Kazuki Yokouchi

A silvering index for the Japanese eel Anguilla japonica

To establish a simple and reliable index for determining silvering stages of the Japanese eel, Anguilla japonica, we observed the colorations of various body parts and biological characteristics of the eels collected in a coastal area of Japan (Mikawa Bay). The four silvering stages are characterized by the colorations of pectoral fins and ventral skin as follows: (1) Y1, yellow eel without a metallic hue at the base of pectoral fins, (2) Y2, late yellow eel with a metallic hue at the base of the pectoral fins but without melanization at the tip of pectoral fins, (3) S1, silver eel with complete melanization at the tip of pectoral fins but without full pigmented belly in black or dark brown, and (4) S2, late silver eel with black or dark brown belly. The body size, eye diameter and sexual maturity of each stage increased in the order of Y1, Y2, S1 and S2 stages, whereas the digestive tract degenerated in the same order, suggesting a sequential development of these ontogenetic stages identified in the study. The Y1, Y2 and S1 stages could be also distinguished by canonical discriminant function analysis using three internal (gonad-somatic index, GSI; hepato-somatic index, HIS; and gut index) and two morphometric (condition factor and eye index) parameters, supporting the significance of these stages. This method of staging for the silvering process of the Japanese eel appeared to be applicable to all specimens of this species, since this index used only simple external characteristics that would be easy to observe during field surveys.

The migratory history of anadromous and non-anadromous tapertail anchovy Coilia nasus in the Yangtze River Estuary revealed by the otolith Sr:Ca ratio

The migratory history of tapertail anchovy Coilia nasus in the Yangtze River Estuary, China was investigated using otolith Sr:Ca ratios and two-dimensional images of the Sr level from an X-ray electron probe microanalyzer (EPMA). The results showed that 17 of the 22 young-of-the-year (YOY) specimens had low Sr:Ca ratios (1.2–2.4 × 10−3;1.5 ± 0.3 × 10−3) at the central otolith area, indicating their riverine origin and initial freshwater residence. In addition, 11 of the 14 adult specimens had low Sr:Ca ratios (1.3–2.2 × 10−3; 1.7 ± 0.4 × 10−3) at the central otolith area but showed alternating changes between high (>4.0 × 10−3) and low (<2.5 × 10−3) values outside of this region, reflecting their riverine origin and the migration between freshwater and estuarine habitats. These 28 specimens represented the anadromous population in this region. The other 5 YOY specimens had high Sr:Ca ratios (3.6–5.9 × 10−3; 4.8 ± 0.8 × 10−3) throughout the life history. Similarly, the other 3 adult specimens had high Sr:Ca ratios (4.0–5.7 × 10−3; 4.8 ± 0.7 × 10−3) at the central otolith area but showed alternating changes between low and high values outside this region, suggesting that estuarine-origin non-anadromous individuals occurred in this region. The average of the otolith Sr:Ca ratios and Sr level mapping along the life-history transects could be used as a scalar for charting the migratory history of the tapertail anchovy in the Yangtze River Estuary: <2.0 × 10−3 for freshwater residence and 3.5–6.0 × 10−3 for estuarine residence.

Head‐shape polymorphism in Japanese eels Anguilla japonica in relation to differences of somatic growth in freshwater and brackish habitats

The age, total length (L(T)), head shape and skull shape were investigated for 379 Japanese eels Anguilla japonica sampled in freshwater and brackish areas of the Kojima Bay-Asahi River system, Okayama, Japan, to learn about the differentiation process of head-shape polymorphism. The relative mouth width (ratio of mouth width to L(T)) of A. japonica > 400 mm L(T) collected in fresh water was significantly greater than that of fish collected in brackish water. Growth rates of mouth width and the distance from the snout to the midpoint of the eyes (the ratio of width and distance to age, respectively) were not significantly different between freshwater and brackish-water samples, whereas the somatic growth rate (the ratio of L(T) to age) of freshwater samples was significantly lower than that of brackish-water eel samples. These results suggest that the factors affecting head and somatic growth of A. japonica are not identical. According to these results and feeding patterns in each habitat reported by another study, it is suggested that somatic growth appears to play a significant role in the differentiation process of the head-shape polymorphism in A. japonica, with the slow-growing fish in fresh water becoming broad-headed and the fast-growing fish in brackish water becoming narrow-headed.

Discrimination of wild and cultured Japanese eels based on otolith stable isotope ratios

Discrimination of wild and cultured Japanese eels based on otolith stable isotope ratios Kenzo Kaifu*, Hikaru Itakura, Yosuke Amano, Kotaro Shirai, Kazuki Yokouchi, Ryoshiro Wakiya, Naoko Murakami-Sugihara, Izumi Washitani, and Takashi Yada Faculty of Law, Chuo University, 724-1 Higashinakano, Hachioji-shi, Tokyo 192-0393, Japan Graduate School of Science, Kobe University, 1-1, Rokkoudaichou, Nadaku, Kobe, Hyogo 657-8501, Japan Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, 3-27-5, Shinhama, Shiogama, Miyagi 985-0001, Japan Atmosphere and Ocean Research Institute, The University of Tokyo 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8564, Japan National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, Yokosuka, Kanagawa 238-0316, Japan Research and Development Initiative, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, Nikko, Tochigi 321-1661, Japan *Corresponding author: tel: þ81 42 674 3243; fax: þ81 42 674 3243; e-mail: kaifu@tamacc.chuo-u.ac.jp

Do Japanese eels recruit into the Japan Sea coast?: A case study in the Hayase River system, Fukui Japan

The Japanese coastlines along the Sea of Japan (Japan Sea) have been thought to be one of the margins of the distribution range of the Japanese eel Anguilla japonica, and there is evidence that eels had naturally recruited into these areas several hundred years ago. However, recruitment there is uncertain recently, because there seems to be no study that reported glass eel or elver recruitment into the coasts along the Japan Sea for a couple of decades, and the eels inhabiting these areas were probably stocked by fisheries cooperatives. In order to improve understanding of the present-day natural geographic distribution range of this species, we searched for naturally recruited wild eels in the Hayase River system, Fukui Prefecture, which flows into the Japan Sea. Multiple approaches including investigation of glass eel recruitment, comparison of body size, and estimation of habitat use types was employed. During the observation period (from January to July 2010), no glass eels were found at the river mouth of the Hayase River in monthly sampling. Of eels collected in this study (n = 127), no eels smaller than the initial body size of eels for stocking were found in this water system and none were identified as being of wild origin based on the habitat use type classifications from otolith microchemistry (n = 48). This lack of evidence of Japanese eels recruiting into Japan Sea coast waters suggests most eels present there may be stocked eels. Japanese eels could have been distributed naturally along the Japan Sea coast more than in recent years, indicating a possible decrease of the natural distribution range of this species.

Location, size and age at onset of metamorphosis in the Japanese eel Anguilla japonica

This study clarifies the location, size and age at the onset of metamorphosis in Japanese eels Anguilla japonica through oceanic surveys, rearing experiments and analyses of the morphology and otoliths of leptocephali and glass eels. Twenty-eight metamorphosing leptocephali were collected in the mesoscale eddy region to the east of Taiwan during research expeditions in 2004. Rearing experiments showed that the total length (LT ) of leptocephali decreased by an average of 12·5% during metamorphosis and 13·9% during the 2-12 h after death. Thus, the mean back-calculated LT at the onset of metamorphosis for 630 glass eels from Taiwan and Japan was estimated at 67·8 ± 2·7 mm (mean ± S.D.). The estimated mean ante-mortem size of the fully grown pre-metamorphic leptocephali collected in 2004 was 64·6 ± 3·4 mm, which was consistent with the LT estimate for glass eels. Otolith analysis showed that the mean age at the onset of metamorphosis was 137 ± 15 days and indicated that Japanese eels may have a recruitment route through the mesoscale eddies to the east of Taiwan in addition to the direct transfer route from the North Equatorial Current to the Kuroshio Current.

Depletion of naturally recruited wild Japanese eels in Okayama, Japan, revealed by otolith stable isotope ratios and abundance indices

To investigate the population dynamics of naturally recruited wild Japanese eels, fisheries data of wild individuals in Okayama Prefecture were investigated as a case study. Wild and stocked eels were discriminated using a recently developed method based on otolith stable isotopes. Of the 161 eels captured in freshwater areas where eels had been stocked, 98.1% were discriminated as stocked. In contrast, 82.8% of 128 eels captured in coastal areas where eels are not stocked were discriminated as wild. There was a significant decrease in longline and set-net catch per unit effort between 2003 and 2016 in the coastal areas where most eels were discriminated as wild, indicating ongoing depletion of wild Japanese eels in these waters.

Low-fidelity homing behaviour of Biwa salmon Oncorhynchus sp. landlocked in Lake Biwa as inferred from otolith elemental and Sr isotopic compositions

Biwa salmon Oncorhynchus sp. is endemic to Lake Biwa, Japan, where it is an important commercial and recreational fisheries species. However, no information is currently available on its population structure and migration ecology. Therefore, here we evaluated whether otolith Sr/Ca, Ba/Ca and 87Sr/86Sr ratios can be used as natural signatures in Biwa salmon and then used these to determine the natal origins of lake-migration-phase individuals and spawning adults, and the homing ability of spawning adults in the Lake Biwa water system. Quadratic discriminant function analysis demonstrated that the lake-migration school comprised individuals with multiple origins, including rivers to the east, west and north of Lake Biwa, and that the homing rate of spawning adults was low (18 out of 80 individuals), with ca. 78% of fish straying into non-natal rivers. However, this straying behaviour was not spatially random, with fish tending to migrate upstream in rivers neighbouring their natal rivers. The high rate of straying in spawning adults is considered important for establishing and maintaining this species, which is highly adapted to life in the Lake Biwa water system where environmental disturbances often occur.

Growth potential can affect timing of maturity in a long-lived semelparous fish

Many diadromous fishes such as salmon and eels that move between freshwater and the ocean have evolved semelparous reproductive strategies, but both groups display considerable plasticity in characteristics. Factors such as population density and growth, predation risk or reproduction cost have been found to influence timing of maturation. We investigated the relationship between female size at maturity and individual growth trajectories of the long-lived semelparous European eel, Anguilla anguilla. A Bayesian model was applied to 338 individual growth trajectories of maturing migration-stage female silver eels from France, Ireland, the Netherlands and Hungary. The results clearly showed that when growth rates declined, the onset of maturation was triggered, and the eels left their growth habitats and migrated to the spawning area. Therefore, female eels tended to attain larger body size when the growth conditions were good enough to risk spending extra time in their growth habitats. This flexible maturation strategy is likely related to the ability to use diverse habitats with widely ranging growth and survival potentials in the catadromous life-history across its wide species range.