Current Dependent Dispersal Characteristics of Japanese Glass Eel around Taiwan

Japanese eel larvae are passively transported to the East Asian Continental Shelf by the North Equatorial Current, Kuroshio and Kuroshio intrusion currents, and coastal currents. Previous studies have investigated the dispersal characteristics and pathways of Japanese glass eels. However, there are still limitations in these studies. According to long-term (2010–2020) catch data from the Fisheries Agency in Taiwan, the distribution and time series of glass eels recruitment to Taiwan are closely related to the surrounding ocean currents. Recruitment begins in eastern Taiwan via the mainstream Kuroshio and in southern Taiwan via the Taiwan Strait Warm Current. In central Taiwan, recruitment occurs from southern Taiwan, as well as from mainland China via the southern branch of the China Coast Current (CCC). The latest recruitment occurred in northern Taiwan and mainly comprised glass eels from mainland China via the northern branch of the CCC. A stronger monsoon during the La Niña phase could affect the recruitment time series in northern and eastern Taiwan. This study suggests that the recruitment directionality of glass eels is an indicator of the flow field of ocean/coastal currents and elucidates the dispersal characteristics of glass eels in the waters around Taiwan.

Aspects of the Biology of Juvenile Freshwater Eels (Anguillidae) in New Zealand

<p>The early freshwater life of the two species of New Zealand freshwater eels, Anguilla australis schmidtii Phillipps and A. dieffenbachii Gray was studied involving an examination of 8131 glass-eels, 5275 migratory elvers, and 4291 resident eels of less than 26 cm. Most eels were collected from the Makara Stream, Wellington by set-net, hand-net and electric fishing. These extensive samples together with subsidiary collections from elsewhere in New Zealand show that glass-eels of both species arrive in fresh-water from July to December. Their otoliths indicate a marine larval life of about 18 months but it is not possible as yet to locate the precise oceanic spawning areas. Migratory movements of glass-eels are in two phases: an invasion of fresh-water from the sea and an upstream migration. The former occurs only at night with a periodicity corresponding to the daily ebb-flood tidal rhythms. There is a seasonal reversal in this response which is attributable to the onset of the behavioural transition taking place prior to the second migratory phase. Increased pigmentation and changes in response to light, flowing fresh-water and schooling tendencies characterise this latter migration which occurs primarily at spring tide periods. Such juvenile eels show specific habitat preferences and a high degree of olfactory differentiation of water types. This behaviour, together with pigment development and physical tolerances, was studied in the laboratory. Measurements of invading glass-eels show that mean length, weight and condition all decline throughout the season of arrival but mean vertebral numbers remain constant. An upstream migration of small eels (elvers) occurs each summer and is readily observed at many hydro-electric stations. These migrations, comprising eels of mixed sizes and age groups, penetrate progressively further upstream each year. In both species, scales begin formation at body lengths of 16.5-20 cm. All features of scale formation, including the number of scale rings, are related to length with relative differences in rate of development occurring between the species. In contrast to scale rings, otolith rings are annual in formation and become visible after grinding or burning the otolith. Growth rates established for 273 eels to 29 cm in length from the Makara Stream, Wellington, are slow, with mean annual increments of 2.2 and 2.1 cm respectively for shortfins and longfins. In contrast, shortfins from a coastal lake near Wellington reach 26 cm in their third year of freshwater life. Length-weight relationships for small eels are given together with mean monthly condition factors. Growth studies on elvers held in a multiple tank unit in which temperature, density, and amount and frequency of feeding could be controlled, show that young eels grow more slowly than normal under such conditions. However, growth appears optimum at 20 degrees C with a feeding rate of 5-7% body weight per day. Feeding efficiency decreases with higher temperatures. At both glass-eel and elver stages, shortfins adapt and survive better under artificial conditions.</p>

Aspects of the Biology of Juvenile Freshwater Eels (Anguillidae) in New Zealand

<p>The early freshwater life of the two species of New Zealand freshwater eels, Anguilla australis schmidtii Phillipps and A. dieffenbachii Gray was studied involving an examination of 8131 glass-eels, 5275 migratory elvers, and 4291 resident eels of less than 26 cm. Most eels were collected from the Makara Stream, Wellington by set-net, hand-net and electric fishing. These extensive samples together with subsidiary collections from elsewhere in New Zealand show that glass-eels of both species arrive in fresh-water from July to December. Their otoliths indicate a marine larval life of about 18 months but it is not possible as yet to locate the precise oceanic spawning areas. Migratory movements of glass-eels are in two phases: an invasion of fresh-water from the sea and an upstream migration. The former occurs only at night with a periodicity corresponding to the daily ebb-flood tidal rhythms. There is a seasonal reversal in this response which is attributable to the onset of the behavioural transition taking place prior to the second migratory phase. Increased pigmentation and changes in response to light, flowing fresh-water and schooling tendencies characterise this latter migration which occurs primarily at spring tide periods. Such juvenile eels show specific habitat preferences and a high degree of olfactory differentiation of water types. This behaviour, together with pigment development and physical tolerances, was studied in the laboratory. Measurements of invading glass-eels show that mean length, weight and condition all decline throughout the season of arrival but mean vertebral numbers remain constant. An upstream migration of small eels (elvers) occurs each summer and is readily observed at many hydro-electric stations. These migrations, comprising eels of mixed sizes and age groups, penetrate progressively further upstream each year. In both species, scales begin formation at body lengths of 16.5-20 cm. All features of scale formation, including the number of scale rings, are related to length with relative differences in rate of development occurring between the species. In contrast to scale rings, otolith rings are annual in formation and become visible after grinding or burning the otolith. Growth rates established for 273 eels to 29 cm in length from the Makara Stream, Wellington, are slow, with mean annual increments of 2.2 and 2.1 cm respectively for shortfins and longfins. In contrast, shortfins from a coastal lake near Wellington reach 26 cm in their third year of freshwater life. Length-weight relationships for small eels are given together with mean monthly condition factors. Growth studies on elvers held in a multiple tank unit in which temperature, density, and amount and frequency of feeding could be controlled, show that young eels grow more slowly than normal under such conditions. However, growth appears optimum at 20 degrees C with a feeding rate of 5-7% body weight per day. Feeding efficiency decreases with higher temperatures. At both glass-eel and elver stages, shortfins adapt and survive better under artificial conditions.</p>

North Equatorial Current and Kuroshio Velocity Variations Affect Body Length and Distribution of the Japanese eel Anguilla japonica in Taiwan and Japan

The larval stage of Japanese eel travels a substantial distance over a long duration through the North Equatorial Current (NEC) and the Kuroshio, and the spawning behavior of mature eels leads to monthly arrival waves in eastern Taiwan between November and February. The total length (TL) of the glass eel relates to its larval duration and age; therefore, the TL can indicate the larval duration. The monthly mean TLs of eels along eastern Taiwan from 2010–2021 were used to estimate the batch age, and the recruitment patterns and relative abundances were compared. The TLs of glass eels followed a normal distribution, and the estimated ages were highly correlated with their mean TLs. Early recruit TLs were significantly greater than those of late recruits. The mean tracer drift time was longer in early recruitment months (November–December) than in later dates (February–March). The recruitment lag was approximately 1–1.5 months, with relative recruitment higher in the early recruitment months than in later months. Cohorts followed the main streams of the NEC and Kuroshio, and the monthly velocity changes of these currents could affect the TLs as well as the distribution patterns of Japanese glass eels in Taiwan and Japan.

Assessing Temporal Patterns and Species Composition of Glass Eel (Anguilla spp.) Cohorts in Sumatra and Java Using DNA Barcodes

Anguillid eels are widely acknowledged for their ecological and socio-economic value in many countries. Yet, knowledge regarding their biodiversity, distribution and abundance remains superficial—particularly in tropical countries such as Indonesia, where demand for anguillid eels is steadily increasing along with the threat imposed by river infrastructure developments. We investigated the diversity of anguillid eels on the western Indonesian islands of Sumatra and Java using automated molecular classification and genetic species delimitation methods to explore temporal patterns of glass eel cohorts entering inland waters. A total of 278 glass eels were collected from monthly samplings along the west coast of Sumatra and the south coast of Java between March 2017 and February 2018. An automated, DNA-based glass eel identification was performed using a DNA barcode reference library consisting of 64 newly generated DNA barcodes and 117 DNA barcodes retrieved from BOLD for all nine Anguilla species known to occur in Indonesia. Species delimitation methods converged in delineating eight Molecular Operational Taxonomic Units (MOTUs), with A. nebolusa and A. bengalensis being undistinguishable by DNA barcodes. A total of four MOTUs were detected within the glass eel samples, corresponding to Anguilla bicolor, A. interioris, A. marmorata, and A. nebulosa/A. bengalensis. Monthly captures indicated that glass eel recruitment peaks in June, during the onset of the dry season, and that A. bicolor is the most prevalent species. Comparing indices of mitochondrial genetic diversity between yellow/silver eels, originating from several sites across the species range distribution, and glass eels, collected in West Sumatra and Java, indicated a marked difference. Glass eels displayed a much lower diversity than yellow/silver eels. Implications for the management of glass eel fisheries and species conservation are discussed.