Sex-biased marine survival and growth in a population of coho salmon

Examination of historical records for coho salmon in Big Beef Creek, in western Washington, U.S.A., indicated that more adult males than females returned to spawn, and that the mean length of the females exceeded that of males. Sex-biased survival and faster growth among females are unusual among salmonids but precedented in some other coho salmon populations. To help determine the stage of life at which sex-biased mortality might occur, the sex-linked GH-ψH pseudogene was used to determine the sex of smolts emigrating from Big Beef Creek in 1995–1997. In each of the 3 years the sex ratio was indistinguishable from 50:50, indicating similar survival rates in fresh water, and implying that the male and female coho salmon follow different foraging strategies when they are at sea. The female strategy apparently results in greater mortality, but benefits survivors with greater size. The male strategy appears to allow greater survival at the cost of reduced size.     

Going with the Flow: Spatial Distributions of Juvenile Coho Salmon Track an Annually Shifting Mosaic of Water Temperature

A critical challenge for ecologists is to understand the functional significance of habitat heterogeneity and connectivity for mobile animals. Here, we explore how a thermo-regulating fish responds to annual variation in the spatial patterning of thermal and trophic resources. In a third-order stream in coastal Alaska, juvenile coho salmon forage on sockeye salmon eggs at night in cold water and then move to warmer water to increase their digestive capacity. We mapped the spatial distributions of water temperature, juvenile coho salmon, and spawning sockeye salmon across a 5-year period during which summer discharge varied by greater than fivefold. In low flow years, warm water (9–12°C) was only available in thalweg (that is, main-channel) habitat at least approximately 400 m upstream of the cooler habitat (3–7°C) where sockeye salmon spawned. In high flow years, the entire stream thalweg was isothermal at 7–8°C, but inundated off-channel areas generated warm habitats (9–12°C) laterally adjacent to the downstream regions where sockeye salmon spawned. The daytime spatial distribution of juvenile coho salmon shifted from headwater thalweg habitats in low flow years, to downstream off-channel habitats in high flow years. In all years, the majority of juvenile coho salmon sampled during the daytime were found in warm habitat units without sockeye salmon present, yet they exhibited diet contents comprised virtually entirely of sockeye salmon eggs. Thus, thermoregulatory movements by coho salmon were able to track an annually shifting mosaic of water temperature. Our results demonstrate how the spatial habitat heterogeneity and connectivity of intact floodplains can in turn buffer aquatic organisms from high levels of temporal variation in habitat conditions and resource abundance.     

Have Native Coho Salmon (Oncorhynchus Kisutch) Persisted in the Nooksack and Samish rivers Despite Continuous Hatchery Production Throughout the Past Century?

For over a century, Washington State Department of Fish and Wildlife has implemented hatchery programs as a means to boost salmon abundance. Concerns have developed that native populations may be replaced by hatchery strains, decreasing the genetic diversity required to respond to environmental changes. We report a comparison of microsatellite DNA variation in wild-spawning and hatchery-strain coho salmon from the Nooksack and Samish rivers in northern Puget Sound. Significant heterogeneity in genotype frequencies was detected between wild-spawning coho salmon from the upper North Fork (NF) Nooksack River and hatchery-strain coho salmon from the Nooksack River (descendants of primarily Nooksack River broodstock). Little difference in genotype frequencies was detected between wild-spawning coho salmon from the Samish River and hatchery-strain coho salmon from the Nooksack River. The 13-locus suite provided high resolution: in assignment tests over 85% of wild-spawning coho salmon from the upper NF Nooksack River were assigned to source. Wild-spawning coho salmon collected below hatcheries in the Nooksack River and 50% of wild-spawning Samish River coho salmon were assigned to hatchery collections. The genetic divergence of wild-spawning coho salmon in the upper NF Nooksack River is remarkable given the extensive stocking history and proximity of a hatchery. We suggest that these upper river fish are native coho salmon and that wild spawners in the lower Nooksack and Samish River are descendants of hatchery productions. We attribute divergence to earlier run timing in upper NF Nooksack River wild spawners, availability of extensive spawning and rearing habitat upstream of a hatchery in the upper NF Nooksack River, and a longer stocking history in the Samish River.     

Skin morphology and humoral non-specific defence parameters of mucus and plasma in rainbow trout,coho and Atlantic salmon

Susceptibility to different diseases among related species, such as coho salmon (Oncorhynchus kisutch), rainbow trout (Oncorhyncus mykiss) and Atlantic salmon (Salmo salar), is variable. The prominence of these species in aquaculture warrants investigation into sources of this variability to assist future disease management. To develop a better understanding of the basis for species variability, several important non-specific humoral parameters were examined in juvenile fish of these three economically important species. Mucous protease, alkaline phosphatase and lysozyme, as well as plasma lysozyme activities and histological parameters (epidermal thickness and mucous cell density, and size) were characterized and compared for three salmonids: rainbow trout, Atlantic salmon and coho salmon. Rainbow trout had a thicker epidermis and significantly more mucous cells per cross-sectional area than the other two species. Rainbow trout also had significantly higher mucous protease activity than Atlantic salmon and significantly higher lysozyme (plasma and mucus) activities than coho and Atlantic salmon, in seawater. Atlantic salmon, on the other hand, had the lowest activities of mucous lysozyme and proteases, the thinnest epidermal layer and the sparsest distribution of mucous cells, compared with the two other salmonids in seawater. Only coho salmon had sacciform cells. Atlantic and coho salmon had higher mucous lysozyme activities in freshwater as compared to seawater. There was no significant difference between mucous lysozyme activities in any of the three species reared in freshwater; however, rainbow trout still had a significantly higher plasma lysozyme activity compared with the other two species. All three species exhibited significantly lower mucous alkaline phosphatase and protease activities in freshwater than in seawater. Our results demonstrate that there are significant histological and biochemical differences between the skin and mucus of these three salmonid species, which may change as a result of differing environments. Variation in these innate immune factors is likely to have differing influences on each species response to disease processes.     

Effects of species, culture history, size and residency on relative competitive ability of salmonids

The relative competitive ability of juvenile farm and wild salmonids was investigated to provide insight into the potential effects of introduction of cultured salmon on wild Pacific salmonid ( Oncorhynchus ) species. Aquarium experiments involving equal contests ( i.e. size matched, simultaneously introduced individuals) indicated that two wild coho salmon Oncorhynchus kisutch populations were competitively equal to a farm coho salmon population. In equal contests between farm Atlantic salmon Salmo salar (Mowi strain) and these wild coho salmon populations or coastal cutthroat trout Oncorhynchus clarki clarki , Atlantic salmon were subordinate in all cases. When Atlantic salmon were given a residence advantage, however, they were competitively equal to both wild coho salmon populations, but remained subordinate to coastal cutthroat trout. When Atlantic salmon were given a 10–30% length advantage, they were competitively equal to one wild coho salmon population but remained subordinate to the other. In equal contests in semi-natural stream channels, both wild coho and farm Atlantic salmon grew significantly more in the presence of the other species than when alone. It appears that coho salmon obtain additional food ration by out competing Atlantic salmon, whereas Atlantic salmon were stimulated to feed more in the presence of coho salmon competitors. These results suggest that wild coho salmon and cutthroat trout should out compete farm Atlantic salmon of a similar size in nature. As the relative competitive ability of Atlantic salmon improves when they have a size and residence advantage, should feral populations become established, they may exist on a more equal competitive footing owing to the long freshwater residence of Atlantic salmon.     

A method for direct measurement of the maximum volume of fish stomachs or digestive tracts

A new method for measuring maximum stomach or digestive tract volume of fish incorporates air injection at constant pressure with water displacement to measure directly the internal volume of a stomach or analogous structure. The method was tested with coho salmon, Oncorhynchus kisutch (Walbaum), which has a true stomach, and northern squawfish, Ptychocheilus oregonensis (Richardson), which has a modified foregut as a functional analogue. Both species were collected during July-October 1987 from the Columbia River, U.S.A. Relationships between fish weight (= volume) and maximum volume of the digestive organ were best fitted for coho salmon by an allometric model and for northern squawfish by an exponential model. Least squares regression analysis of individual measurements showed less variability in the volume of coho salmon stomachs ( R² = 0.85) than in the total digestive tracts ( R² = 0.55) and foreguts ( R² = 0.61) of northern squawfish, relative to fish size. Compared to previous methods, the new technique has the advantage of accurately measuring the internal volume of a wide range of digestive organ shapes and sizes.     

Olfactory sensitivity of coho salmon,Oncorhynchus kisutch (Walbaum): can salinity play a role?

The importance of environmental ions to olfactory sensitivity in fish has not been clarified by electrophysiological experiments. We tested the ability of juvenile coho salmon, Oncorhynchus kisutch (Walbaum), to detect L-serine after acclimation to either fresh water or sea water. The olfactory threshold was identical in the two waters, indicating that the sensitivity of olfaction was not affected by ionic content of the solutions bathing the sensory tissue.     

Chemosensory recognition of siblings in juvenile coho salmon (Oncorhynchus kisutch)

Juvenile coho salmon, Oncorhynchus kisutch, were tested for responses to water conditioned by conspecifics in two-choice tanks. The fish preferred water conditioned by unfamiliar non-siblings over blank water, but preferred water conditioned by both familiar and unfamiliar siblings over non-siblings. These preferences suggest that coho salmon recognize their siblings by matching the phenotype of their tank-mates with unfamiliar conspecifics, and that chemical cues are sufficient for recognition.     

Molecular cloning and sequencing of coho salmon growth hormone cDNA

A cDNA library was constructed using mRNA isolated from coho salmon pituitaries. By employing rainbow trout growth hormone cDNA as a probe, the coho salmon cDNA was isolated and the complete nucleotide (nt) sequence determined. The coding region contains 630 nt while the 5'- and 3'-untranslated regions are 64 and 489 nt in length, respectively. Comparison of the noncoding regions of coho and chum salmon cDNAs reveal identity at the 5' end but significant variation in the 3' end. Chum salmon and rainbow trout have identical amino acid (aa) sequences, but coho salmon growth hormone has a sequence that differs by 6 of the 188 predicted aa. Since salmonids are tetraploid, this difference may be the result of either divergence of the same growth hormone locus or of variation between different loci. Comparisons of the cDNA restriction maps of these three fish species suggest the former possibility.     

Juvenile coho salmon track a seasonally shifting thermal mosaic across a river floodplain

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Xenoma formation during microsporidial gill disease of salmonids caused by Loma salmonae is affected by host species (Oncorhynchus tshawytscha,O. kisutch,O. mykiss) but not by salinity

Host species and salinity often affect the development of disease in aquatic species. Eighty chinook salmon Oncorhynchus tshawytscha, 80 coho salmon O. kisutch and 80 rainbow trout O. mykiss were infected with Loma salmonae. Forty of each species were reared in seawater and 40 in freshwater. The mean number of xenomas per gill filament was 8 to 33 times greater in chinook salmon than in rainbow trout (RBT). Coho salmon had a mean xenoma intensity intermediate to that of chinook salmon and RBT. In contrast to the differences between species, salinity had no significant effect on xenoma intensity in any of these host species. The onset of xenoma formation occurred at Week 5 postexposure (PE) for chinook salmon and RBT, and at Week 6 PE for coho salmon. RBT had cleared all visible branchial xenomas by Week 9 PE, whereas xenomas persisted in coho and chinook salmon at Week 9 PE. Histologically, xenomas were visible in the filament arteries of the branchial arch in chinook and coho salmon gills but were absent from RBT gills. Fewer xenomas were seen in the central venous sinusoids of RBT than in chinook and coho salmon. The lower xenoma intensity, shorter duration of infection and pathological characteristics, common to microsporidial gill disease in RBT, suggest a degree of resistance to clinical disease that is not seen in coho and chinook salmon.     

Assessment of juvenile coho salmon movement and behavior in relation to rehabilitation efforts in the Trinity River, California, using PIT tags and radiotelemetry

Coho salmon (Oncorhynchus kisutch) of the Southern Oregon/Northern California Coast (SONCC) Evolutionarily Significant Unit (ESU) is federally listed as a threatened species. The Trinity River Restoration Program (TRRP) is rehabilitating the Trinity River to restore coho salmon (coho) and other salmonid populations. In order to evaluate the program’s actions, several studies of movements and behavior of coho in the Trinity River were conducted from 2006 to 2009, including snorkel surveys and mark-recapture techniques based on Passive Integrated Transponder (PIT) tags, elastomer tags, and radio transmitters. Catch, recapture, and condition of natural sub-yearlings, along with site fidelity and emigration of hatchery-reared yearlings in rehabilitated and reference habitats, were studied. Location was important because coho were absent from the lower controlled and rehabilitated sites most of the time. However, rehabilitation did not have a significant effect on natural coho salmon at the site level. Apparent survival of radio-tagged, hatchery-reared yearling coho released downstream from Lewiston Dam was much lower in the first 10 km downstream from the release site than in other areas between Lewiston Dam and the Klamath River estuary. Estimated survival of yearling hatchery coho salmon per 100 km down to Blake’s Riffle was estimated at 64 % over the distance of the 239 km study area. Migration primarily occurred at night in the upper Trinity River; however, as yearlings moved through the lower Trinity River towards the Klamath River, estuary nocturnal migration became less. Apparent survival was generally lowest in areas upstream from the North Fork of the Trinity River.     

Use of glacier river-fed estuary channels by juvenile Coho Salmon: transitional or rearing habitats?

Estuaries are among the most productive ecosystems in the world and provide important rearing environments for a variety of fish species. Though generally considered important transitional habitats for smolting salmon, little is known about the role that estuaries serve for rearing and the environmental conditions important for salmon. We illustrate how juvenile coho salmon Oncorhynchus kisutch use a glacial river-fed estuary based on examination of spatial and seasonal variability in patterns of abundance, fish size, age structure, condition, and local habitat use. Fish abundance was greater in deeper channels with cooler and less variable temperatures, and these habitats were consistently occupied throughout the season. Variability in channel depth and water temperature was negatively associated with fish abundance. Fish size was negatively related to site distance from the upper extent of the tidal influence, while fish condition did not relate to channel location within the estuary ecotone. Our work demonstrates the potential this glacially-fed estuary serves as both transitional and rearing habitat for juvenile coho salmon during smolt emigration to the ocean, and patterns of fish distribution within the estuary correspond to environmental conditions.     

The importance of genetic verification for determination of Atlantic salmon in north Pacific waters

Genetic analyses of two unknown but putative Atlantic salmon Salmo salar captured in the Copper River drainage, Alaska, demonstrated the need for validation of morphologically unusual fishes. Mitochondrial DNA sequences (control region and cytochrome b) and data from two nuclear genes [first internal transcribed spacer (ITS-1) sequence and growth hormone (GH1) amplification product] indicated that the fish caught in fresh water on the Martin River was a coho salmon Oncorhynchus kisutch , while the other fish caught in the intertidal zone of the Copper River delta near Grass Island was an Atlantic salmon. Determination of unusual or cryptic fish based on limited physical characteristics and expected seasonal spawning run timing will add to the controversy over farmed Atlantic salmon and their potential effects on native Pacific species. It is clear that determination of all putative collections of Atlantic salmon found in Pacific waters requires validation. Due to uncertainty of fish identification in the field using plastic morphometric characters, it is recommended that genetic analyses be part of the validation process.     

Patterns of intra- and inter-population genetic diversity in Alaskan coho salmon: Implications for conservation

Little is known about the genetic diversityof coho salmon in Alaska, although this arearepresents half of the species' North Americanrange. In this study, nine microsatellite lociwere used to genotype 32 putative coho salmonpopulations from seven regions of Alaska. Theprimary objectives were to estimate andevaluate the degree and spatial distribution ofneutral genetic diversity within and amongpopulations of Alaskan coho salmon. Geneticanalysis yielded four results that provideinsight into forces influencing geneticdiversity in Alaskan coho salmon and haveimportant conservation implications: 1)significant population differentiation wasfound within each region; 2) the degree ofdifferentiation (F _ST = 0.099) amongpopulations was as large or larger than thatreported for other Pacific salmon species inAlaska; 3) phenetic clustering of populationsshowed weak geographic concordance; 4) stronggenetic isolation by distance was only apparentat the finest geographic scale (within adrainage). These results suggest that cohosalmon populations are small relative topopulations of other Pacific salmon, and thegenetic diversity within and among coho salmonpopulations is influenced primarily by geneticdrift, and not gene flow. Resource managementand conservation actions affecting coho salmonin Alaska must recognize that the populationsare generally small, isolated, and probablyexhibit local adaptation to different spawningand freshwater rearing habitats. These factorsjustify managing and conserving Alaskan cohosalmon at a fine geographic scale.     

Pheromones and the water source preferences of adult coho salmon Oncorhynchus kisutch Walbaum

It has been proposed that population-specific pheromones released by juvenile salmonids guide the adults on their homing migration (Nordeng, 1977). To evaluate this hypothesis, adult coho salmon, Oncorhynchus kisutch , were tested in a two-choice apparatus with a variety of water sources. Coho salmon preferred home water without coho odours over an unfamiliar water source (city water). Adults preferred water conditioned by juveniles of their own population over plain city water, but did not prefer water conditioned by juveniles of their own population over that of another.     

Enzymes released from Lepeophtheirus salmonis in response to mucus from different salmonids

Adult and mobile preadult sea lice Lepophtheirus salmonis were incubated with mucus samples from rainbow trout (Oncorhynchus mykiss), coho salmon (O. kisutch), Atlantic salmon (Salmo salar), and winter flounder (Pseudopleuronectes americanus) to determine the response of L. salmonis to fish skin mucus as assessed by the release of proteases and alkaline phosphatase. There was variation in the release of respective enzymes by sea lice in response to different fish. As well, sealice collected from British Columbia responded differently than New Brunswick sea lice to coho salmon mucus. Fish mucus and seawater samples were also analyzed using protease gel zymography to observe changes in the presence of low molecular weight (LMW) proteases after L. salmonis incubation. Significantly higher proportions of sea lice secreted multiple bands of L. salmonis-derived LMW proteases after incubation with rainbow trout or Atlantic salmon mucus in comparison with seawater, coho salmon, or winter flounder mucus. Susceptibility to L. salmonis infections may be related to the stimulation of LMW proteases from L. salmonis by fish mucus. The resistance of coho salmon to L. salmonis infection may be due to agents in their mucus that block the secretion of these LMW proteases or factors may exist in the mucus of susceptible species that stimulate their release.     

Bioaccumulation of organochlorines by yearling coho salmon (Oncorhynchus kisutch walbaum) fed diets containing Great Lakes' coho salmon, and the pathophysiological responses of the recipients

1. Yearling coho salmon were fed diets containing the flesh of sexually mature adult coho salmon from Lakes Ontario, Erie and Michigan; Pacific Ocean coho salmon and a commercial trout chow were used to prepare control diets. The fish were fed the experimental diets for 27 weeks. Samples were taken of fed fish and fish fed for 27 weeks and then food deprived for 4 weeks. 2. Total accumulated organochlorine in the recipient was directly proportional to dietary total organochlorine levels, although there were considerable interlake differences in the degree of clearance of specific organochlorine compounds. 3. There was evidence of changes in thyroid physiology, hematocrit, liver structure and inter-renal gland activity in recipients fed the Great Lakes' coho salmon, apparently correlated with organochlorine bioaccumulation.     

Induced thermotolerance and tissue Hsc70 in juvenile coho salmon, Oncorhynchus kisutch

We examined Hsc70 in gill, liver and caudal fin from coho salmon ( Oncorhynchus kisutch ) before (ambient, ~12 °C) and after a sublethal heat shock of 25 °C for 1 h. Increased levels were observed for at least 48 h in all three tissues. Attempts to demonstrate isoforms of this heat-shock protein were not successful using four different antibodies. However, one of these antibodies recognized isoforms in brine shrimp and oysters, two organisms in which the heat-shock response has been well characterized. Extracts of those organisms and coho salmon tissues were run on the same gels and evaluated on the same Western blots, at the same time. We believe that our results provide a reliable account of Hsc70 in these fish under these conditions. Limited experiments showed that induced thermotolerance was achieved in these coho salmon, lasting for about 1 week after the sublethal heat shock. These data suggest that increased levels of Hsc70 are correlated with induced thermotolerance in these fish.