Ongoing localized extinctions of stream-dwelling white-spotted charr populations in small dammed-off habitats of Hokkaido Island,Japan

Habitat fragmentation caused by damming can greatly reduce the population viability of aquatic organisms, with smaller fragmented populations at higher risk of extinction due to increased demographic, genetic, and environmental stochasticity. However, empirical evidence demonstrating that smaller natural populations are more vulnerable to extinction is limited. We studied the vulnerability to extinction of white-spotted charr (Salvelinus leucomaenis) populations in 30 dammed-off streams in Oshima Peninsula, southwestern Hokkaido Island, Japan, by comparing the incidence of charr populations in streams between 1999 and 2014. Using electrofishing and environmental DNA surveys, we identified three localized extinctions, with the probability of extinction increasing with decreasing watershed area (our surrogate for habitat size). We also found a new population in one dammed-off stream in which white-spotted charr were previously unknown, after installation of a fish ladder, indicating the capacity of white-spotted charr to recolonize reconnected habitat in a short period. Our results suggest that localized extinction of white-spotted charr in small dammed-off streams is ongoing, but that appropriate fish migration corridors can reduce localized extinction risk and increase the probability of species persistence.

     

Rule of age and size at maturity: individual variation in the maturation history of resident white-spotted charr

Individual growth and maturation histories, age, and size at maturity of resident white-spotted charr Salvelinus leucomaenis were examined in a tag-recapture study in a natural river over 3 years. Slow-growing fish reached sexual maturity not only at an older age, but also at a smaller size than fast-growing fish, although females had a larger threshold size at maturity than males at each age. It is suggested that these patterns result from adaptive phenotypic plasticity that depends on individual growth conditions.     

Life history traits of white-spotted charr in an alpine environment: Implications for local adaptation along an altitude gradient

As a case study of local adaptation, we compared the life history traits of white-spotted charr Salvelinus leucomaenis between populations isolated in an alpine environment (altitude above 2000 m) and a normal environment (ca. 1000 m) in the Fuji River basin, near the southern limit of its distribution (35° N, 138° E). Our results showed that white-spotted charr in alpine streams had lower growth rate, longer lifespan, earlier spawning season, and were older and larger size at maturity, compared with those habiting a normal mountain stream. We concluded that charr in alpine streams were adapted to low water temperatures and long starvation periods by specialized life history traits.     

Relationships among tributary length,census population size,and genetic variability of white-spotted charr, <Emphasis Type="Italic">Salvelinus leucomaenis</Emphasis>, in the Lake Biwa water system

The relationships between census population size and tributary length and between haplotype diversity of the mitochondrial DNA and census population size in ten white-spotted charr populations in the Lake Biwa water system and its adjacent basins were investigated. The census population size (number of fish with ≥100 mm in standard length) significantly increased with the tributary length. In the eastern part of the Lake Biwa water system, haplotype diversity increased with the census population size. On other hand, in the western part of the water system and adjacent basins, haplotype diversity was zero irrespective of the census population size. These results suggest that white-spotted charr populations in the eastern and western part of the Lake Biwa water system have undergone different levels of bottlenecks related to the habitat size in the postglacial warming.     

Maturation,growth and fecundity of Arctic charr, <Emphasis Type="Italic">Salvelinus alpinus</Emphasis> (L.), life-history variants co-existing in lake systems of Southern Baffin Island,Nunavut, Canada

Two life-history variants of Arctic charr (Salvelinus alpinus), anadromous and lake-resident, have been previously identified in lakes of Southern Baffin Island, Nunavut, Canada. In accordance with classical life-history theory, it is hypothesised that anadromous charr will delay maturation in both size and age, and have increased fecundity (per spawning event) in comparison with lake-resident charr. Sagittal otoliths and biological data were collected for both life-history variants within the three studied lakes: Iqalugaarjuit, Qasigiat and Qinngu. Sagittal otoliths were embedded in epoxy resin, cross-sectioned for age determination, and imaged for back-calculation (size and age). Back-calculated data in each lake were fit to von Bertalanffy growth models for each life-history variant and compared via analysis of residual sums of squares. Anadromous charr had greater mean size at maturity and experienced a delay in mean age at maturity in comparison with lake-residents. The relationship between size and fecundity or egg diameter did not differ between the two life-history variants. Growth models indicate that the overall growth coefficients of lake-resident and anadromous charr were different in all three studied lakes. The Brody growth coefficient for all lake-resident charr populations was greater than anadromous individuals indicating that maximum length was reached at a rapid rate, resulting in a smaller asymptotic length. Indirect evidence suggests that anadromous and lake-resident charr belong to one reproductive population. Future genetic analysis is necessary to further determine the degree of divergence between the life-history variants.     

Plasticity in probabilistic reaction norms for maturation in a salmonid fish

The relationship between body size and the probability of maturing, often referred to as the probabilistic maturation reaction norm (PMRN), has been increasingly used to infer genetic variation in maturation schedule. Despite this trend, few studies have directly evaluated plasticity in the PMRN. A transplant experiment using white-spotted charr demonstrated that the PMRN for precocious males exhibited plasticity. A smaller threshold size at maturity occurred in charr inhabiting narrow streams where more refuges are probably available for small charr, which in turn might enhance the reproductive success of sneaker precocious males. Our findings suggested that plastic effects should clearly be included in investigations of variation in PMRNs.     

Comparison of competitive ability between native and introduced salmonids: evidence from pairwise contests

Brown trout, Salmo trutta, and rainbow trout, Oncorhynchus mykiss, have been introduced to freshwaters in Hokkaido, Japan. Today, it is recognized that these introduced salmonids have negative impacts on native salmonids such as white-spotted charr, Salvelinus leucomaenis, and masu salmon, O. masou. In particular, interspecific competition may be an important mechanism that could contribute to the exclusion for native salmonids. In this study, experimental pairwise contests were conducted to compare interference competitive ability between native and introduced salmonids. We demonstrated that brown trout were competitively superior to white-spotted charr and masu salmon whereas rainbow trout were superior to white-spotted charr. We suggest that introduced brown trout negatively impact both white-spotted charr and masu salmon, and introduced rainbow trout negatively impact white-spotted charr.     

Latitudinal trends in adult body size of Dolly Varden, with special reference to the food availability hypothesis

The food availability hypothesis (FAH) predicts that the relative productivity of ocean and freshwater habitats changes with latitude, and that anadromy will evolve when ocean productivity is greater than that in neighboring freshwater habitats, or vice versa. In data sets for the anadromous salmonid species Salvelinus malma, we show that the relative body size at maturity of anadromous populations is much larger in the northern limits of their ranges than that of fluvial populations, and, conversely, that of fluvial populations is slightly smaller at the southern limits than that of anadromous populations, supporting the FAH for the evolution of migration behavior in salmonids. Received: October 4, 2001 / Accepted: January 7, 2002     

Skewed reproductive success among male white-spotted charr land-locked by an erosion control dam: Implications for effective population size

In 1996 and 1997, the spawning behavior of fluvial white-spotted charr Salvelinus leucomaenis, was observed in the upstream area of an erosion control dam. A small number of males with relatively large body size mated successfully with females as a pair, while almost all satellite males did not sneak successfully, resulting in a non-random mating system. The low sneaking success of subordinate males, in addition to the monopolization of spawning opportunities by a few dominant males, is one of the most important causes of skewed reproductive success among males. The total number of adult fishes in the study area (N: approximately half of the whole tributary above a dam) was estimated as 148 and 102 in 1996 and 1997, respectively. Based on these findings and some further assumptions, the estimated effective population size (N_e) was low in both years. The N_e/N ratio ranged from 0.33 to 0.36 in both years. In addition to reduced population size by construction of an impassable dam, the above-dam population suffered low N_e due to skewed reproductive success among males. The low N_e may be one cause of extinction in above-dam populations of fluvial charr, especially just after the construction of impassable barriers.     

Redd superimposition by introduced rainbow trout,oncorhynchus mykiss, on native charrs in a japanese stream

Spawning redd superimposition of introduced, spring-spawning rainbow trout,Oncorhynchus mykiss, on native, fall-spawning Dolly varden,Salvelinus malma, and white-spotted charr,S. leucomaenis, were examined in a small stream in Hokkaido, Japan. The stream reaches in which Dolly Varden and white-sported charr redds were observed in fall 1997 greatly overlapped with the reaches in which rainbow trout redds were recorded in spring 1998. Spawning microhabitats were also similar between trout and the two charr species. Thirteen and 3% of Dolly Varden and white-spotted charr redds, respectively, were superimposed by rainbow trout redds. The eggs or alevins in the disturbed charr redds were potentially damaged because charrs were not likely to have emerged from the redds before the superimposition occurred. In sufficiently great abundance, introduced rainbow trout may negatively impact native charr populations by dislodging the latter’s spawning redds.     

Population structure, ecological segregation and reproduction in non-anadromous Arctic charr, Salvelinus alpinus (L), in four unexploited lakes in the Canadian high Arctic

Land-locked populations of Arctic charr in four lakes on Northern Ellesmere Island (80° N) were found to consist of two distinct sizes: 'dwarf' and 'normal' charr. Both groups attained sexual maturity but differed in appearance and habitat. The smaller fish, occupying the more marginal habitats, retained their parr-markings; the larger group had the general characteristics of smolts, being more silvery and without parr-marks. In their juvenile stages, the charr destined to attain the larger group were indistinguishable from members of the smaller group. Although fish in the larger group were capable of cannibalism, this was rarely observed. In general, the fish in the larger group were older than the smaller ones but great variation in size at a given age resulted in certain age classes containing representatives of both groups. The population structure varied considerably between lakes; a high proportion of 'normal' charr correlated well with a high growth rate in the first few years of development. It is postulated that the two groups live in dynamic equilibrium where the advantages of progenesis (retention of juvenile characters by adults) in the smaller type are traded against the larger proportion of the energy resources available to the larger type. The concept of heterochrony in an ecological setting is introduced.     

Effects of habitat fragmentation by damming on salmonid fishes: lessons from white-spotted charr in Japan

Dam construction has serious consequences, and one of the most serious concerns is the fragmentation of riverine ecosystems. We reviewed the influence of habitat fragmentation on white-spotted charr Salvelinus leucomaenis populations. First, habitat fragmentation by damming has serious consequences in terms of alternative life-history strategies. Most fish in dammed-off areas do not migrate to the sea and instead become resident forms. This loss of the anadromous form negatively affects populations through decreased spawning biomass. In addition, the smaller population sizes in dammed-off habitats can negatively affect population dynamics through demographic, environmental, and genetic stochasticity. Therefore, the population viability is reduced in small, dammed-off habitats. White-spotted charr populations also likely experience different selection pressures after damming. Many of these effects of habitat fragmentation due to damming are not immediate but rather occur gradually over several generations. Because most Japanese dams were constructed after 1970, some effects of damming may not yet be obvious. Kentaro Morita is the recipient of the 12th Denzaburo Miyadi Award.     

Migratory patterns of anadromous white-spotted charr Salvelinus leucomaenis in Eastern Hokkaido, Japan: The solution to a mystery?

We evaluated the migratory patterns of white-spotted charr in eastern Hokkaido, Japan using: (1) field observation of fish, (2) salinity-recording archival tags, and (3) analysis of otolith Sr and Ca concentrations. Field observations suggest that the majority of anadromous white-spotted charr outmigrate to the sea in spring (April to June) and ascend the river between late summer and autumn (August to November). Salinity records from archival tags revealed a fluctuating pattern consisting of both low and high values within a month, indicating frequent short-term movements between freshwater (including non-natal rivers) and brackish/salt water habitats. Otolith Sr:Ca ratios exhibited a sharp rise at about 40–50% of the otolith radius, which was followed by several spikes consisting of both low and high values, indicating that fish migrated between freshwater and marine environments multiple times during their life history. However, the peaks in Sr:Ca ratios were generally associated with annulus zones on the otolith, indicating that the anadromous migration was generally an annual event.     

Genetic diversity and population structure of white-spotted charr, Salvelinus leucomaenis, in the Lake Biwa water system inferred from AFLP analysis

The genetic variations—and the time dependence of such variations—of natural populations of the white-spotted charr ,Salvelinus leucomaenis, in the Lake Biwa water system as well as those of a hatchery-reared population were inferred from AFLP. Upon the application of principal coordinate analysis using 118 polymorphic AFLP fragments based on the Jaccard similarity index, specimens of each of six natural local populations from the inlet rivers of Lake Biwa grouped roughly together, suggesting that each local population was genetically differentiated. The hatchery-reared population was shown to be closely related to the local population in the Seri River, suggesting that the Seri River population originated from hatchery-reared charr due to extensive stocking. Furthermore, specimens of the Yasu River grouped in a somewhat different position from the other natural populations, agreeing well with its geographic distance from the other populations. The nucleotide diversities of six natural populations (Harihata River, Ishida River, two reaches of the Takatoki River, Ane River, and Yasu River) in 2002 or 2003 were relatively low (π = 0.067–0.146%) compared with that of the Seri River (0.278%) and the hatchery-reared charr (0.316%). The nucleotide diversity in the five local populations (Ishida River, two reaches of the Takatoki River, Ane River, and Yasu River) remained at a low level from 1994 to 2002/2003, but only the nucleotide diversity in the Harihata River actually decreased. From 1994 to 2002/2003, the nucleotide diversity in the Seri River remained at a higher level among the natural populations from 1994 to 2002/2003; it was enhanced by the artificial release of hatchery-reared charr before 1994. In order to conserve the genetic diversity of the white-spotted charr in the Lake Biwa water system, it is necessary to prevent the stocking of hatchery-reared charr in reaches where hatchery-reared charr have not previously been stocked.     

Habitat fragmentation by damming threatens coexistence of stream-dwelling charr and salmon in the Fuji River,Japan

Habitat fragmentation by damming can affect the persistence of single species population and also coexistence of two or more species through intensified competition. This study examined the effects of habitat fragmentation by damming on the coexistence of two stream-dwelling salmonids: the southern form of white-spotted charr (Salvelinus leucomaenis japonicus) and the red-spotted masu salmon (Oncorhynchus masou ishikawae). We examined charr, salmon, and dam distributions in 27 streams of the Fuji River basin, central Japan. In the 1970s, there were streams with five sympatric and 22 allopatric populations (n = 13 for charr, n = 9 for salmon). However, from the 1970s to 2004, 356 impassable dams were constructed in the surveyed streams, and four of the five sympatric streams became allopatric. In the extant sympatric stream, more than 20 dams fragmented habitat. Species distributions were separated by dams (with decreasing altitude) in the following order: extirpation area, charr-dominant area, and salmon-dominant area. Within the uppermost sympatric section (i.e., situated between the dams), salmon congregated in the largest uppermost pool just below the dam; despite these conditions, salmon frequency increased in the downstream direction at the stream scale. The results suggest that habitat fragmentation threatens the coexistence of stream-dwelling charr and salmon at both the basin and stream scales. We believe that exclusion of one species by another is likely in extremely fragmented habitats with minimal gradients and little variation in physical conditions (through reduced stream gradient and increased sand sedimentation caused by damming). In addition, multiple sites of damming ensure that there are no salmonid refuges from the collapse of metapopulation structure. In such fragmented habitats, even small tributaries serve important roles, as they are used mainly by salmonid fry and juveniles. We propose that habitats of native salmonids should be maximized by reconnecting fragmented habitats as part of a broader management plan.     

Mitochondrial DNA population structure of the white-spotted Charr (Salvelinus leucomaenis) in the Lake Biwa water system

A phylogeographic analysis of mitochondrial DNA sequences was performed in order to elucidate the origin, dispersal process, and genetic structure of white-spotted charr in the Lake Biwa water system. Two haplotypes were most common in the Lake Biwa water system, and were also common in the adjacent inlet rivers of the Sea of Japan. These results suggest that in the glacial periods of the Pleistocene, white-spotted charr dispersed into the northern inlet rivers of Lake Biwa from adjacent inlet rivers of the Sea of Japan by watershed exchanges, colonizing the whole of the Lake Biwa water system. Mitochondrial DNA diversity contrasted sharply between the western and eastern parts of the system, suggesting that the populations in the western part might be more reduced than those in the eastern part in relation to the smaller habitat size. The high overall FST estimate (0.50), together with pairwise comparisons of FST, indicated significant genetic divergence between populations due to isolation and small population size. Hierarchical analysis (AMOVA) also showed that genetic variation was more pronounced among regions (28.39%) and among populations within regions (47.24%) than within populations (24.37%). This suggests that each population in and around the Lake Biwa water system should be treated as a significant unit for conservation and management.     

Seasonal changes in stream salmonid population densities in two tributaries of a boreal river in northern Japan

We examined seasonal changes in population densities of stream salmonids (masu salmon Oncorhynchus masou, white-spotted charr Salvelinus leucomaenis, and rainbow trout O. mykiss) in two tributaries of the Shoro River, eastern Hokkaido, Japan. In one small tributary, water temperature was relatively high during the winter, and populations of salmon and trout increased through immigration at this time of the year, becoming dominant components of the salmonid assemblage; the density of charr in this stream decreased during the winter, but charr was dominant during the summer. In another medium-sized tributary, the water temperature fell to close to 0°C during the winter, and densities of salmon and charr decreased in this season, through emigration; trout were very rare in this stream. Seasonal patterns of stream salmonid densities vary among species and between localities, resulting in seasonal changes in species composition. For a comprehensive understanding of population processes, a whole-river survey across seasons will be necessary.     

The growth history of anadromous white-spotted charr in northern Japan: a comparison between river and sea life

Anadromous white-spotted charr Salvelinus leucomaenis in a natural river in northern Japan migrated to the sea mainly at age 3+ years. Sea-run charr grew faster than parr, particularly as first time migrants. Sea-run charr repeated seaward migration up to three times; the observed maximum age was 5+ years old. The likelihood ratio test identified significant differences in asymptotic size and individual growth variability between parr and sea-run charr. Sea-run charr had an asymptotic size ( L ^∞) twice as large as parr, while the growth parameter ( K ) was similar between sea-run and parr. Growth variability was higher for parr than for sea-run charr.     

Potential fragmentation and loss of thermal habitats for charrs in the Japanese archipelago due to climatic warming

1. The upper thermal limits of the present distributions of two charr species, Dolly Varden, Salvelinus malma , and white-spotted charr, S. leucomaenis , in streams of the Japanese archipelago were examined using groundwater temperature as an index of thermal condition. The lower limits of the altitudinal distributions of Dolly Varden and white-spotted charr were delineated, respectively, by 8 and 16 °C groundwater isotherms.
2. The potential impact of future climatic warming on the geographical distribution, habitat extent and population fragmentation of each species was predicted at both the full archipelago and individual catchment levels.
3. For Dolly Varden, analysis at the full archipelago level indicated a loss of 27.6, 67.2, 79.6 and 89.6% of the current geographical range, respectively, for a 1, 2, 3 and 4 °C increase in mean annual air temperature. The present distribution area of white-spotted charr would likewise reduce by 4.1, 20.5, 33.8 and 45.6%, respectively.
4. Based on the analyses of three individual catchments, one for Dolly Varden and two for white-spotted charr, the lower habitat boundaries for the two charr species could be expected to rise increasingly to higher elevations in each catchment as warming proceeded. As a consequence, there would be large reductions in mean habitat area, with increasing habitat fragmentation followed by localized extinctions of the two species.     

Potential fragmentation and loss of thermal habitats for charrs in the Japanese archipelago due to climatic warming

1. The upper thermal limits of the present distributions of two charr species, Dolly Varden, Salvelinus malma , and white-spotted charr, S. leucomaenis , in streams of the Japanese archipelago were examined using groundwater temperature as an index of thermal condition. The lower limits of the altitudinal distributions of Dolly Varden and white-spotted charr were delineated, respectively, by 8 and 16 °C groundwater isotherms.
2. The potential impact of future climatic warming on the geographical distribution, habitat extent and population fragmentation of each species was predicted at both the full archipelago and individual catchment levels.
3. For Dolly Varden, analysis at the full archipelago level indicated a loss of 27.6, 67.2, 79.6 and 89.6% of the current geographical range, respectively, for a 1, 2, 3 and 4 °C increase in mean annual air temperature. The present distribution area of white-spotted charr would likewise reduce by 4.1, 20.5, 33.8 and 45.6%, respectively.
4. Based on the analyses of three individual catchments, one for Dolly Varden and two for white-spotted charr, the lower habitat boundaries for the two charr species could be expected to rise increasingly to higher elevations in each catchment as warming proceeded. As a consequence, there would be large reductions in mean habitat area, with increasing habitat fragmentation followed by localized extinctions of the two species.