoma life ranged from 245–326 days (mean 289.0 days), and the number of instars ranged from 22–29 (mean 26.2). Body length in the final instar ranged from 28.50–33.10 mm (mean 30.280 mm). For the phyllosoma cultured in groups, relationships between BL and wet/dry body weights ( WW / DW , mg) were expressed as exponential equations: WW  = 0.0686 BL ^2.2023 and DW  = 0.0209 BL ^2.1905.     

渤海中部浮游动物的生态特征

本研究针对2013年5月(春季)、8月(夏季)、11月(秋季)和12月(冬季)在渤海中部海域以浅水Ⅰ型浮游生物网采集的浮游动物样品,分析了浮游动物的种类组成、丰度分布和多样性;通过结合现场环境参数,探讨了环境因子与浮游动物之间的关系。结果显示,4个季节共鉴定浮游动物74种(含浮游幼虫21类),桡足类为绝对优势的类群,在浮游动物的物种丰富度中占25.7%。渤海中部海域全年均出现的浮游动物优势种类共2个,分别为中华哲水蚤(Calanus sinicus)和强壮箭虫(Sagitta crassa)。春季共鉴定浮游动物29种(含浮游幼虫6类),浮游动物平均丰度为782.0 ind/m3,平均湿重生物量为157.1 mg/m3,香农-威纳指数(H’)和物种丰富度指数(D)分别为2.36和1.02;夏季共鉴定浮游动物45种(含浮游幼虫18类),浮游动物平均丰度为199.6 ind/m3,平均湿重生物量为135.8 mg/m3,H’和D分别为1.75和1.78;秋季共鉴定浮游动物42种(含浮游幼虫14类),浮游动物平均丰度为42.1 ind/m3,平均湿重生物量为122.5 mg/m3,H’和D分别为1.83和2.08;冬季共鉴定浮游动物33种(含浮游幼虫12类),浮游动物平均丰度为72.1 ind/m3,平均湿重生物量为151.1 mg/m3, H’和D分别为1.63和1.53。浮游动物丰度与环境因子间的相关性分析表明,春季影响渤海中部海域浮游动物分布的主要环境因子组合为表盐、底溶解氧和水深;夏季影响渤海中部海域浮游动物分布的主要环境因子组合为底温、底盐和叶绿素;秋季影响渤海中部海域浮游动物分布的主要环境因子组合为表温、表pH和底pH;冬季影响渤海中部海域浮游动物分布的主要环境因子组合为底pH和叶绿素。与同期历史数据相比,浮游动物的种类数、丰度和生物量均有所下降。     

Predation on fish larvae by moon jellyfish <Emphasis Type="Italic">Aurelia aurita</Emphasis> under low dissolved oxygen concentrations

ABSTRACT:   Laboratory experiments were conducted to test the hypothesis that low dissolved oxygen concentrations have the potential to enhance the predation rate on fish larvae by moon jellyfish Aurelia aurita which is increasing in abundance in the coastal waters of Japan. Larvae of the red sea bream Pagrus major in four size classes (2.9, 4.1, 6.2 and 8.6 mm in standard length) were used as prey in a short-term predation experiment. No change in the bell contraction rate of the jellyfish (mean bell diameter, 100.4 ± 10.2 mm) was observed at the oxygen concentrations tested (1, 2 and 4 mg/L, and air-saturated, 5.5–6.0 mg/L), suggesting a strong tolerance to a decline in the oxygen concentration. More than 80% of the 2.5 and 4.1-mm size-class larvae were predated on by the jellyfish at all oxygen concentrations during the 15-min trials in 10-L tanks. The 6.2 and 8.6-mm size-class larvae were able to escape from the jellyfish apparently due to their developed swimming ability at the two higher dissolved oxygen concentrations. There was an increase in predation on larvae of these two size classes at the two lower dissolved oxygen concentrations. Predation by moon jellyfish on fish larvae can be more intense at low dissolved oxygen concentrations that commonly occur in the coastal waters of Japan.