Physical structure of habitat network differently affects migration patterns of native and invasive fishes in Lake Biwa and its tributary lagoons: stable isotope approach

A good understanding of how migratory animals use their habitat network is expected to provide important insights for the prediction of population dynamics at both local and regional scales. We focused on how the physical structure of a habitat network could affect fish migration between Lake Biwa and its tributary lagoons. Although the lagoons provide suitable breeding and nursery grounds for native fishes, it is a matter of concern that they can also be a hotbed of invasive exotic fishes. Here, we assessed the migration patterns of native crucian carps (Carassius spp.) and exotic largemouth bass (Micropterus salmoides) and bluegill sunfish (Lepomis macrochirus) using their carbon and nitrogen stable isotopes as migration tracers. As there were marked differences in the carbon isotope ratios of basal food webs between the main lake and its tributary lagoons, stable isotopic signatures of individual fishes collected from each lagoon enabled us to judge whether they were residents of the lagoon or recent immigrants from the main lake. The analysis revealed that native and invasive fishes showed different migration patterns across a variety of lagoons. Exotic fishes frequently immigrated from the main lake to the lagoon as the distance of the channel connecting these two habitats was short. For native crucian carps, in contrast, their migrations were unaffected by the channel distance but were promoted by narrow channels. Physical barriers of weirs and dense vegetation within the channel obstructed their migrations. Such ecological information on migration behavior will be vital to plan designs for habitat restoration to conserve native fishes.     

Malaria vectors in Lake Victoria and adjacent habitats in western Kenya

The prevalence of malaria among the residents of the Lake Victoria basin remains high. The environment associated with the lake may maintain a high number of malaria vectors. Lake habitats including water hyacinths have been suspected to be the source of vectors. This study investigated whether malaria vectors breed in the lake habitats and adjacent backwater pools. Anopheline larvae were collected within the littoral zone of the lake and adjacent pools located along approximately 24.3 km of the lakeshore in western Kenya, and their breeding sites characterized. Three primary vector species, Anopheles arabiensis, Anopheles gambiae s.s. and Anopheles funestus s.s., and three potential vectors, were found in the lake habitats. Unexpectedly, An. arabiensis was the most dominant vector species in the lake sampling sites. Its habitats were uncovered or covered with short grass. A potential secondary malaria vector, Anopheles rivulorum, dominated the water hyacinths in the lake. Most breeding sites in the lake were limited to areas that were surrounded by tall emergent plants, including trees, and those not exposed to waves. Nearly half of adjacent habitats were lagoons that were separated from the lake by sand bars. Lagoons contained a variety of microhabitats. Anopheles arabiensis dominated open habitats, whereas An. funestus s.s. was found mainly in vegetated habitats in lagoons. The current study confirmed that several breeding sites are associated with Lake Victoria. Given that Lake Victoria is the second largest lake in the world, the lake related habitats must be extensive; therefore, making targeted vector control difficult. Further exploration is necessary to estimate the effects of lake associated habitats on malaria transmission so as to inform a rational decision-making process for vector control.     

Range expansion and lineage admixture of the Japanese evergreen tree Machilus thunbergii in central Japan

We investigated the range expansion histories of Machilus thunbergii populations in the Kinki region of central Japan on the basis of nuclear microsatellite data. In the Kinki region, M. thunbergii is typically found in the coastal area, with some fragmented populations inland, around Lake Biwa. Phylogenetic and Bayesian clustering analysis (STRUCTURE analysis) revealed that the inland populations have different genetic components between the west and east sides of Lake Biwa. The population located on the north side of the lake has an admixture of the two genetically differentiated lineages, contributing to an increase in the genetic diversity of the population. Populations around Lake Biwa had lost rare alleles and the F value obtained from STRUCTURE analysis was lower in the coastal populations than in the lake populations. These results suggest that populations around Lake Biwa experienced a bottleneck due to a founder effect during the initial migration to the lake and that glacial refugia of M. thunbergii in the Kinki region existed along the coast.     

Isotope analysis reveals proportional change and site-selection variation of river- and lake-produced eggs of a landlocked migratory fish

Changes in the proportions of river- and lake-produced eggs of a landlocked amphidromous fish, ayu (Plecoglossus altivelis altivelis) in the Lake Biwa water system, Japan, were monitored by stable isotope analysis, based on different δ¹⁵N and δ¹³C values of prey organisms between the lake and its tributaries. During the 3 month reproduction season, the δ¹⁵N values of spawned eggs decreased with time. This result implies that there was a shift from lake-produced eggs to river-produced eggs within a reproductive season, based on the observation that adult fish in the lake had previously been shown to have eggs with distinctly higher δ¹⁵N values in their ovaries than those in the tributaries. This explanation was also supported by the change in δ¹³C values of the spawned eggs. Furthermore, eggs with lower δ¹⁵N and higher δ¹³C values tended to be spawned at less variable depths, suggesting that females spawning river-produced eggs selected the spawning sites from a narrower range. We conclude that stable isotope ratios of spawned eggs can be indicators of the relative contributions of different food chains and can enable comparisons of reproductive characteristics between types of egg.     

Phylogeny and historical demography of endemic fishes in Lake Biwa: the ancient lake as a promoter of evolution and diversification of freshwater fishes in western Japan

To elucidate the origins of the endemic fish of Lake Biwa, an ancient lake in Japan, and the role of the lake in the diversification of freshwater fish in western Japan, we established a molecular phylogenetic framework with an absolute time scale and inferred the historical demography of a large set of fish species in and around the lake. We used mtDNA sequences obtained from a total of 190 specimens, including 11 endemic species of Lake Biwa and their related species, for phylogenetic analyses with divergence time estimations and from a total of 2319 specimens of 42 species (including 14 endemics) occurring in the lake for population genetic analyses. Phylogenetic analysis suggested that some of the endemic species diverged from their closest relatives earlier (1.3–13.0 Ma) than the period in which the present environmental characteristics of the lake started to develop (ca. 0.4 Ma), whereas others diverged more recently (after 0.4 Ma). In contrast, historical demographic parameters suggested that almost all species, including endemic and nonendemic ones, expanded their populations after the development of the present lake environment. In phylogeographic analyses, common or very close haplotypes of some species were obtained from Lake Biwa and other regions of western Japan. The phylogenetic and historical demographic evidence suggests that there was a time lag between phylogenetic divergence and population establishment and that phenotypic adaptation of some endemic species to the limnetic environment occurred much later than the divergences of those endemic lineages. Population structure and phylogeographic patterns suggest that Lake Biwa has functioned not only as the center of adaptive evolution but also as a reservoir for fish diversity in western Japan.     

Genetic homogeneity among breeding grounds and nursery areas of an exploited Lake Malawi cichlid fish

1. Nursery areas are commonly recognized as important habitats for the management and conservation of fish stocks. Here we report the use of nursery areas by an exploited offshore cichlid in Lake Malawi, Rhamphochromis longiceps .
2. Like all haplochromine cichlids that have been studied, the species is a maternal mouthbrooder that broods eggs for several weeks following spawning. We found evidence that during this brooding period females migrate from open water to release juveniles in three shallow peripheral waterbodies (Chia Lagoon, Unaka Lagoon, Lake Malombe). However, it was unclear whether there is geographical population structuring within the species, which could indicate stock differences in their use of these nursery habitats, or if the lake contains a genetically panmictic population that employs nursery habitats opportunistically.
3. To investigate spatial and temporal population structuring within the lake we genotyped populations at seven microsatellite DNA loci. Overall, we found no significant spatial structuring among juveniles from the peripheral lagoons or among breeding males within the main lake body. Moreover, we found no evidence of temporal structuring among males on the breeding grounds within Lake Malawi or females entering Chia lagoon. Together, these results suggest that Lake Malawi contains a genetically homogeneous population of R. longiceps .
4. At present we know little of the distribution of juvenile R. longiceps elsewhere in the Lake Malawi basin, but their absence from surveyed rocky and sandy littoral habitats makes it possible that the species is dependent upon a small number of nearshore nursery areas, including these lagoons. As such, conservation of lagoon habitats and monitoring of exploited fish stocks within them may be important for effective preservation of biodiversity within the catchment.     

Development and characterization of 24 polymorphic microsatellite markers for bluegill sunfish, <Emphasis Type="Italic">Lepomis macrochirus</Emphasis>

The bluegill sunfish (Lepomis macrochirus) is a notorious exotic fish, which causes serious damages to freshwater ecosystems in the world. Therefore, it is a matter of urgency to control this species for the conservation of ecosystem. New microsatellite loci were developed for L. macrochirus using high throughput 454 GS-FLX+ pyrosequencing. We selected 24 primer pairs that were tested on 47 individuals from Lake Biwa, which encompasses the oldest and largest population of L. macrochirus in Japan. Allele per locus varied 2–17. Expected and observed heterozygosities ranged from 0.185 to 0.888 and 0.120 to 0.872, respectively. Applied to L. macrochirus in Lake Biwa, these markers could detect genetic differentiation among populations, which showed a pattern of isolation by distance. These markers are expected to be useful in the estimation of population structure and ecology of L. macrochirus.     

Horizontal distribution of carbon and nitrogen and their isotopic compositions in the surface sediment of Lake Biwa

The horizontal distribution of the abundance and isotopic composition of carbon and nitrogen was studied on surface sediment samples (0–15 cm) collected from the entire area of Lake Biwa, the largest freshwater lake in Japan. As water depth increased, a marked increase in organic matter content was observed at the sampling sites, especially in the western North Basin, characterized by a steep slope. In the northwestern North Basin, which has no major inflowing streams, the sediments contained large amounts of organic matter, suggesting the possibility of lateral transportation of sedimented matter from other places by lake currents. The total amounts of carbon and nitrogen in the top-2 cm of sediment of the entire area of Lake Biwa were estimated to be 9.2 × 10⁴ tC and 1.0 × 10⁴ tN. The δ¹⁵N values in the littoral sediment were low and close to those in the inflowing river sediment, suggesting selective sedimentation of allochthonous organic matter onto the littoral area. In the North Basin, vertical profiles of organic matter content and δ¹³C values of the sediments in the littoral area showed a smaller downward decrease than in the profundal area, whereas δ¹⁵N values decreased with sediment depth in both areas. It was suggested that the littoral sediments contained abundant amounts of allochthonous and relatively refractory organic matter. Further, it was suggested that the autochthonous organic matter originated from primary production deposited mainly on the profundal zone and was easily decomposed in early diagenesis after sedimentation. Received: July 30, 1999 / Accepted: December 10, 1999     

Morphological and Phylogenetic Characterizations of Freshwater Thioploca Species from Lake Biwa, Japan, and Lake Constance, Germany

Filamentous, gliding, sulfide-oxidizing bacteria of the genus Thioploca were found on sediments in profundal areas of Lake Biwa, a Japanese freshwater mesotrophic lake, and were characterized morphologically and phylogenetically. The Lake Biwa Thioploca resembled morphologically Thioploca ingrica, a brackish water species from a Danish fjord. The diameters of individual trichomes were 3 to 5.6 μm; the diameters of complete Thioploca filaments ranged from 18 to 75 μm. The cell lengths ranged from 1.2 to 3.8 μm. In transmission electron microscope specimens stained with uranyl acetate, dense intracellular particles were found, which did not show any positive signals for phosphorus and sulfur in an X-ray analysis. The 16S rRNA gene of the Thioploca from Lake Biwa was amplified by using newly designed Thioploca-specific primers (706-Thioploca, Biwa160F, and Biwa829R) in combination with general bacterial primers in order to avoid nonspecific amplification of contaminating bacterial DNA. Denaturing gradient gel electrophoresis (DGGE) analysis of the three overlapping PCR products resulted in single DGGE bands, indicating that a single 16S rRNA gene had been amplified. With the same method, the Thioploca from Lake Constance was examined. The 16S rRNA sequence was verified by performing fluorescence in situ hybridization targeted at specific motifs of the Lake Biwa Thioploca. Positive signals were obtained with the bacterial probe EUB-338, the γ-proteobacterial probe GAM42a, and probe Biwa829 targeting the Lake Biwa Thioploca. Based on the nearly complete 16S rRNA sequence and on morphological similarities, the Thioploca from Lake Biwa and the Thioploca from Lake Constance are closely related to T. ingrica and to each other.     

How important are trophic state,macrophyte and fish population effects on cladoceran community? A study in Lake Erhai

Environmental controls on cladoceran community structure in lake ecosystems are complex and may involve many environmental parameters including trophic state and fish populations. In Lake Erhai, a plateau lake located in southwest China, it was hypothesized that a combination of lake eutrophication and planktivorous fish introduction would increase the abundance of cladoceran, while also decreasing cladoceran size. To test this hypothesis, we examined temporal changes in cladoceran microfossils in the sediments of Lake Erhai over the past century. The influence of changing macrophyte coverage within the littoral region of the lake was also considered. Results demonstrated that cladoceran abundance (measured as flux of cladoceran fossils in the sediments) increased markedly accompanying eutrophication of the lake. In addition, there was a shift in the dominant cladoceran species from those species that prefer oligotrophic conditions to those that prefer mesotrophic and eutrophic conditions. A reduction in the ephippium length of Daphnia spp. was observed and attributed to the introduction of the planktivorous fish Neosalanx taihuensis. Our findings indicated that eutrophication and fish introduction were the main controls affecting cladoceran community structure during the recent decades, and predation by planktivorous fish had an important impact on Daphnia body size.     

Phenotypic variation and vulnerability to predation in juvenile bluegill sunfish (<Emphasis Type="Italic">Lepomis macrochirus</Emphasis>)

Bluegill sunfish (Lepomis macrochirus) are known to diversify into two forms specialized for foraging on either limnetic or littoral prey. Because juvenile bluegills seek vegetative cover in the presence of largemouth bass (Micropterus salmoides) predators, natural selection should favor the littoral body design at size ranges most vulnerable to predation. Yet within bluegill populations, both limnetic and littoral forms occur where vegetation and predators are present. While adaptive for foraging in different environments, does habitat-linked phenotypic variation also influence predator evasiveness for juvenile bluegills? We evaluate this question by quantifying susceptibility to predation for two groups of morphologically distinct bluegills; a limnetic form characteristic of bluegills inhabiting open water areas (limnetic bluegill) and a littoral form characteristic of bluegills inhabiting dense vegetation (littoral bluegill). In a series of predation trials, we found that bluegill behaviors differed in open water habitat but not in simulated vegetation. In open water habitat, limnetic bluegills formed more dense shoaling aggregations, maintained a larger distance from the predator, and required longer amounts of time to capture than littoral bluegill. When provided with simulated vegetation, largemouth bass spent longer amounts of time pursuing littoral bluegill and captured significantly fewer littoral bluegills than limnetic fish. Hence, morphological and behavioral variation in bluegills was linked to differential susceptibility to predation in open water and vegetated environments. Combined with previous studies, these findings show that morphological and behavioral adaptations enhance both foraging performance and predator evasiveness in different lake habitats.     

Juvenile migration of the exclusively pelagic cyprinid,Gnathopogon caerulescens (Honmoroko) in Lake Biwa,Central Japan

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.     

Genetic variations estimated from PCR-RFLP analysis of two morphs of the freshwater goby <Emphasis Type="Italic">Rhinogobius</Emphasis> in the Lake Biwa water system

The freshwater goby Rhinogobius is the most abundant fish in the shore area of Lake Biwa, Japan. Recently, it has been reported that two morphs of Rhinogobius inhabit this lake. In this study, genetic variations in Rhinogobius sp. OR (Orange-type) and Rhinogobius sp. BW (Biwa-type) in the Lake Biwa water system have been investigated using polymerase chain reaction-based restriction fragment length polymorphism (PCR-RFLP) analysis of mitochondrial DNA, including the variable D-loop region. Samples of Rhinogobius sp. OR were collected from the middle sites of three rivers, two outlets, and two lakeshore sites, whereas samples of Rhinogobius sp. BW were taken from two lakeshore sites. Rhinogobius sp. OR and Rhinogobius sp. BW did not share any haplotypes, suggesting that PCR-RFLP analysis is effective for distinguishing between these species of goby. The haplotype diversity of Rhinogobius sp. OR (0.214–0.543) was lower than that of Rhinogobius sp. BW (0.543–0.682). There were no significant differences in haplotype frequencies between Rhinogobius sp. BW groups from the two localities. In addition, haplotype frequencies in Rhinogobius sp. OR did not differ significantly among samples from the middle sites of rivers, the outlets, and the shores. These results indicate that in Rhinogobius sp. OR there is frequent gene flow among fish inhabiting different sites, and that this species of goby consists of a single population throughout the Lake Biwa water system.     

The relationships between anadromy, sex ratio and parr growth of Arctic charr in a lake in North Norway

Relationships between the degree of anadromy, sex ratio and parr growth of Arctic charr Salvelinus alpinus were investigated in Lake Storvatn in Hammerfest town, by estimating the densities of resident and anadromous charr in the lake, and by comparing the smolt characteristics of fish captured in the outlet river with those of fish of the same age in the lake. About two-thirds of the charr older than 5 years were anadromous. The sex ratio of smolts was approximately 1 : 1 and the frequency of mature male parr in the lake was low. As females tended to dominate the mature portion of the anadromous population, there was probably a relatively higher mortality among sea-run male smolts. The youngest smolts (3+ and 4+) were probably recruited from fast-growing parr in the littoral zone, while older smolts (>4+) may have been recruited from all habitats in the lake.     

Yellow perch genetic structure and habitat use among connected habitats in eastern Lake Michigan

Maintenance of genetic and phenotypic diversity is widely recognized as an important conservation priority, yet managers often lack basic information about spatial patterns of population structure and its relationship with habitat heterogeneity and species movement within it. To address this knowledge gap, we focused on the economically and ecologically prominent yellow perch (Perca flavescens). In the Lake Michigan basin, yellow perch reside in nearshore Lake Michigan, including drowned river mouths (DRMs)—protected, lake‐like habitats that link tributaries to Lake Michigan. The goal of this study was to examine the extent that population structure is associated with Great Lakes connected habitats (i.e., DRMs) in a mobile fish species using yellow perch as a model. Specifically, we tested whether DRMs and eastern Lake Michigan constitute distinct genetic stocks of yellow perch, and if so, whether those stocks migrate between the two connected habitats throughout the year. To do so, we genotyped yellow perch at 14 microsatellite loci collected from 10 DRMs in both deep and littoral habitats during spring, summer, and autumn and two nearshore sites in Lake Michigan (spring and autumn) during 2015–2016 and supplemented our sampling with fish collected in 2013. We found that yellow perch from littoral‐DRM habitats were genetically distinct from fish captured in nearshore Lake Michigan. Our data also suggested that Lake Michigan yellow perch likely use deep‐DRM habitats during autumn. Further, we found genetic structuring among DRMs. These patterns support hypotheses of fishery managers that yellow perch seasonally migrate to and from Lake Michigan, yet, interestingly, these fish do not appear to interbreed with littoral fish despite occupying the same DRM. We recommend that fisheries managers account for this complex population structure and movement when setting fishing regulations and assessing the effects of harvest in Lake Michigan.     

Littoral zone fish communities of two Florida lakes and a comparison with Michigan lakes

Synopsis The abundance and habitat distribution of littoral zone fishes in two small southern Florida lakes were quantified by underwater censuses. The bluegill (Lepomis macrochirus) and large-mouth bass (Micropterus salmoides) accounted for 75–80% of community biomass in both lakes; important coexisting species were predominantly benthic feeders in Lake Sirena and planktivores in Lake Annie. All species were largely confined to areas of macrovegetation which extended to a depth of 4 m in Lake Annie but only to 1.2 m in Lake Sirena. The differences in community structure were related to differences in habitat and also variation in water levels and benthic production.A comparison of community composition with that of small Michigan lakes indicated that similar numbers of species coexist in the littoral zones, despite a three-fold difference in the size of regional species pools. The majority of families and about 50% of the species were common to lakes in both regions; in addition a number of ecological analogues were noted. A major difference was that the small inshore species were members of the Cyprinodontiformes in Florida and Cyprinidae in Michigan. Together the largemouth bass and bluegill comprise similar community proportions in the two regions as do other major feeding groups. Lakes in the two regions that are similar in amount and distribution of vegetation exhibit greater similarity in fish communities than those within regions that differ in littoral vegetation.     

Distribution of CH4 in the north basin of Lake Biwa and tributary rivers

To determine the origin of CH₄, the vertical distribution of CH₄ around the thermocline in the north basin of Lake Biwa and the horizontal distribution in the Yasu and Ado rivers were measured. In 1995, CH₄ concentrations (ranging from 200 to 1000 nM) exceeding the saturation level were observed just above or in the thermocline in the pelagic region off the Yasu River. These values were higher than those just below the thermocline and in the hypolimnion. CH₄ concentrations in the mouths of the Yasu and Ado rivers were much higher than at other stations, around 2500 and 2000 nM, respectively. Due to the drought in 1994 when there was no water from the Yasu River flowing into the north basin, CH₄ concentration just above or in the thermocline in the pelagic region off the Yasu River ranged from 49 to 74 nM. It is thus concluded that the high level of CH₄ observed in the mouth of the Yasu River is one of the sources of the high CH₄ concentrations in the pelagic region off the Yasu River. On the other hand, the CH₄ concentration in the pelagic region off the Ado River was about 50 nM. It is thought that water flowing in from the Ado River diffused readily into the lake water because the depth of the lake in the region off the Ado River declines steeply. The maximum concentration of CH₄ in the river mouths indicates that these areas are important sites for clarifying the mechanism of the decrease in dissolved oxygen in Lake Biwa and the Yodo River watershed. Received: October 25, 2000 / Accepted: May 8, 2001     

Clonal diversity in the Japanese silver crucian carp,Carassius langsdorfii inferred from genetic markers

The Japanese silver crucian carp, Carassius langsdorfii, is a naturally polyploid fish that reproduces by gynogenesis, resulting in a geographically clustered clonal population. To determine the amount of clonal diversity in this species, we used genetic markers to compare fish from several Japanese locations. Samples were collected from Lake Kasumigaura (Ibaraki Pref.), Lake Imba (Chiba Pref.), Lake Fukushimagata (Niigata Pref.), Lake Biwa (Shiga Pref.), Lake Koyama (Tottori Pref.), and the Niyodo and Monobe Rivers (Kochi Pref.). The genetic markers used were the microsatellite loci GF1(*), GF17(*), and GF29(*), as well as isozyme (creatinekinase CK(*)) and mtDNA (D-loop region) RFLP. In the seven locations sampled, 61 clonal lines were identified by combining the genotypes of the three microsatellite loci. Populations at all locations consisted of six to 28 clonal lines. Fourteen common clonal lines were detected in two or more locations, and 13 of these were observed in Lake Biwa. We suggest that these clonal lines have been widely distributed in Japanese freshwaters through the intentional stocking of fish species such as "gengorobuna" (C. cuvieri) and "ayu" (Plecoglossus altivelis altivelis) obtained from Lake Biwa and its adjacent waters. The CK(*) genotypes and mtDNA haplotypes of fish from Lake Kasumigaura differed from those of other locations.     

Multiple colonizations of Lake Biwa by Sarcocheilichthys fishes and their population history

Rapid adaptation and speciation have not been well documented for organisms in Lake Biwa, Japan, the oldest ancient lake in East Asia. To examine these processes, we focused on the divergence of Sarcocheilichthys gudgeons and conducted phylogeographic and population genetic analyses using mitochondrial DNA sequences and microsatellite data. No remarkable genetic differentiation was found between two species, S. biwaensis and S. variegatus microoculus, both endemic to Lake Biwa, whereas this species group, including S. v. variegatus, was genetically divided into three major regional groups: the Lake Biwa, Ise Bay, and western groups. Divergent mtDNA haplotypes were included within the Lake Biwa and western groups, strongly suggesting secondary contacts among allopatrically isolated populations. Dating for mtDNA phylogeny using a geological constraint suggested the colonization of Lake Biwa by multiple Sarcocheilichthys lineages that diverged from each other in the early–middle Pleistocene. Coalescent-based population analyses indicated that the local populations colonized the rocky bottom habitat in Lake Biwa from other habitats after the Last Glacial Maximum, likely reflecting past environmental changes in the lake, including the disappearance of rocky areas during the glacial climate. Divergent morphological adaptation in Sarcocheilichthys associated with substrate type may have rapidly proceeded along with environmental changes.     

Trophic position estimates of formalin-fixed samples with nitrogen isotopic compositions of amino acids: an application to gobiid fish (Isaza) in Lake Biwa, Japan

To evaluate the influence of formalin-fixation on the nitrogen isotopic composition of individual amino acids, we conducted a formalin-fixation experiment over 62 weeks using muscle tissues of largemouth bass (Micropterus salmoides) and freshwater minnow (Zacco platypus) collected in Lake Biwa. The results indicated that the formalin-fixation does not affect the nitrogen isotopic composition of amino acids including glutamic acid and phenylalanine from muscle protein of the fish. We applied this technique to formalin-fixed gobiid fish “Isaza” (Gymnogobius isaza, Tanaka) specimens that had been collected from Lake Biwa through the twentieth century. The pattern of the δ¹⁵N values for these amino acids remains relatively constant, even though each amino acid exhibits a gradual increase by ~3 ‰ from 1916 to 1992. With a formula proposed by Chikaraishi et al. (Limnol Oceanogr Method 7:740–750, 2009), we estimated the temporal variations in the trophic position of Isaza in Lake Biwa. The trophic position of Isaza remained quite constant (3.2–3.3) across a major eutrophication period in 1960–1980. The estimated trophic position of Isaza fish is consistent with the stomach content analysis in the previous reports. The constant trophic position suggests that the eutrophication did not apparently affect the trophic position of Isaza fish, although Isaza’s dietary preference changed from zooplankton to gammarids around 1970.