Hatching time as an indicator of environmental incompatibility and outbreeding depression in intraspecific salmon hybrids

We analysed hatching times of hybrids between two spatially separated pink salmon Oncorhynchus gorbuscha populations. We repeated the experiment in independent even‐ and odd‐year broodlines. In 1996 and in 1997, we made F1 hybrids from Auke Creek (Juneau, Alaska) females and Pillar Creek (Kodiak, 1000 km away) males and F1 controls from Auke Creek parents. Families were reared and released at Auke Creek. F2 hybrids, controls, and backcrosses were made from F1 returns in 1998 and 1999. In 2001, we made F1 hybrids at Pillar Creek with native females and Auke Creek males. Pillar Creek ancestry prolonged development: At Auke Creek, hybrid families (half Pillar Creek ancestry) developed more slowly (more Accumulated Temperature Units between fertilization and hatch; P  < 0·0001) than did controls (only Auke Creek ancestry). At Pillar Creek, families with only Pillar Creek ancestry developed more slowly than did hybrids with half Auke Creek ancestry. Development times of backcrosses were intermediate between those of hybrids and controls. The variation in development times between Auke Creek and Pillar Creek pink salmon has a genetic component that probably results from local adaptation and illustrates a mechanism that can lead to outbreeding depression in intercrosses between salmon populations ( e.g. , occurring between wild and translocated stocks).     

Experimental microevolution: transplantation of pink salmon into the European North

Human-mediated translocations of species beyond their native ranges can enhance evolutionary processes in populations introduced to novel environments. We studied such processes in several generations of pink salmon Oncorhynchus gorbuscha introduced to the European North of Russia using a set of morphological and life-history traits as well as molecular genetic markers with different selective values: protein-coding loci, mtDNA, microsatellites, and MHC. The introduction of reproductively isolated pink salmon broodlines of odd and even years yielded different results. The odd-year broodline established self-reproducing local populations in many rivers of new range, but sustainable changes in external morphology, reproduction, and life-history, as well as the impoverishment of the gene pool occurred. Their successful colonisation of the new range resulted in specialisation manifested in the rapid directional shifts in some highly heritable phenotypic traits accompanied by increased homozygosity at molecular markers as a consequence of genetic drift and selective processes. The returns of transplanted pink salmon of even-year broodline decreased sharply already in the second generation, but there was no marked reduction of genetic diversity. Our data, as well as the analysis of the history of all pink salmon transplantations beyond the species range, demonstrate comparatively greater success of introduced odd-year broodline and permit to assume different adaptive plasticity of the even- and odd-year broodlines in pink salmon, what is most likely determined by differences in their evolutionary histories. Population genetic data suggest that the even-year broodline probably diverged from the odd-year broodline relatively recently and, due to the founder effect, may have lost a part of its genetic variation with which adaptive plasticity potential is associated.     

Genetic variation of Japanese pink salmon populations inferred from nucleotide sequence analysis of the mitochondrial DNA control region

To estimate genetic variation and structure of pink salmon (Oncorhynchus gorbuscha) populations in Hokkaido, Japan, we analyzed the nucleotide sequence of about 500 bp in a variable portion of the 5′ end of the mitochondrial DNA control region for even- and odd-year broodlines. Sixty-seven haplotypes were detected in the examined individuals. Among these, 25 haplotypes were unique to the even-year broodline, while another 30 haplotypes were unique to the odd-year broodline. Five and three length-heteroplasmic haplotypes were detected in the even-year broodline and odd-year broodline, respectively. The distribution pattern of the 67 haplotypes was different among populations between both broodlines, while not different among populations within the same broodline. The haplotype and nucleotide diversity were higher for even-year broodline populations than for odd-year broodline populations, suggesting greater genetic variation within populations of the even-year broodline. Analysis of molecular variance and pairwise fixation index estimates also demonstrated strong genetic differentiation between even- and odd-year broodlines, although there was no genetic differentiation among populations within the same year broodline. The neutrality tests and mismatch distribution analysis indicate that the demographic history of pink salmon in Japan differs between even- and odd-year populations. Together, these results suggest strong reproductive isolation between the even- and odd-year broodlines of pink salmon, and high gene flow with broodlines due to straying.     

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.     

Comparative phylogeography of the two pink salmon broodlines: an analysis based on a mitochondrial DNA genealogy

Over most of their natural northern Pacific Ocean range, pink salmon (Oncorhynchus gorbuscha) spawn in a habitat that was repeatedly and profoundly affected by Pleistocene glacial advances. A strictly two-year life cycle of pink salmon has resulted in two reproductively isolated broodlines, which spawn in alternating years and evolved as temporal replicates of the same species. To study the influence of historical events on phylogeographical and population genetic structure of the two broodlines, we first reconstructed a fine-scale mtDNA haplotype genealogy from a sample of 80 individuals and then determined the geographical distribution of the major genealogical assemblages for 718 individuals sampled from nine Alaskan and eastern Asian even- and nine odd-year pink salmon populations. Analysis of restriction site states in seven polymerase chain reaction (PCR)-amplified mtDNA regions (comprising 97% of the mitochondrial genome) using 13 endonucleases resolved 38 haplotypes, which clustered into five genealogical lineages that differed from 0.065 to 0.225% in net sequence divergence. The lineage sorting between broodlines was incomplete, which suggests a recent common ancestry. Within each lineage, haplotypes exhibited star-like genealogies indicating recent population growth. The depth of the haplotype genealogy is shallow ( approximately 0.5% of nucleotide sequence divergence) and probably reflects repeated decreases in population size due to Pleistocene glacial advances. Nested clade analysis (NCA) of geographical distances showed that the geographical distribution observed for mitochondrial DNA (mtDNA) haplotypes resulted from alternating influences of historical range expansions and episodes of restricted dispersal. Analyses of molecular variance showed weak geographical structuring of mtDNA variation, except for the strong subdivision between Asian and Alaskan populations within the even-year broodline. The genetic similarities observed among and within geographical regions probably originated from postglacial recolonizations from common sources rather than extensive gene flow. The phylogeographical and population genetic structures differ substantally between broodlines. This can be explained by stochastic lineage sorting in glacial refugia and perhaps different recolonization routes in even- and odd-year broodlines.     

Geographical and temporal mitochondrial DNA variability in populations of pink salmon

Pink salmon Oncorhynchus gorbuscha from odd and even year generations in rivers of Sakhalin Island, Kuril Island, Kamchatka Peninsula, and Alaska were investigated with five informative restriction endonucleases for mtDNA variation. The odd and even generations from the same rivers of South Sakhalin differed greatly. The time of divergence between the two broodlines was estimated at 0.9-1.1 Myr. The variability of mtDNA in odd year generations was higher than in even year generations and may have been due to' founder' and/or' bottleneck' effects. The differences among river populations within the Sakhalin region in 1991-1993 were not significant and this confirms the highly migratory nature of pink compared with other Pacific salmon. The mtDNA samples revealed statistically significant differences between regions. The northern populations (Kamchatka, Alaska) were less diverse in number and frequency of haplotypes than the southern populations (Sakhalin). This suggests that pink salmon originated in the Sakhalin-Kuril region and that a founder effect during the spread of this species may have restricted the mtDNA variability in other regions.     

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.     

Straying of hatchery salmon in Prince William Sound,Alaska

The straying of hatchery salmon may harm wild salmon populations through a variety of ecological and genetic mechanisms. Surveys of pink (Oncorhynchus gorbuscha), chum (O. keta) and sockeye (O. nerka) salmon in wild salmon spawning locations in Prince William Sound (PWS), Alaska since 1997 show a wide range of hatchery straying. The analysis of thermally marked otoliths collected from carcasses indicate that 0–98% of pink salmon, 0–63% of chum salmon and 0–93% of sockeye salmon in spawning areas are hatchery fish, producing an unknown number of hatchery-wild hybrids. Most spawning locations sampled (77%) had hatchery pink salmon from three or more hatcheries, and 51% had annual escapements consisting of more than 10% hatchery pink salmon during at least one of the years surveyed. An exponential decay model of the percentage of hatchery pink salmon strays with distance from hatcheries indicated that streams throughout PWS contain more than 10% hatchery pink salmon. The prevalence of hatchery pink salmon strays in streams increased throughout the spawning season, while the prevalence of hatchery chum salmon decreased. The level of hatchery salmon strays in many areas of PWS are beyond all proposed thresholds (2–10%), which confounds wild salmon escapement goals and may harm the productivity, genetic diversity and fitness of wild salmon in this region     

Assessment of concordance among genealogical reconstructions from various mtDNA segments in three species of Pacific salmon (genus Oncorhynchus)

Seven segments of mitochondrial DNA (mtDNA), comprising 97% of the mitochondrial genome, were amplified by polymerase chain reaction (PCR) and examined for restriction site variation using 13 restriction endonucleases in three species of Pacific salmon: pink (Oncorhynchus gorbuscha), chum (O. keta) and sockeye (O. nerka) salmon. The distribution of variability across the seven mtDNA segments differed substantially among species. Little similarity in the distribution of variable restriction sites was found even between the mitochondrial genomes of the even- and odd-year broodlines of pink salmon. Significantly different levels of nucleotide diversity were detected among three groups of genes: six NADH-dehydrogenase genes had the highest; two rRNA genes had the lowest; and a group that included genes for ATPase and cytochrome oxidase subunits, the cytochrome b gene, and the control region had intermediate levels of nucleotide diversity. Genealogies of mtDNA haplotypes were reconstructed for each species, based on the variation in all mtDNA segments. The contributions of variation within different segments to resolution of the genealogical trees were compared within each species. With the exception of sockeye salmon, restriction site data from different genome segments tended to produce rather different trees (and hence rather different genealogies). In the majority of cases, genealogical information in different segments of mitochondrial genome was additive rather than congruent. This finding has a relevance to phylogeographic studies of other organisms and emphasizes the importance of not relying on a limited segment of the mtDNA genome to derive a phylogeographic structure.     

Multigenerational outbreeding effects in Chinook salmon (Oncorhynchus tshawytscha)

Outbreeding, mating between genetically divergent individuals, may result in negative fitness consequences for offspring via outbreeding depression. Outbreeding effects are of notable concern in salmonid research as outbreeding can have major implications for salmon aquaculture and conservation management. We therefore quantified outbreeding effects in two generations (F₁ hybrids and F₂ backcrossed hybrids) of Chinook salmon (Oncorhynchus tshawytscha) derived from captively-reared purebred lines that had been selectively bred for differential performance based on disease resistance and growth rate. Parental lines were crossed in 2009 to create purebred and reciprocal hybrid crosses (n = 53 families), and in 2010 parental and hybrid crosses were crossed to create purebred and backcrossed hybrid crosses (n = 66 families). Although we found significant genetic divergence between the parental lines (F_ST = 0.130), reciprocal F₁ hybrids showed no evidence of outbreeding depression (hybrid breakdown) or favorable heterosis for weight, length, condition or survival. The F₂ backcrossed hybrids showed no outbreeding depression for a suite of fitness related traits measured from egg to sexually mature adult life stages. Our study contributes to the current knowledge of outbreeding effects in salmonids and supports the need for more research to better comprehend the mechanisms driving outbreeding depression.     

Fertility in first-generation hybrids of roach, Rutilus rutilus (L.), and silver bream, Blicca bjoerkna (L.)

Fertility in first‐generation hybrids of roach, Rutilus rutilus, and silver bream, Blicca bjoerkna, was investigated. Sperm and egg production of hybrids at first sexual maturity were examined. Eggs from female hybrids were artificially fertilized with the sperm of a corresponding hybrid male; a hybrid male from the reciprocal crossbreeding; a parental species male R. rutilus; and a parental species male B. bjoerkna. The results revealed that gametogenesis was normal in female hybrids. However, in male hybrids, a low efficiency of gametogenesis was observed. The semen of male hybrids was extremely dilute, with spermatozoa concentration lower than that in parental species. Nevertheless, these F1 hybrids (males and females) from reciprocal crossbreeding were fertile. F2 and backcross generations were produced, but F2 crosses from the female hybrid and corresponding hybrid male displayed a drastically slower hatching rate. Also higher proportions of deformed embryos were hatched than in other post‐F1‐generation crosses.     

Population structure of odd-broodline Asian pink salmon and its contrast to the even-broodline structure

Most of the variation (99%) of Asian odd-broodline pink salmon Oncorhynchus gorbuscha , based on data at 32 variable (46 total) allozyme loci from 35 populations, occurred within populations. The remaining interpopulation variation was attributable to: (1) differences between northern (the northern Sea of Okhotsk, eastern Kamchatka Peninsula and western Kamchatka Peninsula) and southern (Hokkaido Island, Kuril Islands and Sakhalin Island) populations; (2) differences between the southern areas; (3) low variation among populations within some areas. The pattern contrasted strongly with that observed for Asian even-broodline populations, which had a strong structure, possibly related to geographic and oceanographic influences. Isolation-by-distance analyses of each of the two broodlines showed a stronger relationship (x 4·8) among even- than odd-broodline populations. Allele frequency differences between even- and odd-broodlines reflected the reproductive isolation of the broodlines. However, there were no fixed frequency differences which, considered with the differing population structures, suggests that migration-drift equilibrium has not yet obtained in one or both broodlines. The structural differences also suggest it is likely that the even- and odd-broodlines are of different ages and that one is derived from the other. Allozyme data do not provide a genealogical basis for identifying the ancestral lineage.     

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.     

Comparative anatomy of the dorsal hump in mature Pacific salmon

Mature male Pacific salmon (Genus Oncorhynchus ) demonstrate prominent morphological changes, such as the development of a dorsal hump. The degree of dorsal hump formation depends on the species in Pacific salmon. It is generally accepted that mature males of sockeye (O. nerka ) and pink (O. gorbuscha ) salmon develop most pronounced dorsal humps. The internal structure of the dorsal hump in pink salmon has been confirmed in detail. In this study, the dorsal hump morphologies were analyzed in four Pacific salmon species inhabiting Japan, masu (O. masou ), sockeye, chum (O. keta ), and pink salmon. The internal structure of the dorsal humps also depended on the species; sockeye and pink salmon showed conspicuous development of connective tissue and growth of bone tissues in the dorsal tissues. Masu and chum salmon exhibited less‐pronounced increases in connective tissues and bone growth. Hyaluronic acid was clearly detected in dorsal hump connective tissue by histochemistry, except for in masu salmon. The lipid content in dorsal hump connective tissue was richer in masu and chum salmon than in sockeye and pink salmon. These results revealed that the patterns of dorsal hump formation differed among species, and especially sockeye and pink salmon develop pronounced dorsal humps through both increases in the amount of connective tissue and the growth of bone tissues. In contrast, masu and chum salmon develop their dorsal humps by the growth of bone tissues, rather than the development of connective tissue.     

Early life traits of farm and wild Atlantic salmon Salmo salar and first generation hybrids in the south coast of Newfoundland

This study examined fertilization rates, survival and early life‐trait differences of pure farm, wild and first generation (F1) hybrid origin embryos after crossing farm and wild Atlantic salmon Salmo salar. Results show that despite a trend towards higher in vitro fertilization success for wild females, differences in fertilization success in river water are not significantly different among crosses. In a hatchery environment, wild females' progeny (pure wild and hybrids with wild maternal parent) hatched 7–11 days earlier than pure farm crosses and hybrids with farm maternal parents. In addition, pure wild progeny had higher total lengths (L_T) at hatch than pure farm crosses and hybrids. Directions in trait differences need to be tested in a river environment, but results clearly show the maternal influence on early stages beyond egg‐size differences. Differences in L_T were no longer significant at 70 days post hatch (shortly after the onset of exogenous feeding) showing the need to investigate later developmental stages to better assess somatic growth disparities due to genetic differences. Higher mortality rates of the most likely hybrids (farm female × wild male hybrids) at egg and fry stages and their delayed hatch suggest that these F1 hybrids might be less likely to survive the early larval stages than wild stocks.     

Factors affecting male potency in pooled gamete crosses of rainbow trout,Oncorhynchus mykiss

Synopsis Numbers of offspring produced by genetically marked male and female rainbow trout,Oncorhynchus mykiss were determined by an electrophoretic analysis of 1713 progeny from crosses produced by premixing gametes from different parents prior to insemination and associated full- and half-sib controls. We examined whether male potency, defined as the ability of males to sire progeny when their semen is pooled with that of other males, is affected by interaction with female gametes, concentration of sperm cells and the timing of semen application. Male potency differed significantly and was not affected by which of two females was involved in either of two sets of experiments. The differential between males decreased significantly when sperm concentrations were equalized. The first male sired over 75% of progeny when semen from four males was added sequentially to ova at 30 s intervals in each of two experiments. The lack of significant deviations of marker genotypes from expected Mendelian segregation in full-sib families suggested that there was no differential survival of progeny with specific marker genotypes. Furthermore, there was no correspondence between the survival of progeny of specific males in control crosses and the number of progeny sired in matings where sperm was premixed prior to insemination. Thus. variation in male reproductive success appeared the result of differential fertilization rates.     

Population Dynamics of the Pink Salmon Oncorhynchus gorbuschain the Basin of the Sea of Okhotsk in the 1990s

In the 1990s, an extensive body of data was gathered on the size of the Oncorhynchus gorbuschapink salmon populations of the Sea of Okhotsk at all the main developmental stages. A significant increase in numbers was found for juvenile pink salmon migrating into the offshore regions of the Sea of Okhotsk and the Pacific waters around the Kurils: from 250–450 million in 1990–1991 to 807–1570 million fish in 1993–1999. The overall number of migrating pink salmon in even years sharply increased in 1994 up to 215 million fish. After 1994, this estimate exceeded the number of migrating pink salmon in preceding odd years. Ocean survival of juvenile pink salmon gradually declined. This may be related to changes in the North Pacific pelagic ecosystems.     

Evidence for competitive dominance of Pink salmon (<Emphasis Type="Italic">Oncorhynchus gorbuscha</Emphasis>) over other Salmonids in the North Pacific Ocean

Relatively little is known about fish species interactions in offshore areas of the world’s oceans because adequate experimental controls are typically unavailable in such vast areas. However, pink salmon (Oncorhynchus gorbuscha) are numerous and have an alternating-year pattern of abundance that provides a natural experimental control to test for interspecific competition in the North Pacific Ocean and Bering Sea. Since a number of studies have recently examined pink salmon interactions with other salmon, we reviewed them in an effort to describe patterns of interaction over broad regions of the ocean. Research consistently indicated that pink salmon significantly altered prey abundance of other salmon species (e.g., zooplankton, squid), leading to altered diet, reduced total prey consumption and growth, delayed maturation, and reduced survival, depending on species and locale. Reduced survival was observed in chum salmon (O. keta) and Chinook salmon (O. tshawytscha) originating from Puget Sound and in Bristol Bay sockeye salmon (O. nerka). Growth of pink salmon was not measurably affected by other salmon species, but their growth was sometimes inversely related to their own abundance. In all marine studies, pink salmon affected other species through exploitation of prey resources rather than interference. Interspecific competition was observed in nearshore and offshore waters of the North Pacific Ocean and Bering Sea, and one study documented competition between species originating from different continents. Climate change had variable effects on competition. In the North Pacific Ocean, competition was observed before and after the ocean regime shift in 1977 that significantly altered abundances of many marine species, whereas a study in the Pacific Northwest reported a shift from predation- to competition-based mortality in response to the 1982/1983 El Nino. Key traits of pink salmon that influenced competition with other salmonids included great abundance, high consumption rates and rapid growth, degree of diet overlap or consumption of lower trophic level prey, and early migration timing into the ocean. The consistent pattern of findings from multiple regions of the ocean provides evidence that interspecific competition can significantly influence salmon population dynamics and that pink salmon may be the dominant competitor among salmon in marine waters.     

Association of genetic polymorphisms with embryonic mortality in the chicken

Summary Association of the egg-white genotypes of dams with the mortality of their embryos was studied in 1966 in the pure and crossbred embryos of dams of two relatively noninbred Light Sussex substrains, 6D and 6F. These had been derived two generations earlier, in 1964, by equal division of strain 6, which we previously studied in 1962; while the sires of the crosses came from two Rhode Island Red/New Hampshire substrains, 5D and 5F, similarly derived. The effects were estimated parametrically for the three loci Tf, II and III singly and in two- and three-way interaction.In the first week of incubation, Tf had an additive effect in all matings, while II (and perhaps III) had a near-additive effect in the pure matings only. There was an additive X additive interaction effect of II X III in the last six days of incubation, which was consistent over all matings except 5F X 6F. The only effect to influence the total mortality over the whole of incubation was the three-way additive X additive X additive interaction, and there was substantial agreement in this over the matings. Some triple homozygotes were superior and others inferior to the genotypes with one or more loci heterozygous.This evidence of multiple interaction between loci affecting fitness is discussed in relation to hypotheses for the maintenance of genetic variability in populations.     

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.