Takeshi Kikko

Interpopulation variation in egg size of fluvial white‐spotted charr Salvelinus leucomaenis

Interpopulation variation in egg size, which has been reported in many salmonid species and has been considered to be due to adaptations to local environmental conditions, is poorly examined in white-spotted charr Salvelinus leucomaenis. In this paper, egg size variations were analyzed for fluvial white-spotted charr populations in the Lake Biwa water system and its adjacent basins, which are located near the southern limit of its distribution range. Studies on the genetic structure of whitespotted charr populations in the Lake Biwa water system suggest that fluvial white-spotted charr form well-defined populations, despite being geographically close to each other. Field research was conducted in seven isolated reaches where fluvial white-spotted charr occur allopatrically (Fig. 1; Table 1). Because there are no records that charr have ever been artificially released in each reach, it was considered that all individuals to be native. Sampling by electrofishing was conducted for one or two days in each reach (whole period of sampling: 28 October to 4 November) during the spawning season in 2005. Captured fishwere immediatelyanesthetized, thenmeasured (standard length, SL) to the nearest millimeter, and weighed to the nearest gram. Sex was determined based on the secondary sexual characters (color, shape, jaw morphology), the abdomen of mature female fish was pushed softly to obtain fully ovulated eggs, and only the first 10 eggswere preserved in 5% buffered formalin. To identify individual fish, coloring materials were injected (Visible Implant Fluorescent Elastomar; North-west Marine Technology). Finally, the captured fish were released at the catch location after theyhad recovered from the anesthesia. FollowingFleming andNg,wetmass of 10 sampled eggs was weighed to the nearest mg. Egg size of an individual was given by the mass of 10 eggs/10 in the light that egg mass variation is small (coefficient of variation: 3.18–6.75%). Egg size of salmon is positively correlated with female body length. Variation in egg size could reflect covariation with body size. Following Ihssen et al., an allometric adjustments was applied to the observed egg size for comparisons among populations with different body size:

Diel patterns of larval drift of honmoroko Gnathopogon caerulescens in an inlet of Ibanaiko Lagoon, Lake Biwa, Japan.

Diel drift patterns of larvae of the endangered cyprinid Gnathopogon caerelescens in an inlet of the Ibanaiko Lagoon, connected to Lake Biwa in Japan, were assessed in April 2012. Peak occurrence of yolk-sac larvae was within a few hours after dark. Drift of newly hatched larvae is considered to be an important biological mechanism that ensures larval dispersal and recruitment from the inlets (spawning grounds) to the lagoon which functions as a nursery ground.

Introgression of Oncorhynchus masou subsp. (Biwa salmon) genome into lake-run O. m. ishikawae (Amago salmon) introduced into Lake Biwa, Japan

An investigation to determine the extent of introgression between Biwa salmon Oncorhynchus masou subsp. and lake-run Amago salmon O. m. ishikawae from Lake Biwa was conducted using mtDNA sequence and amplified fragment length polymorphism (AFLP) analysis. Hatchery-reared Amago salmon from the Samegai Trout Farm, the main source of stock released into the inflowing rivers of Lake Biwa, were also investigated. The extent of mtDNA introgression between these two subspecies was quite low: an anomalous haplotype was detected in just 1 of 55 Biwa salmon and 2 of 63 lake-run Amago salmon. Many of the mtDNA haplotypes found in lake-run Amago salmon were common to those in hatchery-reared Amago salmon. In the AFLP analysis, the nuclear genome of Amago salmon was hardly detected in Biwa salmon, but the Biwa salmon genome comprised 13.4% of the total amplified fragments from lake-run Amago salmon, being detected in different proportions in 28 of the 63 investigated individuals of the latter subspecies. These results suggest that introgression has occurred between Biwa salmon and lake-run Amago salmon descending from hatchery-reared Amago salmon from the Samegai Trout Farm.

Genetic evaluation of hatchery stocks of Honmoroko Gnathopogon caerulescens by mitochondrial DNA sequence for stock enhancement

Honmoroko Gnathopogon caerulescens is a critically endangered species and important for commercial fisheries; thus stock enhancement programs are being conducted to restore resources. We evaluated the genetic population structure of field collected samples including spawned eggs around spawning areas in Lake Biwa and the extent of genetic diversity in wild samples and hatchery stocks using sequences of mitochondrial cytochrome b gene. Pairwise Φst analysis and AMOVA clearly showed minimal population structure and haplotype network did not reveal any clear geographic pattern in Lake Biwa. It is probable that strays spawn in non-natal spawning areas, resulting in significant levels of gene flow among spawning areas. Genetic characteristics of hatchery stock F1, F2, and F3 were similar to those of wild samples in terms of haplotype diversity, nucleotide diversity and pairwise Φst values. These results indicate that the relatively high genetic diversity at its initiation was retained due to a lot of broodstock over two successive generations. Accordingly we propose that the current Honmoroko breeding method is appropriate for conserving the genetic diversity of Honmoroko and that the hatchery stock are genetically compatable for release and stock enhancement.

Contrasting life history patterns of the goby Rhinogobius similis in central Japan indicated by otolith Sr:Ca ratios

Rhinogobius similis a common goby, widely distributed in Far East Asia, is characterized by an amphidromous life history, adult fish inhabiting the lower reaches of rivers, and larvae in the freshwater area being swept downstream to the sea and returning to the river following a period of pelagic marine existence. Individuals were collected from 6 stations in the Ibi, Nagara and Kamo Rivers, central Japan, and their otolith strontium and calcium environmental signatures examined using electron probe microanalyses. The otolith Sr:Ca ratios of R. similis from the Kamo River suggested a typical amphidromous life history, whereas those of fish collected from the Ibi River suggested two types of migratory life history pattern: amphidromous and river resident. In contrast, the otolith Sr:Ca ratios of fish collected at two stations upriver of a dam in the Nagara River estuary suggested an artificially landlocked life history. Emergent plant vegetation in the lower reaches of the Ibi River created slow currents and pools in the river, thereby possibly enhancing the likelihood of a river resident life history. In the Nagara River, however, landlocked specimens had been obstructed in their downstream migration by the dam. It is suggested that R. similis in central Japan has a variable migratory life history pattern, allowing the species to persist in upstream habitats in large rivers.

Response of sex ratio to timing of breeding in the small cyprinid Gnathopogon caerulescens

The influence of hatching date on the sex ratio of wild Gnathopogon caerulescens was examined. Cohorts reared from eggs collected in the early and middle parts of the spawning season showed almost balanced sex ratios, with female bias in some cohorts. Cohorts born later in the season mostly displayed male bias, and the mean proportion of males later in the season was significantly higher than in early- and mid-season cohorts. These results indicate that the sex ratio of G. caerulescens changes with the time of breeding, increasing along with the ambient water temperature of the lake.

Size at maturity of fluvial white-spotted charr, Salvelinus leucomaenis, around the Lake Biwa water system varies with habitat size

Size at maturity of fluvial white-spotted charr, Salvelinus leucomaenis, was studied in small headwater tributaries of nine rivers around the Lake Biwa water system, Japan. Threshold size at maturity in both sexes showed significant positive relationships with water discharge, indicating that smaller threshold sizes at maturity of fluvial white-spotted charr occurred in smaller habitats. These results provide a link between size at maturity and habitat size and have important implications for the management of both habitats and white-spotted charr populations.

Iteroparity of stream resident masu salmon Oncorhynchus masou

The degree of iteroparity in stream-resident forms of masu salmon Oncorhynchus masou was examined using mark-recapture studies in natural streams. In a partially migratory population, at least 10% of resident males survived after maturation and repeatedly matured for up to 5 years. In the landlocked amago salmon subspecies, the post-maturation survival rate was at least 7% and repeat maturation was observed for up to 3 years for both males and females.

Juvenile migration of the exclusively pelagic cyprinid, Gnathopogon caerulescens (Honmoroko) in Lake Biwa, Central Japan: JUVENILE G. CAERULESCENS MIGRATION

Migration of wild and cultivated juvenile honmoroko Gnathopogon caerulescens of from the spawning and nursery areas in Lake Biwa were investigated, both in the Ibanaiko Lagoon and its outlet to Daido River, using beam-trawl surveys in 2013 and 2014. The study demonstrated migration of G. caerulescens from a nursery lagoon toward Lake Biwa after the juvenile stage. These findings appear to be the first direct evidence for migration of an exclusively pelagic cyprinid species from a littoral nursery to a pelagic adult habitat in a large deep lake.

Trace of outbreeding between Biwa salmon (Oncorhynchus masou subsp.) and amago (O. m. ishikawae) detected from the upper reaches of inlet streams within Lake Biwa water system, Japan

The establishment of fluvial fish populations from anadromous populations by natural or artificial barriers obstructing migration is a good research subject to study life history plasticity. Biwa salmon, Oncorhynchus masou subsp., a salmonid fish endemic to the Lake Biwa water system, exhibit life history variation (e.g., mature stream-resident males) in addition to a typical lacustrine life history type, indicating potential adaptations of life histories in response to emergence of barriers. Currently, fluvial populations that are morphologically similar to both stream-resident Biwa salmon and amago, the fluvial red-spotted masu salmon, Oncorhynchus masou ishikawae, are found upstream of dams which were constructed in the inflowing rivers of Lake Biwa. However, it is unknown whether they are Biwa salmon or amago. To explore that, the genetic characteristics of nine fluvial populations were investigated through AFLP and mtDNA analyses. Bayesian admixture analysis based on the AFLP data revealed that three fluvial populations were admixed populations between Biwa salmon and amago. In addition, a Biwa salmon mtDNA haplotype was detected in some individuals from three populations. However, no genetically pure fluvial populations of Biwa salmon were found, indicating no life history plasticity in this subspecies, and thus hybridization with amago boosted the ability of this subspecies to establish fluvial populations. Nevertheless, other scenarios, such as hybridization after establishment of fluvial populations of Biwa salmon, are also possible. The latter hypothesis could be supported by the fact that amago did not inhabit the river before emergence of barriers. However, a significant genetic population structure was found only in amago, suggesting that this subspecies is native to the Lake Biwa water system. But the possibility that multiple sources of amago have been released into rivers cannot be excluded. Therefore, further studies on the relationships between amago populations in the upper reaches of the Lake Biwa water system and other populations in the surrounding areas of the Lake Biwa water system are needed to clarify the origins of the admixed populations.