Reproductive Ecology and Biodiversity of Freshwater Eels around Sulawesi Island Indonesia

Migration Behavior of Anguilla celebesensis Silver Eels within their Tomini Bay Spawning Area

The tropical Celebes eel, Anguilla celebesensis, has a short migration between its spawning and growth habitats. Its spawning areas were hypothesized to be in Tomini Bay and the Celebes Sea after collecting their small leptocephali. However, there is no information about the silver eel oceanic spawning migration behavior of A. celebesensis. To better understand their short-distance spawning migration behavior, four large female silver eels (Eel 1-4) were equipped with pop-up satellite archival tags (PSATs) and released near the mouth of the Poso River in Tomini Bay of Sulawesi Island on 22 February (Eel 1-3) and 11 March 2010 (Eel 4). All PSATs ascended in Tomini Bay and transmitted their data. Eel 3 and 4 provided clear records of consistent diel vertical migration (DVM: eight days-Eel 3, 13 days-Eel 4) with daytime dives to mean depths of 444.7 m (Eel 3) and 539.0 m (Eel 4), where mean temperatures were 9.1°C (Eel 3) and 7.7°C (Eel 4), and nighttime ascents to mean depths of 132.8 m (Eel 3) and 112.4 m (Eel 4), where mean temperatures were 20.6°C (Eel 3) and 23.4°C (Eel 4). Eel 3 and 4 started to dive to deeper water around nautical dawn and swam up to shallower water around sunset. During nighttime, both eels swam in deeper and colder water during nights with moonlight than during nights without moonlight, and there was a negative linear relationship between experienced water temperatures with the moon in the sky and the lunar age for the eels. The A. celebesensis daily rhythm of DVM behaviors was similar to spawning-migration DVM behaviors of other anguillid species. Essential life history characteristics of A. celebesensis appear to be a short migration between freshwater growth habitat and ocean spawning habitat, and high GSI values with advanced gonadal development in downstream-migrating silver eels.

Effect of size grading on growth of yellow Pacific shortfin eel (Anguilla bicolor pacifica)

Heterogeneous growth, which is common among farmed fish, can be remedied by size grading. This study focused on whether the size grading process, which is commonly practiced in aquaculture, improves the subsequent growth performance of the Pacific shortfin eel Anguilla bicolor pacifica. Eels [338.70 ± 4.70 mm total length (TL) and 84.93 ± 1.87 g body weight (BW)] were initially reared for 60 days (pre-size grading period) and manually divided into three size groups: small (≤115 g), large (>150 g) and mixed (>115 g but <170 g), all maintained at a density of 5 kg m^-3 and reared until day 150 (size grading period). After size grading, mean BW and TL at harvest significantly differed in all eel groups, suggesting growth was size specific. Nonetheless, at day 150, except for biomass, size grading had no significant effect on growth and survival, coefficient of variation of length and weight, and Fulton's condition factor. Large eels had a significantly higher feed intake than mixed or small eels, whereas feed efficiency did not differ among eel groups. Results suggest that size grading promotes size-specific growth but did not improve the overall growth performance of yellow stage of A. bicolor pacifica.

Ecology and evolution of migration in the freshwater eels of the genus Anguilla Schrank, 1798

Scientists have long sought to uncover the secrets of the migration of anguillid eels, genus Anguilla. As catadromous fishes, anguillid eels spend most of their lives in freshwater until they return to their spawning grounds in the tropics, although part of the population never enters freshwater and instead resides in brackish and marine areas close to coastlines. Molecular phylogenetic research suggests that anguillid eels originated from deep-ocean midwater marine anguilliform species and that tropical eels originating from the Indo-Pacific region are the most basal species of anguillid eels. Anguillid eels left the tropical ocean to colonize temperate areas. The yearly spawning of tropical species and constant larval growth throughout the year extend to periods of recruitment in continental habitats to last all year for tropical eels. Tropical eels such as A. celebesensis and A. borneensis have relatively short migrations periods of less than 100 km to their spawning grounds. Conversely, the temperate European eel A. anguilla travels the longest distances and migrates more than 5000 km across the Atlantic Ocean to spawn in the Sargasso Sea. The ancestral state of migration in the genus Anguilla may have been local, short-scale and nonseasonal spawning migration throughout the year as defined in tropical eels. With the expansion of dispersion of global oceanic migration across the world, migration scales can gradually change. Temperate anguillid eels migrate thousands of kilometres from spawning areas to coastal and inland water habitats while retaining spawning areas in tropical areas, accompanied by seasonal downstream and spawning migrations with consequences for seasonal recruitment. Recent advances and the availability of electronic tags such as pop-up satellite archival tag could reconstruct the entire spawning migration from continental growth habitats to spawning sites with detailed migration behaviours and routes. Migration ecology and mechanisms throughout the life of anguillid eels have gradually been revealed in recent decades.

Estimations of Riverine Distribution, Abundance, and Biomass of Anguillid Eels in Japan and Taiwan Using Environmental DNA Analysis

Although populations of anguillid eels have declined remarkably in recent decades, monitoring data on the spatial and temporal variation in their dynamics are often limited, particularly for tropical eel species. As there are often sympatries of multiple eel species in tropical rivers, identifying eel species based solely on morphological characteristics is challenging. Basin-scale surveys were conducted in rivers of southern Japan and northern Taiwan to investigate (1) whether the spatial distribution, abundance, and biomass of the tropical eel species, the giant mottled eel (Anguilla marmorata), can be monitored in rivers by comparing the results obtained from environmental DNA (eDNA) analysis with data from electrofishing and (2) the riverine distribution of the sympatric A. marmorata and the temperate eel species, the Japanese eel (Anguilla japonica), in this region using eDNA analysis. Although we found an much lower abundance of A. marmorata in the study region, we identified the eDNA of the species from all of the study sites (21 sites) where it was collected by electrofishing, in addition to 22 further study sites where it was not collected directly. This indicates that eDNA analysis has a greater sensitivity for detecting A. marmorata, making it a powerful tool for monitoring the spatial distribution of the species in rivers. We found a significant positive relationship between eDNA concentration and both the abundance and biomass of A. marmorata, and eDNA concentration seemed to better reflect the abundance of the species than did biomass. eDNA of both A. japonica and A. marmorata was identified from almost all rivers, indicating the sympatry of these species in this region, although the degree of sympatry differed between rivers. Though the eDNA concentration of A. japonica decreased significantly with increasing distance from the river mouth, no significant relationship was found for A. marmorata. This study is the first to demonstrate the potential usefulness of eDNA analysis for estimating the spatial distribution, abundance, and biomass of tropical eels in rivers and to further apply this method to investigate sympatry among anguillid species. eDNA analysis can help in obtaining data on the population dynamics of tropical eels, providing invaluable information for managing these species.

Age, Growth, and Sex Ratios of the Giant Mottled eel, Anguilla marmorata, in Freshwater Habitats Near Its Northern Geographic Limit: A Comparison to Tropical Regions

To understand the latitudinal difference in the basic ecology of the giant mottled eel Anguilla marmorata, we measured its age, growth, and sex ratios in freshwater areas of Amami-Oshima Island, Japan (28.223°N-28.332°N and 129.329°E-129.439°E), near the northern geographic limit of its range and compared these biological characteristics with those observed in the tropical regions of Indonesia. A total of 109 A. marmorata were captured from three rivers on Amami-Oshima Island. The total length (TL) and age of sampled individuals varied across an order of magnitude (TL range: 119-1320 mm, mean: 385.5 ± 172.6; age range: 3-30 years, mean: 12.8 ± 4.9 years). Neither TL nor age differed among rivers. Male A. marmorata accounted for 88.5% of sexually differentiated individuals, which it contrary to previous results for males living in Indonesian watersheds where males were only found in small rivers and areas close to estuaries. Moreover, we found no males larger than 700 mm in TL and few males that were older than 20 years. The growth rates of male A. marmorata were significantly lower than those of females, and growth rates of sexually undifferentiated fish were significantly less than those of males. The mean growth rate of all individuals was 25.9 ± 6.6 mm/y, which is considerably lower than what has been found at lower latitudes, suggesting that growth differences occur along a latitudinal cline.

Distribution of anguillid leptocephali and possible spawning areas in the South Pacific Ocean

Seven South Pacific anguillid eel species live from New Guinea to French Polynesia, but their spawning areas and life histories are mostly unknown despite previous sampling surveys. A July-October 2016 research cruise was conducted to study the spawning areas and times, and larval distributions of South Pacific anguillid eels, which included a short 155°E station-line northeast of New Guinea and five long transects (5-25°S, 160°E-140°W) crossing the South Equatorial (SEC) and other currents. This survey collected nearly 4000 anguilliform leptocephali at 179 stations using an Isaacs-Kidd Midwater Trawl accompanied by 104 CTD casts. Based on mor-phometric observations and DNA sequencing, 74 anguillid leptocephali were collected, which in the southern areas included 29 larvae of six species: Anguilla bicolor pacifica, A. marmorata, A. australis, A. reinhardtii, A. megastoma, and A. obscura (all anguillid species of the region were caught except A. dieffenbachii). Small A. australis (9.0-16.8 mm) and A. reinhardtii (12.4, 12.5 mm) leptocephali were collected south of the Solomon Islands, other A. australis (10.8-12.0 mm) larvae were caught northwest of Fiji along with an A. obscura (20.0 mm) larva, and an A. marmorata (7.8 mm) larva was collected near Samoa. Considering collection sites, larval ages from otolith analysis, and westward SEC drift, multiple spawning locations occurred from south of the Solomon Islands and the Fiji area (16-20 days old larvae) to near Samoa (19 days old larva) during June and July in areas where high-salinity Subtropical Underwater (STUW, ~150 m depth) and the warm, low-salinity surface Fresh Pool were present. Five long hydrographic sections showed the strong Fresh Pool in the west and the STUW formation area in the east.

Using DNA Barcodes to Aid the Identification of Larval Fishes in Tropical Estuarine Waters (Malacca Straits, Malaysia)

Larval descriptions of tropical marine and coastal fishes are very few, and this taxonomic problem is further exacerbated by the high diversity of fish species in these waters. Nonetheless, accurate larval identification in ecological and early life history studies of larval fishes is crucial for fishery management and habitat protection. The present study aimed to evaluate the usefulness of DNA barcodes to support larval fish identification since conventional dichotomous keys based on morphological traits are not efficient due to the lack of larval traits and the rapid morphological changes during ontogeny. Our molecular analysis uncovered a total of 48 taxa (21 families) from the larval samples collected from the Klang Strait waters encompassing both spawning and nursery grounds of marine and estuarine fishes. Thirty-two (67%) of the larval taxa were identified at the species level, two taxa (4%) at the genus level, and 14 taxa (29%) at family level. The relatively low rate of species-level identification is not necessarily due to the DNA barcoding method per se, but a general lack of reference sequences for speciose and non- commercial fish families such as Gobiidae, Blenniidae, and Callionymidae. Larval morphology remains important in species diagnoses when molecular matches are ambiguous. A lower ethanol percentage (50%) for larva preservation is also useful to keep the body of larvae intact for morphological identification, and to preserve DNA for subsequent molecular analyses. The 10% Chelex resin used to extract DNA is also cost- effective for long term monitoring of larval fishes. Hence, the DNA barcoding method is an effective and easy way to aid the identification of estuarine larval fishes at the species level.