Changes in size and age at maturity in a population of kokanee Oncorhynchus nerka during a period of declining growth conditions

Trends in size distributions and age at maturity of spawning kokanee Oncorhynchus nerka during a 5 year period of declining growth conditions at Bucks Lake, California, U.S.A. were consistent with the hypothesis that reductions in growth rates in successive cohorts induce a shift to an older age at maturity. This forestalls decreases in size at maturity during a transitional period characterized by an increasing proportion of individuals that delay maturation. During the course of the study, kokanee first began declining in size at maturity, and then shifted from a 3 year to a 4 year egg to adult cycle. Individuals that spawned during their fourth year (age 3 years) were significantly larger, on average, than members of their cohort that spawned during their third year (age 2 years). This difference was greatest when age 2 year adults were smallest. The shift to an older age at maturity prevented a steady decline in size at maturity, even though age‐specific size was steadily declining over time. Size at maturity, however, began to decline again once the transition to a 4 year cycle was complete. In addition, there was a general trend of decreasing length‐specific mass. The data indicate that there is a range of growth trajectories over which delayed maturity can prevent a temporal pattern of decreasing size at maturity as growth rates decline.     

Countergradient variation and secondary sexual color: phenotypic convergence promotes genetic divergence in carotenoid use between sympatric anadromous and nonanadromous morphs of sockeye salmon (Oncorhynchus nerka)

Genetically distinct anadromous (sockeye) and nonanadromous (kokanee) morphs of the Pacific salmon, Oncorhynchus nerka, develop identical, brilliant red color at maturity during sympatric breeding in freshwater streams. The marine and lacustrine environments they occupy prior to maturity, however, appear to differ in the availability of dietary carotenoid pigments necessary to produce red coloration. We tested the hypothesis that kokanee, which occupy carotenoid-poor lakes, are more efficient at using the dietary pigments than are sockeye, which occupy the more productive North Pacific Ocean. In a 2-year controlled breeding study, flesh and skin color of mature and immature crosses fed a low-carotenoid diet were quantified with both a chromameter and by chemical extraction of carotenoid pigments. Results revealed striking countergradient variation in carotenoid use, with kokanee approximately three times more efficient at sequestering the pigments to the flesh musculature than similar age sockeye. This difference translated into virtually nonoverlapping differences between pure crosses in secondary sexual color at maturity, when the pigments are mobilized and transported to the skin. Kokanee crosses turned pinkish red over most of their body, whereas sockeye turned olive green. The olive green was similar to the breeding color of residuals in the wild, the progeny of anadromous sockeye that remain in fresh water and are believed to have given rise to kokanee on numerous independent occasions. Reciprocal hybrids were similar to each other and intermediate to the pure crosses, indicating additive genetic inheritance. Mate choice trials with sockeye males in the wild showed the ancestral morph strongly preferred red over green models. These results suggest a preference for red mates maintained in nonanadromous breeding populations drove the reevolution of the red phenotype in kokanee via more efficient use of dietary carotenoid pigments. This is a novel, yet hidden, mechanism by which sexual selection promotes the genetic differentiation of these sympatric populations.     

Radiocaesium concentrations in the muscle and eggs of salmonids from Lake Chuzenji,Japan, after the Fukushima fallout

Approximately 18 months (September to December 2012) after the Fukushima Dai‐ichi Nuclear Power Plant accident, elevated radiocaesium concentrations were measured in samples of muscle and eggs from masu salmon Oncorhynchus masou, kokanee Oncorhynchus nerka, brown trout Salmo trutta and lake trout Salvelinus namaycush from the Lake Chuzenji system, central Honshu Island, Japan (160 km from the station). Mean muscle concentrations were 142·9–249·2 Bq kg^?1 wet mass and mean concentrations in eggs were 38·7–79·0 Bq kg^?1 wet mass. There was no relationship between fork length and muscle radiocaesium concentration in any of the species, but there were significant relationships between individual muscle and egg radiocaesium concentrations from O. masou, S. trutta and S. namaycush.     

Ecological interactions between juveniles of reproductively isolated anadromous and non-anadromous morphs of sockeye salmon, Oncorhynchus nerka, sharing the same nursery lake

Sockeye salmon and kokanee, the anadromous and non-anadromous morphs of Oncorhynchus nerka, spawn in close physical proximity in tributaries to Takla Lake, British Columbia but are reproductively isolated and genetically distinct. Using genetic markers, we were able to investigate, for the first time, ecological interactions between the morphs as juveniles sharing the same nursery lake. Trawl and hydroacoustic surveys conducted in August of 1988 and 1991 revealed that juvenile O. nerka were distributed fairly evenly throughout Takla Lake with average densities ranging from 351–558 fish ha-1 in the north arm to 585–769 fish ha-1 in the west arm. Sockeye salmon were predominant (71–75%) in the west arm whereas kokanee were predominant (82%) in the north arm, a difference attributed to the distribution of spawners in the brood years studied. Within arms, the morphs were intermixed with no detectable difference in relative abundance by depth or among trawl catches. Both morphs were highly selective in their diet, especially in the north arm where fish densities and grazing pressure were lower. As age 0 juveniles, sockeye salmon were significantly larger than kokanee (53 vs. 39 mm on average) but their food habits were virtually identical. Thus we found no evidence of behaviour that would reduce niche overlap between these incipient species.     

Postglacial genetic differentiation of reproductive ecotypes of kokanee Oncorhynchus nerka in Okanagan Lake, British Columbia

Okanagan Lake, south-central interior of BC, contains two reproductive ecotypes of kokanee Oncorhynchus nerka ; individuals spawn in tributary streams ('stream-spawners') as well as on shoreline gravel areas ('beach-spawners'). We tested the hypothesis that these sympatric ecotypes comprise a single panmictic population by assaying variation in morphological traits and at allozyme, mitochondrial and minisatellite DNA loci in fish collected from three stream-spawning and two beach-spawning sites. No morphological traits consistently distinguished the reproductive ecotypes with the exception of the number of anal fin rays which was greater in stream-spawning kokanee. Four of 18 allozyme loci screened were polymorphic, but no significant allele frequency differences were detected among populations within ecotypes or between ecotypes. Similarly, allele frequencies at two minisatellite DNA loci were not significantly different among populations or between ecotypes. By contrast, significant differences in the frequencies of mitochondrial DNA restriction fragment length polymorphism (mtDNA RFLP) haplotypes were detected between stream- and beach-spawners, but not among populations within ecotypes. Further, two RFLPs that distinguished stream- and beach-spawning adults were found in juvenile kokanee sampled from the limnetic zone of Okanagan Lake. The two mtDNA RFLPs and a d-loop sequence variant appear to be unique to Okanagan Lake kokanee because we did not observe these haplotypes in sockeye salmon and kokanee sampled outside of Okanagan Lake. Our data suggest that: (i) there is restricted female-mediated gene flow between stream- and beach-spawning kokanee in Okanagan Lake, (ii) the forms have diverged within the lake basin since the retreat of the Wisconsinian glaciers (< ≊ 11 000 years ago), and (iii) distinct reproductive niches may promote divergence in north temperate freshwater fish faunas.     

EVIDENCE FOR SYMPATRIC GENETIC DIVERGENCE OF ANADROMOUS AND NONANADROMOUS MORPHS OF SOCKEYE SALMON (ONCORHYNCHUS NERKA)

Anadromous and nonanadromous morphs of the Pacific salmon Oncorhynchus nerka spawn in close physical proximity in tributaries to Takla Lake, British Columbia, yet differ in morphology, gill raker number, allozyme allele frequencies, and reproductive traits. Both morphs are semelparous typically maturing at age four, the anadromous morph (sockeye) at fork lengths of 38–65 cm and the nonanadromous morph (kokanee) at 17–22 cm. When reared together, pure and hybrid morphs also exhibited different growth rates and maturity schedules. Collectively, these large differences between the morphs confirm that sockeye and kokanee exist as reproductively isolated populations. Average gene flow (m) was estimated to be 0.1–0.8% between morphs, 1.7–3.7% among tributaries for kokanee, and 0.3–5.6% among tributaries for sockeye. We conclude that divergence has occurred in sympatry and examine potential isolating mechanisms.     

Timing of neurodegeneration and beta‐amyloid (Aβ) peptide deposition in the brain of aging kokanee salmon

Brains of kokanee salmon (Oncorhynchus nerka kennerlyi) in one of four reproductive stages (sexually immature, maturing, sexually mature, and spawning) were stained with cresyl violet and silver stain to visualize neurodegeneration. These reproductive stages correlate with increasing somatic aging of kokanee salmon, which die after spawning. Twenty‐four regions of each brain were examined. Brains of sexually immature fish exhibited low levels of neurodegeneration, whereas neurodegeneration was more marked in maturing fish and greatest in spawning fish. Neurodegeneration was present in specific regions of the telencephalon, diencephalon, mesencephalon, and rhombencephalon. Pyknotic neurons were observed in all regions previously reported to be immunopositive for Aβ. Regions that did not exhibit neurodegeneration during aging included the magnocellular vestibular nucleus, the nucleus lateralis tuberis of the hypothalamus, and Purkinje cells of the cerebellum, all of which also lack Aβ; perhaps these regions are neuroprotected. In 14 of 16 brain areas for which data were available on both the increase in Aβ deposition and pyknosis, neurodegeneration preceded or appeared more or less simultaneously with Aβ production, whereas in only two regions did Aβ deposition precede neurodegeneration. This information supports the hypothesis that Aβ deposition is a downstream product of neurodegeneration in most brain regions. Other conclusions are that the degree of neurodegeneration varies among brain regions, neurodegeneration begins in maturing fish and peaks in spawning fish, the timing of neurodegeneration varies among brain regions, and some regions do not exhibit accelerated neurodegeneration during aging. © 2002 Wiley Periodicals, Inc. J Neurobiol 53: 21–35, 2002     

Critical swimming velocities of juvenile sockeye salmon and kokanee, the anadromous and non-anadromous forms of Oncorhynchus nerka (Walbaum)

In streams tributary to the North Pacific, anadromous sockeye salmon and non-anadromous kokanee, Oncorhynchus nerka (Walbaum), occasionally spawn sympatrically and male kokanee may act as 'sneaks’to spawn with the larger female sockeye. Despite this interbreeding, sockeye and kokanee exhibit persistent biochemical genetic differences at several enzyme loci. Genetic differences between forms may be maintained by selection against‘hybrids’due to the different life histories of sockeye and kokanee; sockeye make extensive smolt, oceanic, and spawning migrations while kokanee reside permanently in fresh water. We tested the sustained swimming abilities of juvenile sockeye, kokanee, and sockeye (female) × kokanee (male) hybrids to see if hybrids were inferior to sockeye in a trait that is probably under stronger selection in an anadromous life history. Sockeye had significantly greater mean critical swimming velocities (U_crit) than kokanee of the same size raised under identical conditions (8.3 v. 7.3 body lengths s^?1 respectively). When tested 1 month later the mean U_crit of sockeye was only marginally greater than that for sockeye × kokanee hybrids (both c. 6.6 body lengths s^?1). Sockeye swimming performance was also less variable than that of either kokanee or hybrids. Sockeye tended to have slimmer bodies and longer caudal regions than kokanee or sockeye × kokanee hybrids of the same size. Sockeye also had significantly more vertebrae than kokanee and hybrids, while hybrids had more vertebrae than kokanee. These morphological differences may have contributed to the differences in swimming performance. We concluded: (i) that juvenile sockeye and kokanee have diverged with respect to sustained swimming performance and that reduced performance by kokanee may be due to relaxed selection for sustained swimming performance associated with their non-anadromous life history, (ii) that sockeye × kokanee hybrids appear to have modestly lower swimming capabilities than pure sockeye, and (iii) if the variability in swimming performance is associated with differences in survival in nature, then such differences may promote divergence between sympatric sockeye and kokanee.     

Resurrecting an extinct salmon evolutionarily significant unit: archived scales, historical DNA and implications for restoration

Archival scales from 603 sockeye salmon (Oncorhynchus nerka), sampled from May to July 1924 in the lower Columbia River, were analysed for genetic variability at 12 microsatellite loci and compared to 17 present‐day O. nerka populations—exhibiting either anadromous (sockeye salmon) or nonanadromous (kokanee) life histories—from throughout the Columbia River Basin, including areas upstream of impassable dams built subsequent to 1924. Statistical analyses identified four major genetic assemblages of sockeye salmon in the 1924 samples. Two of these putative historical groupings were found to be genetically similar to extant evolutionarily significant units (ESUs) in the Okanogan and Wenatchee Rivers (pairwise F_ST = 0.004 and 0.002, respectively), and assignment tests were able to allocate 77% of the fish in these two historical groupings to the contemporary Okanogan River and Lake Wenatchee ESUs. A third historical genetic grouping was most closely aligned with contemporary sockeye salmon in Redfish Lake, Idaho, although the association was less robust (pairwise F_ST = 0.060). However, a fourth genetic grouping did not appear to be related to any contemporary sockeye salmon or kokanee population, assigned poorly to the O. nerka baseline, and had distinctive early return migration timing, suggesting that this group represents a historical ESU originating in headwater lakes in British Columbia that was probably extirpated sometime after 1924. The lack of a contemporary O. nerka population possessing the genetic legacy of this extinct ESU indicates that efforts to reestablish early‐migrating sockeye salmon to the headwater lakes region of the Columbia River will be difficult.     

Latitudinal cline in allele length provides evidence for selection in a circadian rhythm gene

Divergent natural selection across a heterogeneous landscape can drive the evolution of locally adapted populations in which phenotypic variation is fine‐tuned to the environment. At the molecular level, such processes can be inferred by identifying correlations between genetic variation and environmental variables. We demonstrate that allele length and allele frequency at a regulatory circadian rhythm gene, OtsClock1b, are highly correlated (R² = 0.86, P = 1.25 × 10^?5) with latitude (a surrogate for photoperiod) in kokanee, the freshwater resident form of sockeye salmon (Oncorhynchus nerka). Two OtsClock1b alleles were identified that differed in length by seven amino acids, with the frequency of the shorter allele increasing from 50% in southern British Columbia (49°N) to complete fixation in Alaska (62°N). No such associations were detected for neutral microsatellite loci. In addition, a kokanee population sampled from Kamchatka, Russia (55°N) fits within the North American latitudinal cline, suggesting that this pattern may be convergent across large longitudinal spatial scales. This correlation provides evidence that natural selection rather than demographic processes may drive the distribution of genetic variation at OtsClock1b in kokanee. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 869–877.