Aya Kotake

Occurrence of sea eels of Anguilla japonica along the Sanriku Coast of Japan

Abstract The age and migratory history of the Japanese eel, Anguilla japonica, collected along the Sanriku Coast of Japan, were examined using otolith microstructure and analysis of strontium (Sr) and calcium (Ca) concentrations. 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, including sea eels that never entered freshwater and others which had entered freshwater for brief periods but returned to the estuary or bay. This first evidence of the occurrence of sea eels in this northern area indicates that Japanese eels of the Sanriku Coast do not necessarily migrate into freshwater rivers.

Ecological Aspects of the Japanese Eel, Anguilla japonica, Collected from Coastal Areas of Japan

Abstract The ecological characteristics of 597 yellow and silver-stage Japanese eels, Anguilla japonica, were examined and compared among collection sites located at three different latitudes of Japan (Amakusa Islands, Mikawa Bay, and Sanriku Coast) to provide basic data on this unusual catadromous fish species. Eels were sexed and their total length, body weight, age, and growth rate based on otolith analysis was compared among sexes, stages, and collection sites. The overall sex ratio favored females (94%), but the sex ratio differed among the three locations. The frequency of females was highest in the coastal waters at Sanriku in the north (100%), next highest at Mikawa Bay in central Japan (95%), and lowest in the Amakusa Islands in the south (70%). Silver eel males ranged from 41.2–66.3 cm in length and 4–10 years in age, and silver eel females from 44.3–97.2 cm in length and 5–17 years in age. Female eels generally grew faster (8.7±2.2 cm/year) than males (6.4±2.6 cm/year), and the growth rate slowed in the older eels. The growth rate of A. japonica at all three sites was much faster than that of other temperate anguillid species (<4 cm/year), and their age at maturation was younger than that of other temperate species (~7 to >50 years), suggesting this species has important ecological differences from other similar species.

Evidence of downstream migration of Sakhalin taimen, Hucho perryi, as revealed by Sr:Ca ratios of otolith

The migratory history of Sakhalin taimen, Hucho perryi, was examined in terms of strontium (Sr) and calcium (Ca) uptake in the otolith by using wavelength dispersive X-ray spectrometry on an electron microprobe. Otolioth Sr : Ca ratios of freshwater-reared samples remained consistently at low levels throughout the otolith. The Sr : Ca ratios of samples from Lake Aynskoye of Sakhalin Island showed a low value from the core up to a point of 700–2140 µm. Thereafter, the ratios increased sharply and remained at higher levels up to the outermost regions. The difference in Sr : Ca ratio might be the result of the presence of individuals that underwent seawater and freshwater life history phases, probably reflecting the ambient salinity or the seawater–freshwater gradient in Sr concentration. Otolith Sr : Ca ratio analysis revealed downstream migration history in H. perryi.

Identifying sea-run brown trout, Salmo trutta, using Sr : Ca ratios of otolith

Abstract The migratory history of the brown trout, Salmo trutta, collected from Japanese rivers, was examined in terms of strontium (Sr) and calcium (Ca) uptake in the otolith, by means of wavelength dispersive X-ray spectrometry on an electron microprobe. Sea-run (anadromous) and freshwater-resident (nonanadromous) types of S. trutta were found to occur sympatrically. Otolith Sr concentration or Sr : Ca ratios of anadromous S. trutta fluctuated strongly along the life history transect in accordance with the migration (habitat) pattern from sea to freshwater. In contrast, the Sr concentration or the Sr : Ca ratios of nonanadromous fish remained at consistently low levels throughout the otolith. The higher ratios in anadromous S. trutta, in the otolith region from the core to 1500 μm, corresponded to the initial seagoing period, probably reflecting the ambient salinity or the seawater–freshwater gradient in Sr concentration. The findings clearly indicated that otolith Sr : Ca ratios reflected individual life histories, enabling the sea-run S. trutta to be distinguished from the freshwater-resident brown trout.

Migration of anadromous white-spotted charr Salvelinus leucomaenis, as determined by otolith strontium : calcium ratios

The migratory history of anadromous white-spotted charr Salvelinus leucomaenis collected from japanese coastal waters, was examined in terms of strontium (Sr) and calcium (Ca) uptake in the otolith, by means of wavelength dispersive X-ray spectrometry using an electron microprobe. Otolith Sr concentration or Sr∶Ca ratios, of anadromous S. leucomaenis, fluctuated strongly along the life history transect in accordance with the migration (habitat) pattern from sea to fresh water. The anadroous S. leucomaenis showed phase L (low Sr∶Ca ratio) from the core to the point 1000–2500 μm distant, averaging 1.3×10−3 to 2.7×10−3 and thereafter, the ratios increased sharply, being higher than 5.0 × 10−3 to 10.0 ×10−3. These findings indicated that otolith Sr∶Ca ratios reflected individual life histories, enabling a sea habitat to be identified from a freshwater habitat in this species.

Movements and life history patterns of the skipjack tuna Katsuwonus pelamis in the western Pacific, as revealed by otolith Sr:Ca ratios

The strontium (Sr) and calcium (Ca) concentrations in the otoliths of the skipjack tuna Katsuwonus pelamis collected in the western Pacific Ocean were examined by wavelength dispersive X-ray spectrometry on an electron microprobe. Otolith Sr:Ca ratios of the tunas collected off the Marshall Islands in the tropical waters were constant over the life history transect of the otolith. In contrast, ratios in most tunas collected off Sanriku in the temperate waters in October fluctuated from low to high at 1100 1800 μm from the core. A similar fluctuation of otolith Sr:Ca ratio from low to high was found in a skipjack tuna that was tagged and released off Sanliku, and then recaptured off the Palau Islands. Therefore, at least two life history patterns of the skipjack tunas, of global migration toward northern temperate regions and local residence in.tropical spawning grounds, are suggested by specimens collected in the western Pacific Ocean.

Variation in migratory history of Japanese eels, Anguilla japonica , collected in the northernmost part of its distribution

In order to examine the variation of migratory histories in the Japanese eel, Anguilla japonica , we measured otolith strontium (Sr) and calcium (Ca) concentrations by X-ray electron microprobe analysis in A. japonica collected in a coastal brackish water lake in the northernmost part of its distribution. Two migratory types that were categorized as river eels and estuarine eels were found. Estuarine eels were dominant (85%), while ordinary diadromous eels that had entered the freshwater habitat made up only 15% of the population. The low proportion of river eels suggested that the estuarine eels that inhabit the nearby coastal areas might make a larger reproductive contribution to the next generation in this area. There was no significant difference in growth between the river and estuarine eels, which suggested that the limited carrying capacity of the adjacent river and geographical features might be more effective in determining the habitat use of the Japanese eel than the genetic feature and food abundance at the northern edge of its distribution.

Migratory history and habitat use by New Zealand freshwater eels Anguilla dieffenbachii and A. australis, as revealed by otolith microchemistry

Abstract The migratory history of Anguilla dieffenbachii and A. australis, collected from a coastal lake of New Zealand, was examined using analysis of strontium (Sr) and calcium (Ca) concentrations. Line analysis of Sr : Ca ratios along the life history transect of each otolith showed a peak (Ca. 16–20 × 10−3) 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 eels had different migratory histories, which included freshwater residency in some eels (average Sr : Ca ratios, 1.7 × 10−3–2.4 × 10−3) but not in others (average Sr : Ca ratios, 3.1 × 10−3–6.5 × 10−3). These findings suggest that New Zealand freshwater eels have a flexible migration strategy and an ability to adapt to various habitats and salinities.

Environmental habitat use and migration of Plecoglossidae and Osmeridae fish

The life histories of Plecoglossidae and Osmeridaefish collected from Japanese fresh, brackish, and seawaters were studied by examining the strontium (Sr) to calcium (Ca) ratios in their otoliths. The Sr:Ca ratios in the otoliths changed with the salinity of the habitat. The fish living in a freshwater environment showed consistently low Sr:Ca ratios throughout the otolith. The fish were identified as a standard freshwater type. In contrast, fish collected from the intertidal zone showed higher otolith Sr:Ca ratios than those in the standard freshwater type, and the ratios fluctuated along the growth phase. In the present study, in addition to the representative migration pattern reported previously, other migration patterns were found to show consistently high Sr:Ca ratios throughout the otolith in several Osmeridae fish. Those results indicate that these fish have a flexible migration strategy with a high degree of behavioural plasticity and an ability to utilize the full range of salinity throughout their life history.