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Contribution of amphidromous (Ooayu) and lake types (Koayu) of the landlocked Ayu ( Plecoglossus altivelis altivelis) fish population in Lake Biwa to the next generation was investigated by isotopic approaches. δN values of eggs in adult ovaries distinctively differed between the two types. Contribution of Koayu, calculated using δN values of spawned Ayu eggs, was much greater than that of Ooayu in general, as previously reported. Ooayu spawned more eggs in the upper area of the spawning ground and did so late in the season. These results show the potential of isotopic approaches for identifying maternal life-history types of spawned eggs of migratory animals. [ABSTRACT FROM AUTHOR]

Female ayu Plecoglossus altivelis were sampled from the Mogami River System, Yamagata Prefecture to investigate the occurrence of multiple spawning that has previously been reported in small rivers. Spawning frequencies of the females were estimated by histological observations of the ovary mainly from the degree of degeneration of postovulatory follicles. In the middle and late part of the spawning period, most females showed characteristics of having spawned two or three times, indicating that multiple spawning is a common phenomenon also in a large river. Mean relative batch fecundity (RBF) and condition factor (without gonads) decreased with the number of spawning batches. Size dependency in the occurrence of the multiple spawning and the RBF values was not observed. These results are similar to those reported in previous studies of ayu from a small river near the Mogami River System, and indicate that the occurrence and properties of multiple spawning are almost the same regardless of the river scale. However, it is possible that the ratios of multiply spawned females may differ due to variations in survival rates during the spawning season. [ABSTRACT FROM AUTHOR]

Abstract: Hydraulic structures such as a dam, weir and water gate affect on the flow depth, velocity, bed topology, water quality and so on, so that the hydraulic structures may affect on the water environment for plants and aquatic lives in and around rivers. Therefore, before construction of such hydraulic structures, it is necessary to conduct the environmental assessment. However, the accuracy of the environmental assessment is not so high at present, because the preference curves of the flow depth, velocity and so on are not clear. Recently, Nagaya et al. [Nagaya, T., Onitsuka, K., Higashino, M., Takami, T., Otsuka, N., Akiyama, J., Matsumoto, K., 2004. Evaluation of weight of parameter for spawning of ayu, Plecoglossus altivelis, and prediction of spawn density per unit area. In: Lee, J.H.W., Lam, K.M. (Eds.), Environmental Hydraulics and Sustainable Water Management, vol. 2. AA. Balkema Publishers, UK, pp. 947–954] pointed out that the velocity, flow depth and substrate affect on the suitability of spawning for ayu, Plecoglosses altivelis altivelis on the basis of statistical investigation. Preference curves of velocity and flow depth were made clear by Onitsuka et al. [Onitsuka, K., Nagaya, T., Higashino, M., Takami, T., Otsuka, N., Akiyama, J., Ozeki, H., Matsumoto, K., Shiraishi, Y., 2005. Suitable flow depth and velocity for spawning of ayu, Plecoglossus altivelis. In: Proceedings of the 31th Congress of IAHR, Korea, pp. 1850–1858]. On the other hand, the preference curve of substrate has not been investigated. In this study, the preference curve of substrate was investigated on the basis of field survey. Further, a numerical simulation by making use of a horizontal 2D numerical model was carried out with changing the discharge at the Gokasegawa River due to a lot of ayu inhabits in the Gokasegawa River. The numerical model is based on the model which was suggested by Nagata et al. [Nagata, N., Hosoda, T., Muramoto, Y., 2000. Numerical analysis of river channel processes with bank erosion. Journal of Hydraulic Engineering, ASCE 126 (4), 243–252]. The suitability of spawning for ayu is predicted with the preference curves of the flow depth, velocity and substrate. [Copyright &y& Elsevier]

Adult ayu Plecoglossus altivelis were sampled from the Nezugaseki River, Yamagata Prefecture to investigate the reproductive parameters and analyze their variations related to multiple spawning. Ovarian histology (mainly based on the occurrence of postovulatory follicles) indicated that female ayu can be divided into four groups, which are considered to correspond with unspawned, once spawned, twice spawned, and three (or more) times spawned individuals. Comprehensive analysis of the ovarian state with field surveys revealed that most of the female ayu achieve second spawning and that third spawning is very common in this river. Mean relative batch fecundity (RBF) for the second, third, and fourth spawning indicated 69, 42, and 22%, respectively, of the value for the first spawning, showing a decline with the number of spawning batches. Mean gonadosomatic index (GSI) values of non-ovulating fish showed a similar decline during the main spawning period (October). Condition factor (without gonads) during autumn also decreased with the number of batches spawned, suggesting that multiple spawning is achieved partly by transfer of body mass nutrients to the ovary. Size dependency in reproductive parameters such as occurrence of the multiple spawning, RBF values, and GSI values of the unspawned fish, was not evident except for GSI values in early September (generally during the early phases of ovarian development), when larger fish had larger GSI. [ABSTRACT FROM AUTHOR]

In order to study the physiological characteristics of neoteny in teleosts, the development of the thyroid axis and digestive tract of the ice fish,Salangichthys microdon(Salmonoidei), were examined using wild caught samples in the Yura River and its estuary. In all developmental stages examined in this study, the digestive tract was straight, and no gastric gland was observed, even in adults. Since these features are only observed in larval stages of a related species, ayu,Plecoglossus altivelis(Salmonoidei), it is suggested that the ice fish retains larval features, not only in appearance but also in internal organs. Additionally, the thyroid axis exhibited a unique characteristic. Until the end of the juvenile stage, thyroid glands remained inactive, and no thyroid stimulating hormone (TSH) cells were observed. Activation of the thyroid gland and the first appearance of TSH cells occurred in adults. Since activation of the thyroid axis was generally known to occur by the time larval have transformed to juveniles in several teleosts, a delayed functioning of the thyroid axis was suggested as a specific characteristic, and probably a major factor contributing to neotenic development of the ice fish. [ABSTRACT FROM AUTHOR]

Ayu Plecoglossus altivelis including amphidromous and landlocked populations, are known for batch spawning. Unusual clutches at ovulation were observed from rearing stocks that were naturally induced to sexual maturation. Two peaks occurred simultaneously in the frequency distribution of egg size, compared to normal fish, which have clutches with unimodal size distribution. The eggs in the small size class could be fertilized, and small-sized larvae hatched from these eggs. To explain the alteration in egg size through divergence from amphidromous to landlocked populations, the role of batch allocation among oocytes is discussed. [ABSTRACT FROM AUTHOR]

This article presents a study on the occurrence of multiple spawning in the wild amphidromous type ayu. Adult ayu were collected from the Nezugaseki River in Yamagata, Japan by cast net or by electrofishing during autumn of 2001. The collected fish were killed with tricaine methanesulfonate and sectioned. Female ayu were divided into three groups by the occurrence of post ovulatory follicles. The results demonstrate the occurrence of multiple spawning in wild amphidromous type ayu, and also indicate that multiple spawning is a common phenomenon in the Nezugaseki River.

Abstract Ayu Plecoglossus altivelis is a typical amphidromous fish with ontogenic requirements for habitats among lower reaches of a river, the marine coastal area, and middle reaches of a river at the immature adult stage. Mature fish that had a small body size like ordinary young were found around the spawning ground. In comparison with standard adults, the migratory histories of the dwarf adults were examined using otolith microchemistry analysis. Sr?:?Ca ratio estimated that the dwarf fish were exposed to saline or brackish water after once moving into the river, whereas the standard-size fish migrated upstream. [ABSTRACT FROM AUTHOR]

Examines the otolith strontium-calcium ratio of spawning ayu to determine the contribution of stocked populations to reproduction in rivers in Japan. Measurement of body length and weight; Gonad index of ayu; Genetic similarity to the amphidromous population.