Outbreeding Depression in Hybrids Between Spatially Separated Pink Salmon, Oncorhynchus gorbuscha, Populations: Marine Survival, Homing ability, and Variability in Family Size

Hybridization between distinct populations and introgression of nonnative genes can erode fitness of native populations through outbreeding depression, either by producing a phenotype intermediate to that of both contributing genomes (and maladapted in either population's environment) or by disrupting distinct coadapted complexes of epistatic genes. In salmon, fitness-related traits such as homing ability or family-size distribution may be eroded. We investigated geographically separated pink salmon populations in repeated trials in independent broodyears (odd and even). Hybrids were made between female Auke Creek (Southeast Alaska) pink salmon and Pillar Creek (Kodiak Island, ～1 000 km away) males; hybrids and their offspring were compared to offspring of control crosses of the same females with Auke Creek males. Parentage assignment from microsatellite analysis was used to improve estimates of survival and straying and to examine variation of family size. Hybridization reduced return rates of adults (a proxy for survival at sea) in the F₁ generation in the odd-year broodline (p < 0.0001) but not in the even-year broodline (p = 0.678). Hybridization reduced survival in both the odd- and even-broodyear F₂ (p < 0.005 and p < 0.0001). Hybridization did not appear to impair homing ability; weekly surveys revealed similar straying rates (～2%) by both hybrid and control fish into nearby (～1 km) Waydelich Creek in both generations in both trials. Hybridization did not increase the index of variability (σ²/μ) in family size. Decreased survival in the hybrid F₂ generation supports an epistatic model of outbreeding depression; nonepistatic effects may have contributed to reduced survival in the odd-broodyear F₁ hybrid fish. Outbreeding depression in hybrids of geographically separated populations demonstrates that introgression of nonnative fish can erode fitness, and should be recognized as a potential detriment of both aquaculture and management practices.     

Climate warming causes phenological shift in Pink Salmon, Oncorhynchus gorbuscha, behavior at Auke Creek, Alaska

Thirty‐four years (1972–2005) of water temperature data and extensive biological observations at Auke Creek, Alaska indicate a general warming trend that affected the native pink salmon (Oncorhynchus gorbuscha) population. Serial environmental records at nearby Auke Bay, Alaska over 46 years show trends of increasing air and sea surface temperatures. Trends of increased total precipitation and earlier date of ice out on nearby Auke Lake also occurred, but not at significant rates. Average water temperatures during the incubation of pink salmon in Auke Creek increased at a rate of 0.03 °C yr^?1 over the 34‐year period. For the 1972–2005 broods, midpoints of fry migrations from Auke Creek ranged between April 2 and May 7, and there was a trend of earlier migration of pink salmon fry at a rate of ? 0.5 days yr^?1. The migration timing of adult salmon into Auke Creek also showed a trend toward earlier timing. The earlier adult migration combined with warmer incubation temperatures are related to earlier migration of pink salmon fry. If the observed warming trend continues, Auke Creek may become unsuitable habitat for pink salmon. Given the trend for salmon fry to migrate earlier, a larger portion of the population may become mismatched with optimum environmental conditions during their early marine life history. If salmon adults continue to migrate into the creek earlier when water temperatures are commonly high, it will result in increased prespawning mortality.     

Developmental progression of gill rakers as a post-hatch developmental marker in pink salmon, Oncorhynchus gorbuscha

We evaluate the usefulness of gill rakers as a post-hatch developmental marker in salmon by tracking development in undisturbed and stressed yolk-bearing salmon embryos. Native pink salmon (Oncorhynchus gorbuscha) from Auke Creek, Juneau, Alaska and genetically stressed outbred hybrids between Auke Creek and Pillar Creek (Kodiak Island) salmon were incubated in ambient-temperature Auke Creek water. Environmentally stressed native embryos were reared in water that was 2 to 4 °C warmer than ambient. The sum of rakers on the first left and right branchial arches of natives reared at ambient temperatures averaged 23.20 (SD?±?1.64) per embryo when post-hatch sampling began. The subsequent increase in raker counts was linear and positively correlated with the accumulation of thermal units until counts reached maxima 223 days after fertilization, which coincided with the complete consumption of yolk reserves. The average maximum raker count was 39.55 (SD?±?1.76) per embryo, which is substantially fewer than the 60 rakers typically observed in adults. Neither raker development nor yolk consumption patterns were affected by hybridization. Elevated incubation temperatures accelerated raker formation and yolk consumption in native embryos, but the number of rakers associated with a given amount of yolk was the same regardless of temperature suggesting that changes in yolk consumption rate and raker development rate did not influence raker counts. These results indicate that rakers are easily observed and counted, grow in a predictable sequence, and are developmentally stable in the face of both genetic and environmental stress, thereby making them potentially reliable post-hatch developmental markers.     

Timing of Development During Epiboly in Embryos of Second-Generation Crosses and Backcrosses between Odd- and Even-Broodyear Pink Salmon, Oncorhynchus gorbuscha

Synopsis Odd- and even-year-spawning pink salmon (Oncorhynchus gorbuscha) are genetically isolated; their broodlines differ even in the same natal stream. Hybrids between broodlines exhibit outbreeding depression in survival. Variation in the time to completion of epiboly in embryos appears to be adaptive in both broodlines. We compared stage of development at a time near the completion of epiboly in families of second-generation offspring from crosses between odd- and even-year broodlines with development stages of within-broodyear controls and of backcrossed families. We observed embryos derived from matings of mature fish that were the results of fertilizations made 2 years earlier of eggs from females from the even brood year with semen from males from the even broodyear and with cryopreserved semen from males of the odd broodyear. The resulting fry had been released to the Pacific Ocean and recovered at maturity. Second generation embryos were produced by factorial matings of these mature fish involving (1) female and male controls, (2) female and male hybrids, and (3) both backcrosses. Analysis of variation of development time detected no effect of outbreeding, i.e., differences between controls and second generation hybrids (p > 0.05), but did detect variation between individual female parents (p < 0.03). Neither epistatic nor additive outbreeding depression could be detected in the rate of early embryonic development of pink salmon. However, effects on development rate attributable to female parents indicate that either a maternal effect or early additive genetic effects occurred before the expression of the paternal genome in embryos.     

Use of a genetic marker to examine genetic interaction among subpopulations of pink salmon (Oncorhynchus gorbuscha)

In 1979 and 1981, a genetic marker was bred into one of the five identifiable subpopulations of pink salmon [Oncorhynchus gorbuscha (Walbaum)] in the Auke Lake drainage in Southeast Alaska. As a result of the marking effort, the frequencies of two malate dehydrogenase (MDH-B1, 2^*) alleles were changed in the marked subpopulation, but not in other subpopulations that spawn at different times or places. Between 1983 and 1989, the marker allele frequencies were monitored in many of these subpopulations and in early- and late-run pink salmon spawning in nearby Waydelich Creek, located approximately 1km away. Changes in allele frequencies at MDH-B1, 2^*, used to obtain direct estimates of average migration rates (m) from the marked to the unmarked subpopulations, revealed little or no introgression into early subpopulations or into nearby Waydelich Creek. Moreover, spatially distinct late-run Auke Creek subpopulations were not immediately overrun by the more abundant marked subpopulation. These observations suggest that genetic isolation exists between temporally distinct spawning runs and that small temporal and spatial (or ecological) differences contribute to population structure. These observations should be considered in taking actions that affect conservation and harvest management or extensive culture of salmonids.     

Spatial partitioning and asymmetric hybridization among sympatric coastal steelhead trout (Oncorhynchus mykiss irideus), coastal cutthroat trout (O. clarki clarki) and interspecific hybrids

Hybridization between sympatric species provides unique opportunities to examine the contrast between mechanisms that promote hybridization and maintain species integrity. We surveyed hybridization between sympatric coastal steelhead (Oncorhynchus mykiss irideus) and coastal cutthroat trout (O. clarki clarki) from two streams in Washington State, Olsen Creek (256 individuals sampled) and Jansen Creek (431 individuals sampled), over a 3-year period. We applied 11 O. mykiss-specific nuclear markers, 11 O. c. clarki-specific nuclear markers and a mitochondrial DNA marker to assess spatial partitioning among species and hybrids and determine the directionality of hybridization. F1 and post-F1 hybrids, respectively, composed an average of 1.2% and 33.6% of the population sampled in Jansen Creek, and 5.9% and 30.4% of the population sampled in Olsen Creek. A modest level of habitat partitioning among species and hybrids was detected. Mitochondrial DNA analysis indicated that all F1 hybrids (15 from Olsen Creek and five from Jansen Creek) arose from matings between steelhead females and cutthroat males implicating a sneak spawning behaviour by cutthroat males. First-generation cutthroat backcrosses contained O. c. clarki mtDNA more often than expected suggesting natural selection against F1 hybrids. More hybrids were backcrossed toward cutthroat than steelhead and our results indicate recurrent hybridization within these creeks. Age analysis demonstrated that hybrids were between 1 and 4 years old. These results suggest that within sympatric salmonid hybrid zones, exogenous processes (environmentally dependent factors) help to maintain the distinction between parental types through reduced fitness of hybrids within parental environments while divergent natural selection promotes parental types through distinct adaptive advantages of parental phenotypes.     

Widespread hybridization among species of Indian major carps in hatcheries, but not in the wild

Twenty‐one allozyme loci in samples of wild‐caught and hatchery‐reared Indian major carps from Bangladesh were analysed. Bayesian model‐based clustering analysis revealed the presence of four taxa, corresponding to the three known species along with a fourth unknown taxon present in two hatchery samples. Individual admixture coefficients showed that 24% of all hatchery‐reared fishes were hybrids, whereas a single hybrid was observed in the wild‐caught samples. Only catla Catla catla × rohu Labeo rohita and mrigal Cirrhinus cirrhosus × rohu hybrids were observed, the vast majority of which were F₁ hybrids, though five individuals represented putative backcrosses. Mitochondrial DNA analysis revealed that catla × rohu hybridization primarily involved catla males and rohu females, whereas mrigal × rohu hybrids primarily resulted from rohu males and mrigal females. Despite the high percentage of F₁‐hybrids in hatchery samples, reproductive barriers among species have so far precluded widespread introgression. Continued hybridization may eventually lead to a breakdown of species barriers, thereby compromising the genetic integrity of the species in the wild, and leading to production losses in aquaculture.     

Growth and gonadal development of gynogenetic diploid Scophthalmus maximus

The survival, growth, gonadal development and sex ratios of gynogenetic diploid turbot Scophthalmus maximus between 9 and 36 months of age were investigated. Gynogenesis was induced by activation of freshly collected eggs with diluted (1 : 10 with Ringer) and UV‐irradiated (30000 erg mm⁻²) sperm, followed by a cold shock at −1 to 0° C for 25 min, starting at 6·5 min after fertilization. The exclusive maternal inheritance of the resulting gynogenetic diploids was confirmed in all individuals used in the experiment by microsatellite markers. Mean total length and mass throughout the experiment was higher in the controls than in the gynogenetics. The gonadal development in gynogenetic males proceeded normally, and in gynogenetic females it was delayed during the first sexual maturation but was normal during the second one. The sex ratio was 1 male (M) : 1 female (F) in the controls, as expected, but 1 M : 3 F in the gynogenetics. In a second, independent experiment, carried out with fish originating from different broodstock, however, the sex ratio was 1 M : 1 F in the controls and 0 M : 1 F ( i.e . 100% females) in the gynogenetics. Together, these results suggested that the main sex‐determining genetic component in turbot fits well to female homogamety, in accordance with data from other flatfishes. These results indicate that it will be possible to obtain near or all‐female turbot stocks through induced gynogenesis or by crosses involving hormonally sex‐reversed fish.     

Upstream migration rates of radio‐tagged adult Chinook salmon in riverine habitats of the Columbia River basin

Upstream migration rates were assessed for 1801 radio‐tagged adult spring–summer Chinook salmon Oncorhynchus tshawytscha through 12 unimpounded river reaches in the Columbia River basin from 1997 to 2002. Reaches were 36 to 241 km long (mean = 130 km) and included sections of the large Columbia and Snake Rivers and smaller free‐flowing tributaries. Median Chinook salmon migration rates ranged from <10 km day⁻¹ in the Deschutes and Clearwater Rivers to >35 km day⁻¹ in the Columbia and Snake Rivers. Using multivariate analyses, migration date explained the most variance in Chinook salmon migration rates while river discharge, migration year and migration reach were secondary. Both within and between years, Chinook salmon migrated more rapidly as migration date increased and more slowly when discharge was high. Arrival at high elevation spawning grounds at appropriate times and increased metabolic activity and reproductive maturation may explain the greater power of migration date, relative to river discharge, in predicting migration rates of Columbia basin spring–summer Chinook salmon.     

Temporal variation in abundance, return rate and life histories of previously spawned Atlantic salmon in a large subarctic river

The 30 year time series analyses revealed large temporal variation in the return rates and a recent increase in abundance of previous spawning Atlantic salmon Salmo salar in the River Teno, northern Scandinavia. The mean proportion of repeat spawners was 7 and 4% in the total Atlantic salmon catch and 9 and 22% in multi‐sea‐winter (MSW) catch component for females and males, respectively. Previous spawners constituted on the average 7% of the catch in mass but up to 20%(31 t) and 30%(19 t) in 2003 and in 2004, respectively. In 1975–2000, the proportion of previous spawners varied between 1 and 6%(3–12% of MSW Atlantic salmon), whereas in 2001–2004, they accounted for 8–21%(16–35% of MSW Atlantic salmon) of the total Atlantic salmon catch. The number of previous spawners in the catch correlated significantly with the preceding numbers of respective 1–3 sea‐winter (SW) maiden Atlantic salmon 2 years earlier. The recent increase in the numbers of 1S1 and 2S1 (1 or 2 years at sea followed by first spawning and 1 year reconditioning period at sea) alternate spawning Atlantic salmon was a consequence of higher numbers of maiden 1SW and 2SW Atlantic salmon in the catches and increased sea temperatures. Similarly, the return rate of 1SW Atlantic salmon to second spawning has improved in recent years. Most previous spawners ascended and were captured early in the fishing season. The smolt and sea‐age combinations of repeat spawners comprised 68 age groups contributing with the annual mean of 15 age groups to the great diversity of the River Teno Atlantic salmon population complex.     

Hybrids between chum Oncorhynchus keta and pink Oncorhynchus gorbuscha salmon: age,growth and morphology and effects on salmon production

Mature hybrids between chum salmon Oncorhynchus keta and pink salmon Oncorhynchus gorbuscha, which were identified by an intermediate colour pattern, were caught at the Kurilsky Hatchery, Iturup Island, Russia. Most of them were female and 3 years old (a partial freshwater year and 2 marine years), which is intermediate between the ages of maturity of the parental species. The hybrids exceed both parental species in the rate of growth, are large in size and robust and might successfully compete for mating in the wild or be chosen for artificial reproduction. The ratio of the scale length over width, R, is oblate (R < 1), whereas scales of the parental species are prolate (R > 1). From scale analyses, the c.v. in body size of hybrid females at the second marine year is twice that of O. keta, which suggests developmental instability in the hybrid. A dynamic model predicted that continuing hybridization at a low rate does not produce a substantial hybrid load due to selection against advanced‐generation hybrids and backcrosses. A high hybridization rate, however, may be an additional risk for genetic management and should be taken into account in programmes of artificial reproduction of Pacific salmon Oncorhynchus spp., although such hybrids might have commercial use in confined production systems.     

Lacustrine growth of juvenile pink salmon and a comparison with sympatric sockeye salmon

The growth rates of naturally sympatric juvenile pink Oncorhynchus gorbuscha and sockeye Oncorhynchus nerka salmon were compared in a common lacustrine environment in south‐west Alsaka, an unusual opportunity given the normal disparity in freshwater residence time of these two species. Fork length ( L _F) frequency distributions of juvenile pink salmon caught in the lake during the summer in 1991 and 1999–2003 indicated a growth rate of 0·54 mm day⁻¹, 54% greater than the estimated growth rate of juvenile sockeye salmon sampled from 1958 to 2003 (0·35 mm day⁻¹). Examination of daily growth rings on otoliths indicated that pink salmon in Lake Aleknagik grew an average of 1·34 mm day⁻¹ in 2003 but sockeye salmon grew only 0·63 mm day⁻¹(average specific growth rates were 3·0 and 1·8% day⁻¹, respectively). Pink salmon increased from c . 32 mm L _F and 0·2 g at emergence to 78 mm L _F and 3·0 g within 3–4 weeks. After experiencing these rapid growth rates, the pink salmon appeared to leave the lake by late July in most years. The diets of pink and sockeye salmon in the littoral zone of the lake were very similar; >80% of the stomach contents consisted of adult and pupal insects and the remainder was zooplankton. This high degree of diet overlap suggested that the observed differences in growth rate were not attributable to variation in prey composition.     

Embryo survival and smolt to adult survival in second‐generation outbred coho salmon

Outbreeding depression was not detectable in observations of embryonic survival and of survival of smolts to adulthood relative to controls in coho salmon Oncorhynchus kisutch experimentally outbred over two generations by crossing fish from three widely separated populations. Survival in outbred coho salmon was not detectably less than that in second‐generation hybrid controls or second‐generation parental controls. Variation of embryonic survival was affected by the maternal source population and by individual females but not by the paternal source population or by individual males. Survival of smolts to adulthood at sea was greater in one second‐generation control group than in two others, but survival in second‐generation outbred groups was no less than in parental controls or hybrid controls.     

Tests of reproductive isolation among species in the <Emphasis Type="Italic">Fundulus notatus</Emphasis> (Cyprinodontiformes: Fundulidae) species complex

We assessed prezygotic (probability of spawning) and postzygotic (hatching success) reproductive isolation among the three ecologically and morphologically similar species in the Fundulus notatus species complex. We employed a multi-generation breeding experiment to test the hypotheses that karyotypic differences, body size differences, or geographic isolation among populations will increase pre or postzygotic reproductive barriers. Overall, prezygotic barriers were strong and postzygotic barriers weak in crosses of non-hybrid heterospecifics (F1 hybrid crosses) while prezygotic barriers were weaker and postzygotic barriers stronger in crosses involving hybrid individuals (F2 hybrid crosses and backcrosses). Prezygotic barriers among the two smaller species (Fundulus notatus and F. euryzonus) broke down rapidly; first generation hybrids spawned (F2 hybrid crosses and backcrosses) as frequently as parental forms in intraspecific crosses. There was no increase in postzygotic barriers among species with cytogenetic differences. There were increased prezygotic, but not postzygotic, barriers among geographically isolated populations of one species. While pure males and females were just as likely to spawn with hybrids, some types of hybrid females suffered from increased sterility, but not inviability, over hybrid males. Female sterility was only seen in hybrids with a Fundulus euryzonus parent, while other female hybrids produced viable eggs.     

Assessing patterns of hybridization between North Atlantic eels using diagnostic single-nucleotide polymorphisms

The two North Atlantic eel species, the European eel (Anguilla anguilla) and the American eel (Anguilla rostrata), spawn in partial sympatry in the Sargasso Sea, providing ample opportunity to interbreed. In this study, we used a RAD (Restriction site Associated DNA) sequencing approach to identify species-specific diagnostic single-nucleotide polymorphisms (SNPs) and design a low-density array that combined with screening of a diagnostic mitochondrial DNA marker. Eels from Iceland (N=159) and from the neighboring Faroe Islands (N=29) were genotyped, along with 94 larvae (49 European and 45 American eel) collected in the Sargasso Sea. Our SNP survey showed that the majority of Icelandic eels are pure European eels but there is also an important contribution of individuals of admixed ancestry (10.7%). Although most of the hybrids were identified as F1 hybrids from European eel female × American eel male crosses, backcrosses were also detected, including a first-generation backcross (F1 hybrid × pure European eel) and three individuals identified as second-generation backcrosses originating from American eel × F1 hybrid backcrosses interbreeding with pure European eels. In comparison, no hybrids were observed in the Faroe Islands, the closest bodies of land to Iceland. It is possible that hybrids show an intermediate migratory behaviour between the two parental species that ultimately brings hybrid larvae to the shores of Iceland, situated roughly halfway between the Sargasso Sea and Europe. Only two hybrids were observed among Sargasso Sea larvae, both backcrosses, but no F1 hybrids, that points to temporal variation in the occurrence of hybridization.     

Genetic linkage mapping of allozyme loci in even- and odd-year pink salmon (Oncorhynchus gorbuscha)

We constructed genetic linkage maps of allozyme loci in even- and odd-year pink salmon (Oncorhynchus gorbuscha), using the total of 320 families (each female was crossed with two different males, and 80 females and 160 males were used for each of even year and odd year). The maps include eight linkage groups involving 22 loci. We observed substantial variation in recombination frequencies among different families within broodline and between sexes within broodlines. In the linkage analysis between sAAT-3* and sMDH-B1,2*, two even-year families and one odd-year family exhibited evidence of association, but two even-year and one odd-year families did not. Recombination rate tends to be reduced in males in pink salmon. The ratio of recombination rate (female/male), which ranged from 1.7 to infinity, averaged 2.8 in the even-year crosses and 3.2 in the odd-year crosses. The linkage groups (LG) I and II involving sAAT and mAH loci, which probably duplicated in the recent tetraploidization event, and the orders of loci in the LGs I (sAAT-3* --> mAH-4*) and II (mAH-3* --> sAAT-4*) were reversed, suggesting the possible paracentric inversion during salmonid evolution after the duplication.     

Reproductive trait divergence and hybrid fertility patterns between chromosomal races of the house mouse in Tunisia: analysis of wild and laboratory-bred males and females

The divergence in reproductive features and hybrid fertility patterns between two chromosomal races (2 n  = 40, 40St, and 2 n  = 22, 22Rb) of the house mouse in Tunisia were re-assessed on a larger sample of wild and laboratory-bred individuals than studied hitherto. Results showed that litter sizes were significantly smaller in 40St than in 22Rb mice, contrary to previous analyses. This suggests that variation in litter size between the two chromosomal races is more likely related to selective and/or environmental factors acting locally than to interracial reproductive trait divergence. However, the significantly reduced litter size of F₁ hybrids compared with parental individuals was confirmed, and further highlighted a sex difference in hybrid infertility, as F₁ females produced fewer litters and of smaller size than males. Histological analyses of F₁ and backcrosses showed a breakdown of spermatogenesis in males and a significantly reduced primordial follicle pool in females. The degree of gametogenic dysfunction was not related to the level of chromosomal heterozygosity per se , but a significant effect of two Rb fusions on follicle number was observed in hybrid females. These results suggest that genetic incompatibilities contribute to primary gametogenic dysfunction in hybrids between the chromosomal races in Tunisia.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 407–416.     

A genetic analysis of body size in pink salmon (Oncorhynchus gorbuscha)

Two small-sized and two large-sized male pink salmon (Oncorhynchus gorbuscha) were mated to each of four females, producing eight families sired by small males and eight sired by large males. The juveniles were reared for 500 d after fry emergence. Juvenile weight in the two male size classes was similar until the spring of the year of maturity, when juveniles sired by large males grew faster than those sired by small ones. Heritability estimates of weight based upon the dam component of variance increased during 500 d of rearing from 0.4 to 0.8. Heritability of weight based upon the sire component of variance generally ranged between 0.1 and 0.3. The large variation in male body size in spawning pink salmon populations may have resulted from different male breeding strategies.     

Behavioral sterility of hybrid males in acoustically communicating grasshoppers (Acrididae,Gomphocerinae)

The effectiveness of hybridization barriers determines whether two species remain reproductively isolated when their populations come into contact. We investigated acoustic mating signals and associated leg movements responsible for song creation of hybrids between the grasshopper species Chorthippus biguttulus and C. brunneus to study whether and how songs of male hybrids contribute to reproductive isolation between these sympatrically occurring species. Songs of F1, F2, and backcross hybrids were intermediate between those of both parental species in terms phrase number and duration. In contrast, species-specific syllable structure within phrases was largely lost in hybrids and was produced, if at all, in an irregular and imperfect manner. These divergences in inheritance of different song parameters are likely the result of incompatibility of neuronal networks that control stridulatory leg movements in hybrids. It is highly probable that songs of hybrid males are unattractive to females of either parental species because they are intermediate in terms of phrase duration and lack a clear syllable structure. Males of various hybrid types (F1, F2, and backcrosses) are behaviorally sterile because their songs fail to attract mates.     

Reproduction of interspecific hybrids of Atlantic salmon and brown trout in a stream environment

1. Reproduction between Atlantic salmon males and interspecific hybrid Salmo salar × Salmo trutta females was monitored in a controlled flow channel diverted from a south European river located at the edge of Atlantic salmon natural geographic distribution in Europe. 2. Post‐F1 hybrids were viable and survived in the wild, at least until dispersal from redds. After transfer to hatchery conditions, 67% survived into the second year. 3. The hybrids possessed 98 chromosomes: two sets of Atlantic salmon(2n = 58) and one set of brown trout (n = 40) chromosomes. 4. The existence of a low proportion of allotriploid individuals can be expected in rivers where Atlantic salmon and brown trout populations coexist.