Amphidromous Life History Research Articles

Movement and Growth of Translocated Coastrange Sculpin and Prickly Sculpin in Two Urban Lake Washington Tributaries, Washington

Abstract Urbanization and other forms of land conversion have led to dramatic changes in freshwater ecosystems. We assessed the movement and growth of Coastrange Sculpin Cottus aleuticus and Prickly Sculpin C. asper, two species that are considered migratory based on their potamodromous and amphidromous life history strategies. We determined critical aspects of their ecology in two urban streams where they are rare or absent. Both species were collected near the stream mouth or in Lake Washington, implanted with passive integrated transponder (PIT) tags, and translocated to an upper stream reach where their downstream movements and growth were monitored. Detections at stationary antennas indicated most downstream movements occurred from January to April, as has been observed in other Pacific Northwest systems. About 80% of the translocated sculpins remained in the study reach for at least 2 months before moving downstream and 10% remained more than a year. Smaller Prickly Sculpin generally remained in the study reach longer than larger Prickly Sculpin; however, no size effect was observed for Coastrange Sculpin. Most downstream movements occurred during low and moderate streamflow levels during each season for both streams; however, downstream movements occurred at relatively higher flows compared to the cumulative distribution of observed flows during most seasons. Both species appeared to grow well in the study streams; although, Prickly Sculpin growth rates were approximately double that of Coastrange Sculpin. Coastrange Sculpin often inhabit riffles where increased fine sediments and embeddedness from urbanization may impact them more than Prickly Sculpin, which typically inhabit pools. The overall survival and growth of both species in the two highly urbanized streams was better than expected. This study suggests that the lack of sculpins in some urban streams may be a result of stream barriers to movement rather than poor habitat conditions.

Modelling the longitudinal distribution, abundance, and habitat use of the giant freshwater shrimp (Macrobrachium spinipes) in a large intermittent, tropical Australian river to inform water resource policy

Abstract Water development threatens rivers and their biodiversity. Amphidromous shrimp are particularly vulnerable as they require migration between freshwater and estuaries to complete their life cycle. The Fitzroy River is a large tropical intermittent river undergoing water development that is home to the amphidromous shrimp Macrobrachium spinipes (cherabin), yet little is known about its habitat use and flow‐ecology making it difficult to inform sustainable water‐take. We investigated habitat associations, distributional patterns suggestive of amphidromy, and the influence of water availability by sampling main channel and floodplain pools along a 350‐km river length during 2 contrasting flow years. Applying a size‐specific abundance model, we estimated abundance per size class, site, and year. We then predicted abundance at the landscape scale with remotely sensed water to reveal the impact of water availability on the meta‐population. Our model revealed that juveniles were in greatest abundance in downstream main channel pools, whereas adults were in greatest abundance in upstream floodplain pools. Abundance varied by year with lower numbers predicted in the low‐flow year. Longitudinal and habitat patterns remained when our pool‐level results were scaled to the landscape, and the positive relationship of abundance to wet‐season flow was strengthened. The predominance of smaller cherabin in the lower reaches of the river provides indirect support for an estuarine nursery and amphidromous life history; however, small individuals observed in landlocked pools, during late dry season suggests possible within‐river recruitment. The importance of water development policies that protect wet‐season flow and passage along the Fitzroy River is supported by this work. These types of policies are likely to be important for this and other amphidromous shrimp species across Australia, Southeast Asia and further afield. Further research detailing the species life history and describing flow–recruitment relationships will be important contributions to understanding this important taxonomic group and refining policies for current and future water resource development.

Diversity in Habitat Use by White Sturgeon Revealed Using Fin Ray Geochemistry

Understanding life-history diversity in a population is imperative to developing effective fisheries management and conservation practices, particularly in degraded environments with high environmental variability. Here, we examined variation in habitat use and migration patterns of White Sturgeon (Acipenser transmontanus), a long-lived migratory fish that is native to the San Francisco Estuary, CA, United States. Annual increment profiles were combined with respective geochemical (87Sr/86Sr) profiles in sturgeon fin rays to reconstruct annual salinity chronologies for 112 individuals from 5 to 30 years old. Results indicated a complex and diverse amphidromous life history across individuals, characterized largely by estuarine residence, a general ontogenetic trend toward higher-salinity brackish habitats, and high variability in habitat use across all age groups. Hierarchical clustering based on fin ray geochemistry during the first 10 years of life, prior to sexual maturation, indicated at least four distinct migratory phenotypes which differed largely in the timing and duration of juvenile to subadult movements between fresh- and brackish-water habitats. This study provides information regarding habitat use and migration in sub-adult fish that was previously lacking. Different migratory phenotypes vary in exposure to stressors across time and space and populations. Understanding White Sturgeon habitat distributions through space and time at different life stages can help identify areas where habitat restoration would be most effective and develop management actions to reduce stressors associated with specific areas where White Sturgeon are present.

Amphidromous but endemic: Population connectivity of Rhinogobius gigas (Teleostei: Gobioidei).

Rhinogobius gigas is an amphidromous fish endemic to eastern Taiwan. Fishes with the diadromous behavior are expected to have a broader distribution range and higher genetic homogeneity despite that some amphidromous fishes with limited distribution are observed and R. gigas is an additional exception with a limited distribution range. Rhinogobius gigas has been documented to be retained inshore near the river plume with a short pelagic larval duration of 30–40 days, which may account for the endemism of this species. The short marine larval stage of R. gigas may imply a population genetic structure and the aim of the present study is to test whether the population genetic structure is present in R. gigas. To test the population genetic structure, fragments of mitochondrial displacement loop and cytochrome c oxidase subunit I were sequenced to provide molecular inference for genetic structure among populations. Sixty-nine haplotypes were identified among 191 R. gigas from 10 populations of eastern Taiwan and the mean haplotype and nucleotide diversities for all samples were 0.956 and 0.0024, respectively, implying a bottleneck followed by a recent population expansion further supported by Fu’s Fs (-26.6; p < 0.001) and Tajima’s D (-1.5; p = 0.037) values. The phylogenetic analysis revealed lack of genetic structure and the bush-like median joining network without commonly shared haplotypes supports the same scenario. The genetic homogeneity is probably due to the amphidromous life history providing the opportunity for passive larval transportation among the rivers through coastal currents in eastern Taiwan. The endemism to eastern Taiwan may be a consequence of complicated interactions among short pelagic larval duration, interspecific competition and coastal currents.

Living in an amphidromous world: Perspectives on the management of fish passage from an island nation

Abstract Amphidromy is a form of migratory life history typified by the reproduction of fish in freshwater environments, the early downstream dispersal of post‐hatch larvae to marine environments, and the return of small‐bodied young juveniles to freshwater environments for growth to adulthood. Island freshwater fish communities are frequently dominated by fish species with amphidromous life histories. Amphidromous life cycles leave fish communities highly susceptible to habitat modification and disruptions to connectivity across marine and freshwater environments. This means that managing waterway connectivity is fundamental to their conservation; however, the unique and often geographically restricted amphidromous communities that characterize many small island nations have received little consideration in the development of strategies for the management of fish passage. The ecology and locomotory capabilities of amphidromous species are often poorly studied, partly because their small size at migratory life stages renders current state‐of‐the‐art in situ biotelemetry methods unsuitable. The small size of fish also means that seemingly small obstructions can severely impede migrations. The steps necessary to advance the management of fish passage for island fish communities are: curating and maintaining barrier inventories; evaluating barrier permeability; developing effective barrier mitigation options; and prioritizing restoration and conservation efforts. New methods for understanding the ecology and locomotory capabilities and behaviour of amphidromous fishes are required to advance the management of fish passage for island fish communities. Fish passage solutions that imitate natural streams, such as those promoted in new guidelines in New Zealand, may be the most effective way of improving waterway connectivity; however, integrated approaches to freshwater fish conservation that account for meta‐population dynamics, in combination with the management of fish passage, are necessary to optimize conservation outcomes for amphidromous species.

Morphological, distributional, and genetic characteristics of Cottus pollux in the Kyushu Island, Japan: indication of fluvial and amphidromous life histories within a single lineage

We evaluated the morphological, distributional, and genetic characteristics of the freshwater sculpin, Cottus pollux, in Kyushu Island, Japan. Based on pectoral fin ray number, the sculpins inhabiting 30 rivers were divided into two types [modes 13 (M13) and 15 (M15)]. We evaluated four environmental parameters: length of main river (L-MR), average gradient of main river (G-MR), distance from sampling site to river mouth (D-SM), and gradient around sampling site (G-S), and compared the two types. L-MR, D-SM, and G-S were significantly larger for the rivers that contained M13 fish than for those containing M15 fish. M13 individuals were distributed in the upstream areas of large river systems, suggesting a fluvial life history, whereas M15 individuals were distributed in the downstream areas of small river systems, indicating an amphidromous life history. We conducted phylogenetic analysis based on mitochondrial 12S rRNA [788 base pair (bp)] and control regions (386 bp). Mitochondrial DNA analysis showed that both M13 and M15 groups were genetically of C. pollux middle-egg type (ME). Our findings proposed the hypothesis that C. pollux ME in Kyushu Island exhibits dimorphism in both morphological and distributional traits. Additionally, haplotype distribution indicated that the fluvial M13 populations had higher genetic specificity in each river in contrast to the existence of one genetic group of amphidromous M15 individuals in Kyushu Island.

Contrasting life histories contribute to divergent patterns of genetic diversity and population connectivity in freshwater sculpin fishes

BackgroundLife history characteristics are considered important factors influencing the evolutionary processes of natural populations, including the patterns of population genetic structure of a species. The sister species Cottus hangiongensis and C. koreanus are small bottom-dwelling freshwater sculpin fishes from South Korea that display marked life history divergence but are morphologically nearly indistinguishable. Cottus hangiongensis evolved an ‘amphidromous’ life history with a post-hatching pelagic larval phase. They spawn many small eggs in the low reaches of rivers, and hatched larvae migrate to the sea before returning to grow to maturity in the river mouth. In contrast, C. koreanus evolved a ‘fluvial’ landlocked type with benthic larvae. They release a smaller number of larger eggs, and the larvae undergo direct development, remaining benthic in the upstream rivers throughout their entire lives. We tested whether there were differences in patterns and levels of within-population genetic diversities and spatial population structure between the two closely related Korean sculpins using mitochondrial DNA control region sequences and seven nuclear microsatellite loci.ResultsThe combined analyses of both marker sets revealed that C. hangiongensis harboured considerably higher levels of within-population genetic diversities (e.g. haplotype/allelic richness, heterozygosities) than C. koreanus. In contrast, the fluvial sculpin exhibited noticeably more spatial population structure than did the amphidromous sculpin, as suggested by pairwise FST statistics. The finding that C. hangiongensis individuals comprised a single random mating population across the east-flowing river basins in the Korean Peninsula, whereas C. koreanus individuals comprised genetically discrete individual populations, was further supported by an individual-based Bayesian population assignment and also factorial correspondence analyses.ConclusionsThe higher genetic diversity, but lower population structure, of the amphidromous sculpin relative to the fluvial sculpin may have resulted from its greater larval dispersal and also possibly, higher fecundity accompanied by an amphidromous life history. Hence, we conclude that contrasting early life histories – including the presence or absence of the pelagic larval phase – may have led to divergent patterns of within-population genetic diversities and spatial population structure between the sister Cottus species following speciation from a common ancestor of marine sculpin.

Effects of Temperature and Salinity on Early Ontogeny of Bigmouth Sleeper Larvae

AbstractDeclines in the abundance of Bigmouth Sleeper Gobiomorus dormitor within its range warrant directed conservation efforts, which may include population supplementation with hatchery‐produced fish. However, previous attempts at Bigmouth Sleeper culture resulted in limited larval development and lack of successful exogenous feeding after yolk sac depletion, presumably in response to environmental conditions. The effects of temperature and salinity on hatch success, survival, growth, and development of Bigmouth Sleeper yolk sac larvae were examined in controlled, replicated experiments. Hatch success was greater at lower temperatures (23°C) and salinities (0, 5, and 10 g/L), larvae survived longer at lower temperatures (23 and 26°C), and growth metrics were greatest in freshwater (salinity = 0 g/L). Larvae held at salinities of 5 g/L and higher experienced 100% mortality within 12 h posthatch. Survival and development were inversely correlated and dependent on temperature; survival was greater at 23°C, but larvae developed slowly, while development was rapid at 29°C, yet all fish died by 48 h posthatch. The results suggests that the optimal hatching temperature and salinity for Bigmouth Sleeper is 23°C and 0 g/L, respectively, and the optimum rearing temperature for yolk sac larvae is 26°C. However, the amphidromous life history strategy of this species may require an increasing temperature and salinity regimen that mimics the environmental conditions larvae would experience following hatch during downstream transport from cooler freshwater streams to warmer brackish and marine waters in their natural habitats.

Complete mitochondrial genome of the Sakhalin sculpin Cottus amblystomopsis (Cottoidei: Cottidae)

The complete mitochondrial genome was sequenced in two individuals of the Sakhalin sculpin Cottus amblystomopsis. The genome sequences are 16,526 and 16,527 bp in size, and the gene arrangement, composition, and size are very similar to the other sculpin mitochondrial genomes published previously. The difference between the two genomes studied is low, 0.28%, in spite of the relatively long distance separating the localities. The data are consistent with the amphidromous life history of C. amblystomopsis, promoting gene flow even between distantly located rivers.

Importance of the natural flow regime to an amphidromous shrimp: a case study

Amphidromous shrimp inhabiting large river systems are thought to migrate downstream to hatch larvae close to the estuary or sea. Recent work on Macrobrachium spinipes in northern Australia challenged this hypothesis, with the discovery of abundant females in reproductive condition over 400km upstream. We investigated the early life history of M. spinipes by using a combination of field sampling, laboratory experiments and modelling. We determined the spatial and temporal production of larvae, larval salinity requirements, and, finally, the potential for transport of larvae to the estuary during the wet season. Larvae occurred in high abundances across a 400-km river reach. However, successful larval development was contingent on access, by drift, to saltwater within 7 days. This interval is longer than that found for the majority of amphidromous shrimp species and we propose that larvae in the study river are able to successfully moult through the early stages in freshwater. Large, wet-season discharge events were critical for transporting larvae from all sampled sites to the estuary. The present study confirmed the amphidromous life history of M. spinipes, highlighted the importance of natural flows for maintaining existing populations and provided evidence of a novel type of amphidromous life-history strategy.

Immediate impact of the tsunami associated with the 2011 Great East Japan Earthquake on the Plecoglossus altivelis altivelis population from the Sanriku coast of northern Japan

The huge tsunami associated with the 2011 Great East Japan Earthquake severely impacted the ecosystems on the Sanriku coast of Japan. The life history traits of ayu, Plecoglossus altivelis altivelis, which have a one-year amphidromous life history, were investigated by otolith analyses in two rivers of the Sanriku coast just after the tsunami compared with results before the tsunami to reveal the immediate impact of the tsunami. Hatching date compositions differed between the upstream migrants before and after the tsunami. The migrants after the tsunami were solely composed of fish hatched during October and November, whereas the migrants before the tsunami were mostly composed of fish hatched in September. Discrepancies between the hatching dates of the migrants and drifting larvae (hatched larvae) indicate that selective mortality of early-hatched fish occurred during the tsunami. Differences in otolith Sr:Ca ratios between upstream migrants before and after the tsunami suggest that fish surviving the tsunami inhabited saline water and early-hatched fish that inhabited the estuary decreased selectively in number because of the severe erosion around the river mouth. The oceanic growth period shortened in accordance with the change in hatching date. These results show that the tsunami drastically altered the ecology of P. altivelis altivelis on the Sanriku coast.

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.

Amphidromy and marine larval phase of ancestral gobioids Rhyacichthys guilberti and Protogobius attiti (Teleostei: Rhyacichthyidae)

Even if amphidromous fish species contribute most to the diversity of fish communities in the tropical insular rivers, their biological cycle remain poorly known. For the first time, the otolith elemental composition and microstructure of two ancestral gobioids, Rhyacichthys guilberti and Protogobius attiti, were investigated to describe their biological cycle and pelagic larval duration (PLD). The otolith analysis using a femtosecond laser ablation coupled to an inductively coupled plasma–mass spectrometer (fs-LA-ICP-MS) revealed an amphidromous life history for R. guilberti and it suggested a progressive habitat shift from a marine habitat to a freshwater environment for P. attiti. For the first time, an endemic species, P. attiti, showed longer and more variable PLD (55.2 ± 13.5 days) than did a widespread one (R. guilberti: ~30 days). These results need to be confirmed by analysing more samples but suggest that factors other than the PLD control endemism and dispersal processes. In association with this first description of the biological cycle for both species, such an approach is a prerequisite for the management and conservation of both patrimonial species.

Amphidromy Links a Newly Documented Fish Community of Continental Australian Streams, to Oceanic Islands of the West Pacific

BackgroundIndo-Pacific high island streams experience extreme hydrological variation, and are characterised by freshwater fish species with an amphidromous life history. Amphidromy is a likely adaptation for colonisation of island streams following stochastic events that lead to local extirpation. In the Wet Tropics of north-eastern Australia, steep coastal mountain streams share similar physical characteristics to island systems. These streams are poorly surveyed, but may provide suitable habitat for amphidromous species. However, due to their ephemeral nature, common non-diadromous freshwater species of continental Australia are unlikely to persist. Consequently, we hypothesise that coastal Wet Tropics streams are faunally more similar, to distant Pacific island communities, than to nearby faunas of large continental rivers.Methods/Principal FindingsSurveys of coastal Wet Tropics streams recorded 26 species, 10 of which are first records for Australia, with three species undescribed. This fish community is unique in an Australian context in that it contains mostly amphidromous species, including sicydiine gobies of the genera Sicyopterus, Sicyopus, Smilosicyopus and Stiphodon. Species presence/absence data of coastal Wet Tropics streams were compared to both Wet Tropics river networks and Pacific island faunas. ANOSIM indicated the fish fauna of north-eastern Australian coastal streams were more similar to distant Pacific islands (R = 0.76), than to nearby continental rivers (R = 0.98).Main Conclusions/SignificanceCoastal Wet Tropics streams are faunally more similar to distant Pacific islands (79% of species shared), than to nearby continental fauna due to two factors. First, coastal Wet Tropics streams lack many non-diadromous freshwater fish which are common in nearby large rivers. Second, many amphidromous species found in coastal Wet Tropics streams and Indo-Pacific islands remain absent from large rivers of the Wet Tropics. The evolutionary and conservation significance of this newly discovered Australian fauna requires clarification in the context of the wider amphidromous fish community of the Pacific.

Population Dynamics of a Landlocked and Amphidromous Freshwater Shrimp, Caridina gracilipes (Decapoda: Caridea) in Subtropical Waters

The freshwater shrimp Caridina gracilipes is widely distributed in the Indo-Pacific, and both landlocked and amphidromous populations occur in southern Taiwan. In this study, we collected monthly samples between September 2006 and August 2008 from a lake and a stream, representing landlocked and amphidromous populations, respectively. We found the amphidromous individuals were larger and grew faster than the landlocked individuals. The mature females in the landlocked population are in general small compared to the amphidromous population, and their eggs were larger but fewer than the amphidromous females. Ovigerous females could be found year-round in both populations, but the main recruitment was in September. The life span of females was longer than males; that of amphidromous were longer than landlocked populations. We found that populations of C. gracilipes in the subtropical region have evolved different biological and reproductive strategies based on their landlocked and amphidromous life histories.

Migratory history of the freshwater goby Rhinogobius sp. CB in Japan

Tsunagawa T, Arai T. Migratory history of the freshwater goby Rhinogobius sp. CB in Japan. Ecology of Freshwater Fish 2011: 20: 33–41. © 2010 John Wiley &amp; Sons A/SAbstract – The migratory histories of Rhinogobius sp. CB collected from 32 locations connected to the sea were studied by examining the strontium (Sr) and calcium (Ca) concentrations in the otoliths using X‐ray electron microprobe (EPMA) analysis. The Sr and Ca contents in the riverine water were also determined using inductively coupled plasma mass spectrometry (ICP‐MS) analysis. The Sr:Ca ratios in the riverine waters were highly different among rivers and correlated with those in the otolith outer region corresponding to the river life period. Consequently, almost all samples (159 of 160 individuals) showed a typical amphidromous life history, with one individual showing a landlocked life history. These results suggest that the clear differences of migration history in population or locality were not found in Rhinogobius sp. CB. Therefore, it was thought that Rhinogobius sp. CB most commonly had an amphidromous life history.

Movement and Population Size of the River Sculpin Cottus hangiongensis in the Daitobetsu River of Southern Hokkaido

Individual movements and population size of the amphidromous sculpinCottus hangiongensis, excluding young-of-the-year smaller than 50 mm in body length, were studied from October 1983 to December 1984 in 5 separate sections of the Daitobetsu River of southern Hokkaido, by using mark-recapture methods. During the non-breeding season, distinct inclinations in density, body length distribution and sex ratio ofC. hangiongensis populations were found along the course of the river. The population density was the highest, 3.45 per m2, in the lowest section and decreased in the upper sections. Larger males were found in larger numbers toward the lower reaches, whereas the sex ratio, which was biased in favor of females, was generally more striking upstream. These characteristics of the population structure may result from the amphidromous life history and the polygynous mating system of this species. Many marked fish were recaptured within the original sections, where they had been marked and released, throughout the year. During the non-breeding season, especially, the mean movement was 40.6 m, with the greatest movement being 92 m. During the breeding season, on the other hand, some sculpins appeared to move downstream before spawning and upstream after spawning. Such downstream spawning migration may increase the chance of encountering a mate, and for females it may enhance the chance of encountering larger males. Moreover, it may also contribute to a decrease in the mortality rate of their flowing larvae.

멸종위기에 처한 한둑중개(Cottus hangiongensis)의 산란습성 및 초기생활사

Spawning behavior and early life history of the tuman river sculpin, Cottus hangiongensis were studied in the laboratory and in the field at Wangpi Stream, Gyeongsangbuk-do, Korea, from January to December, 2007. The spawning ground was in the lower Wangpi Stream, which is a shallow region about 40cm or less in depth. During the spawning period, from March to April, mature males made nest cavities under stone 10 which they led a gravid female. The male and female then turned upside down, and spawning and fertilization occurred onto the ceiling of the nest cavity. After spawning, the male chased the female from the nest and mated with several other females. Fertilized eggs were spherical in shape, demersal, adhesive, transparent and yellow in color, measuring 1.86 mm (1.79~1.93 mm) in diameter. A mean of 17(12~22) various-sized oil globules were counted in the yolk. Granular materials formed a mass in the yolk. Fertilized eggs hatched at 256 hrs, 10 minutes after the morula stage under water temperature of <TEX>$15.0{\sim}18.0^{\circ}C$</TEX>. Newly hatched larvae 9.34 mm (9.02~9.69 mm. n=10) in total length (TL) had a large yolk At 14 days after hatching, larvae 11.40 mm (11.07~11.72 mm, n=10) in TL transformed to the postlarval stage. At 41 days after hatching, postlarvae of 18.42 mm (17.31~18.62 mm, n=10) in TL had reached the juvenile stage. The result of this study indicate that Cottus hangiongensis has the spawning ground in the lower stream and the amphidromous life history which is the different from that of Cottus poecilopus.

Survival and behavioral characteristics of amphidromous goby larvae of Sicyopterus japonicus (Tanaka, 1909) during their downstream migration

To understand the ecology and environmental tolerances of newly hatched larvae of the amphidromous fish Sicyopterus japonicus during their downstream migration, the salinity tolerance of eggs, 0–15 day old larvae, and adults, and the temperature tolerance, specific gravity and phototaxis of hatched larvae were examined. Tolerances of adults were measured as survival after a 24 h challenge in freshwater (FW), brackish water (1/3 SW) and seawater (SW). The survival rate of adult S. japonicus was 100% in FW and 1/3 SW, while none survived in SW. Hatching success of eggs (30 eggs each) was significantly higher in FW (mean: 73%) and 1/3 SW (73%) than in SW (19%). Tolerance of newly hatched larvae to salinity and temperature was investigated in different combinations of salinities (FW, 1/3 SW and SW) and temperatures (18, 23 and 28 °C). Larval survival was significantly different in each salinity and temperature. Survival rate was significantly higher in 1/3 SW than in FW and higher in SW than in FW at 23 °C and 28 °C. At the latter part of the experiment, there was no survival in FW and at 28 °C. Survival was higher in lower temperatures, but larval development did not occur in FW. Specific gravity of newly hatched larvae was 1.036 at 28 °C and 1.034 at 23 °C. When exposed to a light source on one side of an aquarium, larval distribution was not affected. Our results indicated larval S. japonicus are more adapted to brackish water and seawater than freshwater, while the adults and eggs are more adapted to freshwater and brackish water than seawater. This is consistent with their amphidromous life history with growth and spawning occurring in freshwater and the larval stage utilizing marine habitats.

Migration diversity of the freshwater goby Rhinogobius sp. BI, as revealed by otolith Sr:Ca ratios

The migratory histories of the endangered freshwater goby Rhinogobius sp. BI (Bonin Island type) collected from 6 stations on Chichijima Island, part of the Bonin Islands chain in Japan, were studied by examining the environmental signatures of strontium and calcium concentrations in otoliths using electron probe microanalyses. The Sr:Ca ratios in the otoliths showed when the gobies had experienced various salinity environments such as freshwater, brackish water, and seawater. The otolith Sr:Ca ratios of almost all Rhinogobius sp. BI from rivers connected to the sea reflected the typ- ical amphidromous life history. In contrast, fish collected upriver from a dam, disconnected from the downstream area, utilized freshwater habitats only, reflecting an artificially landlocked life history. Further flexible migration patterns were found in fish from areas downstream of the aforementioned dam (in a river connected to the sea), indicating that they utilized freshwater, brackish water, and seawater in their larval and adult life period. The present study clearly suggests that the migratory histories of Rhinogobius sp. BI are highly variable within habitats and that they have flexible migra- tory patterns, allowing them to utilize the full range of salinity during their life history.