Effects of density, starvation and size difference on aggressive behaviour in juvenile yellowtails (Seriola quinquevadiata)

The effects of density, starvation and size difference on aggressive behaviour in juvenile yellowtails, Seriola quinqueradiata (Temminck and Schlegel), wee investigated. When fish were acclimated to densities of 1, 2, 4, 8, and 16 fish L-1, the frequency of aggressive behaviour per fish decreased significantly. Following starvation for 0, 4, 8, 12 and 24 h, the highest aggression was observed at 24 h, while levels of aggression were equal between 0 and 8 h. According to a pair match test from one batch among three groups of different size (total length mm: 16.6 ± 2.0, 21.1 ± 4.9), the highest aggression was observed in the group with the largest size difference. therefore, in view of the three parameters investigated, aggressive behaviour in the yellowtail was concluded to be positively starvation- and size-dependent, but negatively density-dependent. Synthesizing all relevant results, including abiotic factors such as the light condition, a practical method is presented to reduce mortality caused by aggressive behaviour in yellowtail cultures

Studies on eels and leptocephali in Southeast Asia : A new research frontier

Leptocephali are the unique type of fish larvae of eels and their close relatives, which are abundant in the upper few hundred meters of the world\u27s tropical and subtropical oceans. Historically there have been few studies on leptocephali in Southeast Asia because these larvae require special fishing techniques to collect and are difficult to identify and match with their adult species. Recent cooperative sampling surveys for leptocephali in the Indonesian Seas region have indicated that there is a high biodiversity of eels in the region, so more taxonomic research to distinguish the many species of leptocephali of more than 20 families of eels and their relatives is needed to facilitate detailed studies on the biodiversity and ecology of these fishes. Several techniques such as fishing large plankton nets offshore or using set nets in coastal areas with strong tidal flow can be used to collect leptocephali, and a new field guide to identifying the major taxa of leptocephali in the region has been recently published. Examination of metamorphosing larvae entering coastal areas is one method to help match larval and adult forms, and a more modern technique is to compare molecular genetic characters between larval and adult specimens, but this requires the collection and proper tissue preservation of both leptocephali and adults. These types of studies and those on the ecology of eels and their larvae represent a new research frontier in marine biodiversity science in Southeast Asia, which is the center of marine biodiversity worldwide

Evaluation of the population structure of Anguilla bicolor bicolor using total number of vertebrae and the mtDNA control region

The population structure of Anguilla bicolor bicolor was evaluated using their total number of vertebrae and genetic analyses of the mtDNA control region. Based on the likely geographic population structure of this subspecies in the Indian Ocean, the data were combined into East (Madras, Sumatra Island and Australia) and West groups (South Africa, Madagascar, Reunion and Seychelles) according to their sampling localities. Significant differences were not found in the range and mean total number of vertebrae between the East (N=74) and West groups (N=47) in the Indian Ocean, which were 107 to 113 (108.29±1.26), and 106 to 113 (109.60±1.47), respectively. Furthermore, the polymerase chain reaction-restriction fragment length polymorphism analysis for the mitochondrial control region using a total of 18 specimens from three localities showed no genetic differences between the East (N=14, Myanmar and Sumatra Island) and West groups (N=4, Madagascar) in the Indian Ocean. Morphological and genetic characters examined in the present study suggested no population structure for A. bicolor bicolor in the east and west side of the Indian Ocean, whereas they were ecologically assumed to be different populations. This contradiction suggested that these populations were in the beginning of speciation

Migratory history of wild and released ayu (Plecoglossus altivelis) in the Kurobe River, Japan

In order to discuss the influence of dam discharge on the movement of ayu Plecoglossus altivelis in a river and the stocking effectiveness of released ayu, the migratory history of 83 juveniles of ayu (standard length=67.9-142.4mm sL) collected in the Kurobe River, Japan during 25 June-23 August 2004 was determined by otolith microchemistry, and the origin of fish was identified as being either native amphidromous ayu or released amphidromous or landlocked fish. Except for otoliths with abnormal morphology (N=10), most specimens (96%, N=70) had inner layers (radius:>400μm) of higher Sr: Ca ratios and outer ones of low values. These specimens were identified as native amphidromous fish that migrated upstream from the sea. Only 4% (N=3) were released ayu (reared fish), which were further divided into two origins, amphidromous fish (N=2) with smaller inner layers (radius:<400μm) of high Sr: Ca ratios, and landlocked fish (N=1) with constantly low values over the whole otolith. There were no specimens with an increase in Sr: Ca ratios in the outer layers of the low Sr: Ca region of the otolith after their upstream migration in spring, suggesting that it is unlikely that any of these ayu had been washed-out from the river by cold and turbulent river water

First record of a leptocephalus larva ingested by a chaetognath

An approximately 17mm Nemichthys scolopaceus leptocephalus was found in the gut of a 25mm chaetognath, Sagitta (Flacdsagitta) hexaptera, collected in the North Equatorial Current region of the western North Pacific. The chaetognath was photographed before preservation. The leptocephalus was folded in half and was positioned deep within the hindgut. Although the chaetognath was caught during sampling for leptocephali, its location in the hindgut suggests that the leptocephalus could have been ingested prior to capture by the net. This first record of ingestion of this unique type of fish larva suggests that if contact is made with small leptocephali, it is possible for them to be ingested by relatively large sized chaetognaths that may identify them as potential prey

Genetic diversity of Sicyopterus japonicus as revealed by mitochondrial DNA sequencing

To examine the genetic diversity of Sicyopterus japonicus, 448 sites of the control region of the mitochondrial DNA were analyzed in 77 specimens from the four localities of Okinawa, Kochi, Wakayama, and Shizuoka in Japan. A total of 74 haplotypes were found in the individuals examined. The same haplotypes occurred in Okinawa and Kochi, Kochi and Wakayama, and two specimens in Wakayama. The average sequence in genetic characteristics within localities varied from 1.7% in Kochi, 1.6% in Wakayama to 1.4% in Okinawa. There was no significant difference in genetic characteristics among the three locations of Wakayama, Kochi, and Okinawa (10,000 times permutation test, P>0.05). The number of specimens from Shizuoka was too small to compare to the other three locations. The neighbor-joining tree of the mitochondrial DNA haplotypes for all specimens constructed from the Kimura\u27s two-parameter distances suggested no evidence of genetic subdivision of S. japonicus. These results suggested that this species has a single panmictic population and their larvae probably have a high dispersal ability during their oceanic stage

Environmental history of Japanese eel, Anguilla japonica collected in Miyako Bay, northeastern Japan

The environmental history in combination with age of the Japanese eel, Anguilla japonica, collected in Miyako Bay along the Sanriku Coast of Japan, was examined using the otolith microstructure and analysis of strontium (Sr) and calcium (Ca) concentrations with wavelength dispersive X-ray spectrometry by an electron microprobe. The line analysis of Sr : Ca ratios along the life history transect of each otolith showed a peak (ca 12-17×10^) between the core and elver mark, which corresponded to the period of their leptocephalus and early glass eel stages in the ocean. The mean Sr : Ca ratios from the elver mark to the otolith edge indicated that there were eels with several general categories of migratory history, that included sea eels that never entered freshwater (average Sr : Ca ratios, >6.0×10^) and others that had entered freshwater for brief periods, but returned to the estuary or bay. This evidence of the occurrence of sea eels indicates that Japanese eels in this northern area do not necessarily migrate into freshwater rivers during recruitment as glass eels at the beginning of their growth phase, and even those that do enter freshwater may later return to the marine environment

Age and growth of Anguilla interioris leptocephali collected in Indonesian waters

Anguilla interiorisis has been only known to be distributed in northern New Guinea, but recent sampling surveys for leptocephali and the development of species identification techniques using DNA analysis have discovered that this species also appears to occur off Sumatra in the eastern Indian Ocean, and may be present in other areas of the Indonesian Archipelago. To reveal the ages and early life histories of this species, the otolith microstructure of the leptocephali collected near the Sulawesi Island and off Sumatra of Indonesia were examined. The otolith microstructure of the A. interioris leptocephali was similar to other anguillid species and showed narrow increment widths (0.49-0.58μm) near the core that increased (1.14-1.34μm) before decreasing again until the otolith edge (0.51-0.83μm), except in the smallest specimen (12.4mm TL) collected off Sumatra, which had no peak. The range of Sr: Ca ratios in their otoliths were 8.37 to 14.01. Their ages were 19d for the smallest specimen from off Sumatra, 85d for the specimen (48.9mm TL) from Tomini Bay, and 85 to 94d for the three specimens (43.4-46.5mm TL) from the Molucca Sea. The overall growth rate of the leptocephali was 0.48mm/d, and this value was intermediate compared with the growth rates of the other anguillid species. The age of the smallest A. interioris leptocephalus collected off Sumatra and the geographic patterns of currents in the regions, suggest that it was spawned in the Indian Ocean and that it may belong to a different population than the New Guinea population