Sympatric spawning but allopatric distribution of Anguilla japonica and Anguilla marmorata: temperature- and oceanic current-dependent sieving

Anguilla japonica and Anguilla marmorata share overlapping spawning sites, similar drifting routes, and comparable larval durations. However, they exhibit allopatric geographical distributions in East Asia. To clarify this ecological discrepancy, glass eels from estuaries in Taiwan, the Philippines, Indonesia, and China were collected monthly, and the survival rate of A. marmorata under varying water salinities and temperatures was examined. The composition ratio of these 2 eel species showed a significant latitude cline, matching the 24 °C sea surface temperature isotherm in winter. Both species had opposing temperature preferences for recruitment. A. marmorata prefer high water temperatures and die at low water temperatures. In contrast, A. japonica can endure low water temperatures, but their recruitment is inhibited by high water temperatures. Thus, A. japonica glass eels, which mainly spawn in summer, are preferably recruited to Taiwan, China, Korea, and Japan by the Kuroshio and its branch waters in winter. Meanwhile, A. marmorata glass eels, which spawn throughout the year, are mostly screened out in East Asia in areas with low-temperature coastal waters in winter. During summer, the strong northward currents from the South China Sea and Changjiang River discharge markedly block the Kuroshio invasion and thus restrict the approach of A. marmorata glass eels to the coasts of China and Korea. The differences in the preferences of the recruitment temperature for glass eels combined with the availability of oceanic currents shape the real geographic distribution of Anguilla japonica and Anguilla marmorata, making them "temperate" and "tropical" eels, respectively.     

Oceanic migration and spawning of anguillid eels

Many aspects of the life histories of anguillid eels have been revealed in recent decades, but the spawning migrations of their silver eels in the open ocean still remains poorly understood. This paper overviews what is known about the migration and spawning of anguillid species in the ocean. The factors that determine exactly when anguillid eels will begin their migrations are not known, although environmental influences such as lunar cycle, rainfall and river discharge seem to affect their patterns of movement as they migrate towards the ocean. Once in the ocean on their way to the spawning area, silver eels probably migrate in the upper few hundred metres, while reproductive maturation continues. Although involvement of a magnetic sense or olfactory cues seems probable, how they navigate or what routes they take are still a matter of speculation. There are few landmarks in the open ocean to define their spawning areas, other than oceanographic or geological features such as oceanic fronts or seamounts in some cases. Spawning of silver eels in the ocean has never been observed, but artificially matured eels of several species have exhibited similar spawning behaviours in the laboratory. Recent collections of mature adults and newly spawned preleptocephali in the spawning area of the Japanese eel Anguilla japonica have shown that spawning occurs during new moon periods in the North Equatorial Current region near the West Mariana Ridge. These data, however, show that the latitude of the spawning events can change among months and years depending on oceanographic conditions. Changes in spawning location of this and other anguillid species may affect their larval transport and survival, and appear to have the potential to influence recruitment success. A greater understanding of the spawning migration and the choice of spawning locations by silver eels is needed to help conserve declining anguillid species.     

Coral spawning in the western Pacific Ocean is related to solar insolation: evidence of multiple spawning events in Palau

Coral reproductive schedules have long been related to concomitant increases in regional sea surface temperature (SST). Yet, corals also mass spawn in the tropics, where SSTs vary little throughout the year. Here we show that the rise toward and fall from solar insolation maxima, which is the electromagnetic energy incident on the surface of the earth, coincide with coral spawning events in tropical Palau. Two insolation maxima (a consequence of the vernal and autumnal equinoxes) in the tropics lead to multiple spawning events per year. SSTs are poor predictors of coral spawning in the tropics. Multiple spawning events increase the rate of genetic recombination, which in turn may facilitate the likelihood of speciation and explain, in part, the high coral diversity in the tropics. We also show that mass spawning coincides with near-maximum solar insolation (5.0–7.2 kWh/m²/day) events from 33°N to 30°S in the western Pacific Ocean, and should be considered as one of the primary variables driving coral reproductive cycles.     

Wavelength-specific thresholds of artificially reared Japanese eel Anguilla japonica larvae determined from negative-phototactic behaviours

We report wavelength-specific thresholds of leptocephali of Japanese eels Anguilla japonica determined from their negative-phototactic behaviour. Leptocephali are most sensitive to wavelengths 400–500 nm and at very short wavelengths. Their visual sensitivity decreases more sharply at wavelengths >500 nm than it does at wavelengths <400 nm. The spectral sensitivity of leptocephali adapts to the optical conditions of their habitat. The mean visual sensitivity threshold of leptocephali is 7.22 × 10⁻⁴ μmol m⁻² s⁻¹ between 400 and 500 nm. Based on visual sensitivity thresholds of 475 nm, the most transparent wavelength in waters where these leptocephali occur, the daytime depth of occurrence of these larvae may exceed 250 m. LEDs emitting light of wavelength 625 nm in culture environments would minimise disturbance to leptocephali during facility maintenance.     

Anguilla japonica is already magnetosensitive at the glass eel phase

Magnetosensitivity of the Japanese eel Anguilla japonica at the glass eel phase (newly metamorphosed juveniles) was examined by conditioning and electrocardiography. The glass eels were conditioned to an imposed magnetic field of 192 473 nT parallel to the fish body placed along the earth's west‐east axis. After 10 to 40 conditioning runs, all the glass eels exhibited a significant conditioned response ( i.e . slowing of the heart beat) to a 192 473 nT magnetic field and even to a 12 663 nT magnetic field that combined with the geomagnetic field (32 524 nT) at the laboratory and produced a resultant magnetic field of 21° easterly. These results indicate that glass eels have high magnetosensitivity and probably acquire geomagnetic information early in life. It is hypothesized that silver‐phase adult eels find their way back to the oceanic spawning ground by reversing the geomagnetic direction that had been detected and 'memorized' during the glass eel phase when migrating from the open ocean towards the continental shelf and coastal waters.     

Age and growth of Anguilla bicolor bicolor leptocephali in the eastern Indian Ocean

A sampling survey for leptocephali was conducted in the eastern Indian Ocean to the west of Sumatra from 5 to 20 June 2003 in an overlapping area with the historical survey in September to November 1929 during the Danish Round the World Expedition. The age and larval growth rate of 34 Anguilla bicolor bicolor collected in 2003 were estimated using their otolith microstructure to obtain new information about the early life history of this sub-species. The total lengths and ages of these leptocephali were 44·1–55·5 mm and 114–158 days, respectively. Their hatching dates ranged from 3 January to 20 February 2003. Combining these age data and the estimated age of leptocephali collected by others in the same area, this sub-species was estimated to have a wide range of spawning periods. Individual growth rates of the leptocephali in 2003 ranged from 0·32 to 0·39 mm day⁻¹ with a mean ± s . d . of 0·35 ± 0·02 mm day⁻¹. These values were lower than the growth rates of leptocephali of other tropical eels such as Anguilla celebesensis or Anguilla borneensis , suggesting that A. b. bicolor from the eastern Indian Ocean have a longer leptocephalus period of oceanic migration.     

Advanced sexual maturation before marine migration of Anguilla bicolor bicolor and Anguilla marmorata at Réunion Island

Maturing sub-adults of two species of anguillid eels (a female Anguilla bicolor bicolor and a male Anguilla marmorata ) were collected for the first time at Réunion Island, western Indian Ocean. Both were silver eels, i.e. maturing eels at the onset of their spawning migration, and characterized by advanced sexual maturation that has been only observed in Anguilla dieffenbachii from New Zealand.     

Temperature influence on the spawning performance of artificially-matured Japanese eel, Anguilla japonica, in captivity

We studied the influence of temperature on the spawning performance of artificially matured Japanese eels, Anguilla japonica, in captivity. We used routine hormone injections to bring females and males to maturity in separate aquaria. We recorded the behavior of three pairs of such hormone-treated matured eels in an aquarium (2 replicates) at four temperatures: 14, 18, 22, and 27°C, respectively. They became active and frequently left the bottom swimming in the water column, and spawning events occurred. Females released eggs in the water column around the activity peaks. Males preceded females in reaching activity peaks (presumably the timing of sperm ejection and egg release), possibly resulting in the low fertilization we observed in this experiment. Males and females returned back to the aquarium bottoms and became quiet after spawning. On several occasions, male-female or female-female pairs were observed to ‘cruise together’ in the water column for several to tens of seconds prior to egg releasing, but no courtship behavior indicative of spawning such as pairing and chasing was observed in the eels in our study. Our results suggest that 18–22°C might be the thermal preference for spawning for Japanese eels, which approximates the temperature range of the 500 m deep water layer around the Mariana Islands seamount area, the presumed spawning site for the Japanese eel.

First observations of the burrows of Anguilla japonica

The burrow morphology of the Japanese eel Anguilla japonica was studied using in situ resin‐casting in a mud bottomed tidal drainage channel adjacent to the Fukui River in Tokushima, Japan. Two eels (62·5 and 56·3 cm total length) were initially fished from the burrows to verify that they were being used by A. japonica . Casts were made of 10 burrows that were found to have from one to three openings and main tunnels that were parallel to the axis of water flow in the channel. The maximum depths of the tunnels in the mud were 17·8–30·0 cm. The diameters of main tunnels ranged from 1·2 to 7·9 cm, were almost always wider than the bodies of the Japanese eels examined, and were more variable in the horizontal axis than in the vertical axis. There were no other animals capable of constructing a long and thin burrow in this channel, so these observations indicate that anguillids are able to construct their own burrows in soft mud sediments that may be used for extended periods of time.     

The effects of seasonal and interannual variability of oceanic structure in the western Pacific North Equatorial Current on larval transport of the Japanese eel Anguilla japonica

As the North Equatorial Current (NEC)–bifurcation is known to be related to El Niño-Southern Oscillation (ENSO) events, the influence of the position of the NEC bifurcation on transport success of the larval Japanese eel Anguilla japonica was investigated. Using a Lagrangian modelling approach, larval transport was simulated and the relative influence of El Niño and La Niña events and the NEC-bifurcation position on the success of particle transport analysed. The number of particles transported from the NEC to the Kuroshio tended to be lowest during El Niño years, and differences between La Niña and regular years were small. The transport success observed in simulations showed some relationships to annual A. japonica glass eel recruitment to Tanegashima Island over 1993 to 2001, but not in 2002. The study shows that particle tracking simulations can be used to improve knowledge of the oceanic migration of A. japonica but further studies are required, including comparisons with the effects on larval survival of fluctuations in temperature and food availability.     

Location,size and age at onset of metamorphosis in the Japanese eel Anguilla japonica

This study clarifies the location, size and age at the onset of metamorphosis in Japanese eels Anguilla japonica through oceanic surveys, rearing experiments and analyses of the morphology and otoliths of leptocephali and glass eels. Twenty‐eight metamorphosing leptocephali were collected in the mesoscale eddy region to the east of Taiwan during research expeditions in 2004. Rearing experiments showed that the total length (L_T) of leptocephali decreased by an average of 12·5% during metamorphosis and 13·9% during the 2–12 h after death. Thus, the mean back‐calculated L_T at the onset of metamorphosis for 630 glass eels from Taiwan and Japan was estimated at 67·8 ± 2·7 mm (mean ± S.D.). The estimated mean ante‐mortem size of the fully grown pre‐metamorphic leptocephali collected in 2004 was 64·6 ± 3·4 mm, which was consistent with the L_T estimate for glass eels. Otolith analysis showed that the mean age at the onset of metamorphosis was 137 ± 15 days and indicated that Japanese eels may have a recruitment route through the mesoscale eddies to the east of Taiwan in addition to the direct transfer route from the North Equatorial Current to the Kuroshio Current.     

Reproductive biology of female bigeye tuna Thunnus obesus in the western Pacific Ocean

The reproductive biology of female bigeye tuna Thunnus obesus was assessed by examining 888 fish (ranging from 84·9 to 174·4 cm fork length, L_F) caught by Taiwanese offshore longliners in the western Pacific Ocean from November 1997 to November 1998 and November to December 1999 and 258 gonad samples from these fish. The overall sex ratio of the catch during the sampling differed significantly from 0·5, but males were predominant in sizes >140 cm L_F. Reproductive activity (assessed by histology), a gonado‐somatic index, and the size‐frequency distributions of whole oocytes indicated that spawning occurred throughout the year and the major spawning season appeared to be from February to September. The estimated sizes at 50% maturity (L_F50) of females was 102·85 cm (95% c.i .: 90·79–110·21 cm) and the smallest mature female was 99·7 cm L_F. They are multiple spawners and oocytes develop asynchronously. The proportion of mature (0·63) and reproductively active (0·70) females with ovaries containing postovulatory follicles indicated that they spawn almost daily. Batch fecundity for 15 females with the most advanced oocytes (>730 µm) ranged from 0·84 to 8·56 million eggs (mean ± s.d . = 3·06 ± 2·09). The relationships between batch fecundity (F_B, in millions of eggs) and L_F (cm) and round mass (M_R, kg) were (r² = 0·84) and (r² = 0·80), respectively. The parameters estimated in this study are key information for stock assessments of T. obesus in the western Pacific Ocean and will contribute to the conservation and sustainable yield of this species.     

Summer feeding habits of the Pacific pomfret Brama japonica in the transitional and subarctic waters of the central North Pacific

Feeding habits of the Pacific pomfret Brama japonica , ranging from 30 to 40 cm in standard length, were quantitatively evaluated using the index of relative importance and were compared between the transitional domain and subarctic region in the central North Pacific. In the transitional domain (between the subarctic boundary and the subarctic front), this species fed mainly on subarctic and transitional‐water gonatid squids such as Berryteuthis anonychus , Gonatus middendorffi and Gonatopsis borealis , the transitional‐water squid Onychoteuthis borealijaponica , and a subtropical and transitional‐water amphipod, Phronima sedentaria . These prey items accounted for 52·8% by wet mass of total stomach contents. In the subarctic region (north of the subarctic front), Pacific pomfrets preyed primarily on subarctic and transitional‐water squids, such as B. anonychus and G. middendorffi , and secondarily on subarctic amphipods such as Hyperia medusarum and Hyperia glaba . These four prey items comprised 71·6% by mass of total stomach contents. Major transitional domain prey items, such as G. borealis, O. borealijaponica and P. sedentaria , were almost non‐existent in stomachs of fish from the subarctic region. The mean wet stomach content mass expressed as a percentage of body mass was significantly higher in the subarctic region (2·35%) than in the transitional domain (0·75%), suggesting that, for this fish, prey items are more readily available in the subarctic region during summer and their northward migration is a feeding migration. Feeding strategy of the Pacific pomfret was discussed in relation to their seasonal south‐north migration.     

Crevice spawning behavior in Dionda dichroma,with comments on the evolution of spawning modes in North American shiners (Teleostei: Cyprinidae)

The crevice spawning behavior of Diondadichroma is described for the first time, andcompared to spawning behavior in the genusCyprinella. The evolution of crevice spawningwith respect to other spawning behaviors ofNorth American shiners is examined usingexplicitly phylogenetic hypotheses for thisgroup. We present evidence that broadcastspawning is plesiomorphic and all otherspawning behaviors are independently derived. There is evidence that crevice spawning hasevolved independently at least three timeswithin the shiner clade. There is no supportfor an evolutionary transition between eggclustering and crevice spawning. Nestassociation, spawning on habitat prepared byother species, has also evolved multiple timeswithin this clade. Evolution of spawning inshiners is best described by phylogeneticstasis with several independent origins ofspecialized spawning stragegies.     

Age and growth of migrating tropical eels,Anguilla celebesensis and Anguilla marmorata

The age and growth of migrating tropical eels, Anguilla celebesensis and Anguilla marmorata from central Sulawesi, Indonesia, were examined. Migrating eels (63 A. celebesensis and 38 A. marmorata ) were obtained from weirs near the Poso Lake outlet and non‐migrating eels (35 A. celebesensis and 119 A. marmorata ) were captured by baited hooks, eel pots, scoop net and electro‐fishing in the Poso River system, Laa River system, Baluga River, Tongku River and Padapu River from February 2009 to October 2010. In both species, the proportion of eels with opaque otolith edges showed a single peak in July, suggesting that one annulus (a pair of translucent and opaque zones) was formed each year in their otoliths. Mean ± s.d . and range of total length (L _T) and age was 785·2 ± 114·9 (585–1083) mm and 7·5 ± 1·6 (5–11) years in migrating female A. celebesensis and 1132·2 ± 173·7 (800–1630) mm and 11·6 ± 3·3 (7–23) years in A. marmorata . The age of migrating female eels was negatively correlated with annual growth rate, 100·7 ± 17·2 (68·1–145·0) mm year^?1 in A. celebesensis and 97·9 ± 19·3 (66·6–131·6) mm year^?1 in A. marmorata , but there was no significant correlation between the L _T and annual growth rate in either species. The annual growth rates of these female tropical eels were typically higher than those of temperate anguillid species, suggesting a latitudinal cline in growth rate in the genus Anguilla reflecting the environmental conditions of their growth habitat.     

Evaluation of downward movements of Japanese eel Anguilla japonica inhabiting brackish water areas

This study evaluated the size and age distributions and otolith microchemistry of the Japanese eel Anguilla japonica in freshwater and brackish water areas in the Aki and Tsuchikawa rivers for 1 year, and in brackish water areas in the Asahi River for 3 years to understand the movements of Japanese eels between continental habitats of different salinity after recruitment (n = 759). For all three rivers, the total length (L_T) and age distributions were consistent; yellow eels captured in the upper brackish water (Aki River: 353.5 ± 77.4 mm and 3.0 ± 0.8 years; Tsuchikawa River: 287.7 ± 87.3 mm and 3.7 ± 1.3 years; Asahi River: 418.2 ± 112.1 mm and 4.2 ± 1.7 years) were smaller and younger than not only those in the fresh water of the two rivers but also those in the lowest brackish water sampling areas (Aki River: 436.0 ± 71.6 mm and 3.8 ± 1.1 years; Tsuchikawa River: 370.9 ± 121.7 mm and 4.9 ± 2.3 years; Asahi River: 558.5 ± 85.9 mm and 5.7 ± 1.7 years). In the Asahi River, these tendencies were found throughout the 3 years. Otolith analysis indicated that the majority of the eels captured in the lowest brackish water areas had moved down from upstream. These results suggest that Japanese eels inhabiting saline water generally move from the upper estuary as they grow. The upper estuary can be an important area for the management of this species because these eels spend their early continental growth life there.     

Mapping the spawning grounds of North Sea cod (Gadus morhua) by direct and indirect means

Despite recent evidence for sub-stock structuring, North Sea cod are assessed as a single unit. As a consequence, knowledge of sub-stock trends is poor. In particular, there are no recent evaluations of which spawning grounds are active. Here we report results from the first ichthyoplankton survey to cover the whole North Sea. Also, this survey, conducted in 2004, was the first to make extensive use of DNA-based molecular methods to unambiguously identify early developmental stage cod eggs. We compare the findings from the plankton survey with estimated egg production inferred from the distribution of mature cod in contemporaneous trawl surveys. Results from both approaches were in general agreement and showed hot spots of egg production around the southern and eastern edges of the Dogger Bank, in the German Bight, the Moray Firth and to the east of the Shetlands. These areas broadly coincide with known spawning locations from the period 1940 to 1970. We were, however, unable to directly detect significant numbers of cod eggs at the historic spawning ground off Flamborough (northeast coast of England). The results demonstrate that most of the major spawning grounds of cod in the North Sea are still active but that some localized populations may have been reduced to the point where it is now difficult to detect the presence of eggs in the plankton.     

Aggregation and spawning of the humphead wrasse Cheilinus undulatus (Pisces: Labridae): general aspects of spawning behaviour

The humphead wrasse Cheilinus undulatus formed resident spawning aggregations daily after high tide at specific locations along the seaward edge of the Palau barrier reef. The location and extent of one aggregation site remained consistent for 6 years with no physical features distinguishing it from adjacent areas. Spawning was documented most months and probably occurred year round with possible seasonal and lunar variation. Spawning males arrived first at the site, followed by females and potentially small primary males. The aggregation female to male sex ratio was estimated to be between 6:1 and 10:1. A maximum of 15 males and 100–150 females were observed at the site. A male courtship posture with the anal fin pointed, the caudal fin folded down and the dorsal fin folded against the body was maintained while swimming a few metres off the bottom in view of females. When ready to spawn females rose up as the posturing male passed and the pair released gametes in a relatively sedate fashion near the surface along the shelf break. No attempted predation on spawning adults was seen. Egg predation after spawning was uncommon. On days with early to mid‐day high tides the spawning period started 2·0–2·5 h after high tide when the speed of lagoon–ocean tidal currents peaked and lasted c. 1 h. On days with later afternoon high tides, spawning occurred sooner after high tide and before current speeds peaked. Other fishes with planktonic eggs spawned at the site as pairs or small groups in a rough succession after high tide with C. undulatus, the last species to spawn.     

Age and growth of eels Anguilla japonica in a Chinese river

Age and growth of Anguilla japonica sampled from the Qiulu River in southern China were studied. Mean body lengths and weights of the eels at each age were back-calculated from their otoliths. The growth parameters for the von Bertalanffy function gave L _∞= 98–2cm, K =0·07 and t ₀=− 1·94. Scales were observed to form firstly at the middle part of the lateral line, at a minimum body length of 160 mm and were inappropriate for ageing.     

Larval growth and drift pattern and the separation of herring spawning groups in the North Sea*

Data from two internationally co-ordinated survey programmes are used in an attempt to display the events in the early life that took place during the ‘recovery period’ of the North Sea herring Stock, 1979–1986. During the period, an increasing importance of the spawning grounds off Buchan and off Yorkshire is evident from distributions of newly hatched herring larvae. The dramatic changes in relative importance of spawning grounds are traced in the data on both late larvae (6 months old) and juveniles (18 months old). The onset of extended spawning off Buchan and off Yorkshire was followed by increase in distinct groups of late larvae of large mean length, and by enlargement of a group of medium-sized juveniles. It is hypothesized that the identified groups are interconnected. Thus, larvae spawned off Buchan and off Yorkshire are found to have growth rates superior to those attained by larvae from west of Scotland and around the Orkney/Shetland Isles, whereas the relationship is reversed in the juveniles. In the investigated period, larval drift routes exhibited the same trends from year to year, the drift of the northerly spawned larvae being the most variable. Thus, changes in the relative contributions from spawning grounds apparently influence the overall spatial distribution as well as size composition of larval and juvenile North Sea herring. The findings indicate that groups of larvae retain, to a large extent, separate distributions until metamorphosis, and point to larval drift pattern as a determinant of spawning group distinctness.