Gastric evacuation rates and daily ration of piscivorous coho salmon, Oncorhynchus kisutch Walbaum

Effects of temperature and meal size on gastric evacuation rates of juvenile coho salmon, Oncorhynchus kisutch , consuming sockeye salmon, O. nerka , fry were examined and used in the estimation of daily meal, daily ration and number of fry consumed by coho in Chignik Lake, Alaska. Evacuation of fry by coho was best described by a negative exponential model (average R² = 0.93). A square root model also provided a good fit (average R² = 0·93), but the y-intercepts deviated more from the expected value than did the y-intercepts of the exponential model. The effect of temperature ( T , 5–13° C) and meal size (MS, 0·166–0·367 g) on the exponential evacuation rate (r_e, h^-1) could be described as
In the lake, coho fed continuously during the 24-h period in early June 1986 and 1987. Estimates of daily meal and ration of coho calculated by the Eggers method and the geometric mean of prey weight ranged from 0·224 to 0·435 g (2.1–4.4% body wt) depending on location and year. The Elliott & Persson method provided similar estimates of food consumption, whereas estimates based on the Pennington method and square root evacuation of prey differed from the exponential models. Sockeye fry represented 93% of the total prey weight. The average number of sockeye fry consumed per coho per 24 h, based on the arithmetic mean of prey weight, was 3·0–3·9 fry.     

Sub-surface drift feeding by coho salmon (Oncorhynchus kisutch, Walbaum): a model and test

The results of a test of a mechanistic encounter model that predicts the size-frequency composition of the diet of drift-feeding coho salmon ( Oncorhynchus kisulch , Walbaum) are described. When all taxa in the drift were included in the model the predicted and actual diets differed significantly, although the model explained most of the variance between the diet and the predictions of a null model. When non-consumed taxa, including the distasteful and aposematic taxon Hydracarina, were excluded from the drift composition there was no significant difference between predicted and actual diets. The model's goodness of fit increased when it was modified to predict the biomass size-frequency diet composition, explaining 99% of the weighted sum of squared deviations between the diet and a null model. A number of alternate models are tested and a potentially useful simplification of the model is identified.     

Discovery and characterization of single nucleotide polymorphisms in coho salmon,Oncorhynchus kisutch

Molecular population genetic analyses have become an integral part of ecological investigation and population monitoring for conservation and management. Microsatellites have been the molecular marker of choice for such applications over the last several decades, but single nucleotide polymorphism (SNP) markers are rapidly expanding beyond model organisms. Coho salmon (Oncorhynchus kisutch) is native to the north Pacific Ocean and its tributaries, where it is the focus of intensive fishery and conservation activities. As it is an anadromous species, coho salmon typically migrate across multiple jurisdictional boundaries, complicating management and requiring shared data collection methods. Here, we describe the discovery and validation of a suite of novel SNPs and associated genotyping assays which can be used in the genetic analyses of this species. These assays include 91 that are polymorphic in the species and one that discriminates it from a sister species, Chinook salmon. We demonstrate the utility of these SNPs for population assignment and phylogeographic analyses, and map them against the draft trout genome. The markers constitute a large majority of all SNP markers described for coho salmon and will enable both population‐ and pedigree‐based analyses across the southern part of the species native range.     

Genetics of size and growth rate through sexual maturity in freshwater-reared coho salmon (Oncorhynchus kisutch)

Genetic parameters of size through sexual maturity have been relatively unexplored for Pacific salmon. In this study, individually tagged coho salmon were raised in freshwater, and the heritabilities of size and growth rate were estimated at several intervals between 13 and 24 months of age (spawning). Heritability estimates for size were moderate to high from 13 to 19 months of age, ranging from 0.36 to 0.50, and lower from 21 months to spawning at 24 months, ranging from 0.17 to 0.32. Heritabilities of specific growth rates estimated over 3-month intervals were moderate from 16 to 21 months of age, ranging from 0.21 to 0.34. Genetic and phenotypic correlations between sizes measured at different ages were moderate to high, ranging from about 0.7 to 1.0. Correlations between growth rate and size indicated that the larger fish were the fastest growing between 16 and 19 months of age and were slower growing between 19 and 21 months of age.     

Transient serotonin-immunoreactive neurons coincide with a critical period of neural development in coho salmon (Oncorhynchus kisutch)

Summary In coho salmon (Oncorhynchus kisutch), smolt transformation has been shown to be associated with sequential surges of neurotransmitters in the brain. In order to determine if the surge of serotonin (5-HT) is correlated with structural changes, we have used immunocytochemistry to observe changes in the serotonin immunoreactivity before, during and after the 5-HT surge. The following stages were studied: 12-month-old freshwater presmolts, 17-month-old freshwater presmolts, 18-month-old saltwater smolts, 19-month-old saltwater postsmolt, 24-month-old postsmolt, and adult spawners. In the 17-month-old samples, but not at any other stage, we found a set of transient (serotonin-immunoreactive) 5-HT-immunoreactive neurons in the lateral preoptic area, as well as a discrete population of 5-HT-immunoreactive neurons in the lateral part of the dorsal right habenular nucleus. In addition, a higher density of serotonergic fibers was found in the telencephalon at this stage compared to the following two stages. Since the transient 5-HT-immunoreactive structures presented here do not appear simultaneously with the 5-HT total brain concentration surge, we conclude that they are unlikely to be the source of the 5-HT surge, but are probably related to other developmental changes in the brain associated with smolt transformation.     

Interspecific relationships between emerging Atlantic salmon, Salmo salar, and coho salmon, Oncorhynchus kisutch, juveniles

Interspecific relationships between Atlantic salmon and coho salmon were studied at early life stages in laboratory and semi-natural stream channels. During emergence, the survival and dispersal patterns were similar for the two species in single or mixed populations. Survival of Atlantic salmon fry was reduced in the presence of older coho fry. However, no predation was observed. Microdistribution differed between the two species, with Atlantic salmon fry more numerous in riffles when coho were present.
Coho juveniles had a pelagic and gregarious distribution, in contrast to the benthic behaviour of the Atlantic salmon. In laboratory streams, Atlantic salmon fry moved out or adopted a subordinate cryptic behaviour which allowed them to escape predation while negatively affecting their growth.     

Retinal projections in sockeye salmon smolts (Oncorhynchus nerka)

Summary The retinal projections in 2-year-old salmon smolt (Oncorhynchus nerka) are significantly different from those observed in other teleosts examined to date in that the projections are more extensive. Very noticeable are extensive projections to most of the dorsal thalamus, to all layers of the optic tectum, and into the periaqueductal gray of the torus semicircularis. The salmon smolt has bilateral retinal projections to the diencephalon and pretectum. A small retinal projection to the lateral habenular nucleus has not been described previously. Although these findings suggest striking differences in retinal projections among teleosts, this variation may relate to age differences since the previously studied teleosts were adults.     

Distribution of FMRFamide-like immunoreactivity in the brain,retina and nervus terminalis of the sockeye salmon parr,Oncorhynchus nerka

Summary Neurons displaying FMRFamide(Phe-Met-Arg-Phe-NH₂)-like immunoreactivity have recently been implicated in neural plasticity in salmon. We now extend these findings by describing the extent of the FMRF-like immunoreactive (FMRF-IR) system in the brain, retina and olfactory system of sockeye salmon parr using the indirect peroxidase anti-peroxidase technique. FMRF-IR perikarya were found in the periventricular hypothalamus, mesencephalic laminar nucleus, nucleus nervi terminalis and retina (presumed amacrine cells), and along the olfactory nerves. FMRF-IR fibers were distributed throughout the brain with highest densities in the ventral area of the telencephalon, in the medial forebrain bundle, and at the borders between layers III/IV and IV/V in the optic tectum. High densities of immunoreactive fibers were also observed in the area around the torus semicircularis, in the medial hypothalamus, median raphe, ventromedial tegmentum, and central gray. In the retina, immunopositive fibers were localized to the inner plexiform layer, but several fiber elements were also found in the outer plexiform layer. The olfactory system displayed FMRF-IR fibers in the epithelium and along the olfactory nerves. These findings differ from those reported in other species as follows: (i) FMRF-IR cells in the retina have not previously been reported in teleosts; (ii) the presence of FMRF-IR fibers in the outer plexiform layer of the retina is a new finding for any species; (iii) the occurrence of immunopositive cells in the mesencephalic laminar nucleus has to our knowledge not been demonstrated previously.     

Food evacuation and models for multiple meals in fishes

Synopsis A model describing evacuation of multiple meals in fishes is experimentally tested. It is shown that the total food quantity is evacuated according to the pattern and at the rate predicted by the model. The mathematical assumption inherent in the model is, however, erroneous as the evacuation of different meals is shown to occur at different rates. The consequences of this for studies of food consumption rates in fishes are discussed.     

Upper thermal tolerance of wild‐type,domesticated and growth hormone‐transgenic coho salmon Oncorhynchus kisutch

In coho salmon Oncorhynchus kisutch, no significant differences in critical thermal maximum (c. 26·9° C, C_Tmax) were observed among size‐matched wild‐type, domesticated, growth hormone (GH)‐transgenic fish fed to satiation, and GH‐transgenic fish on a ration‐restricted diet. Instead, GH‐transgenic fish fed to satiation had significantly higher maximum heart rate and Arrhenius breakpoint temperature (mean ± s.e. = 17·3 ± 0·1° C, T_AB). These results provide insight into effects of modified growth rate on temperature tolerance in salmonids, and can be used to assess the potential ecological consequences of GH‐transgenic fishes should they enter natural environments with temperatures near their thermal tolerance limits.     

The blood vasculature of the gastrointestinal tract in chinook, Oncorhynchus tshawytscha (Walbaum), and coho, O. kisutch (Walbaum), salmon

The venous and arterial vasculature of the chinook and coho salmon gastrointestinal tract were examined using corrosion casts and India ink injection techniques. Observations derived from 28 individuals of various sizes and of both sexes were used to construct simplified venous and arterial plans. Examination of the blood vasculature revealed the presence of a variety of anastomoses hitherto undescribed in teleosts.     

Modeling of the Oncorhynchus nerka Population of Dal'neye Lake (Kamchatka) Applying an Individual-Based Method

A computer model of the sockeye salmon (Onchorhynchus nerka) population from Dal'neye Lake (Kamchatka) based on the results of long-term research was developed. As opposed to an earlier published model [4], the stochastic approach and individual-based method were used in the present model. The application of such techniques in the model has enabled study of the dynamics of the genetic structure of the population under the effect of varying environments, both in the sea and in the fresh-water period of the sockeye's life, and discovery of patterns of transition from the cyclic fluctuation mode to the mode of chaotic change of number.     

Riverine, estuarine and marine migratory behaviour and physiology of wild and hatchery-reared coho salmon Oncorhynchus kisutch (Walbaum) smolts descending the Campbell River, BC, Canada

Eighty coho salmon Oncorhynchus kisutch smolts (40 wild and 40 hatchery-reared) were surgically implanted with acoustic transmitters and released into the Quinsam River over 2 days. Differences in physiology, travel time and migratory behaviour were examined between wild and hatchery-reared fish. In addition, tagged and control fish of both wild and hatchery-reared stock were raised for 3 months following surgery to compare survival and tag retention. Detection ranges of the acoustic receivers were tested in the river, estuary and ocean in a variety of flow conditions and tide levels. Receivers were placed in the river, estuary and up to 50 km north and south from the river mouth in the marine environment. Wild smolts were significantly smaller by mass, fork length and condition factor than hatchery-reared smolts and exhibited significantly higher levels of sodium, potassium and chloride in their blood plasma than hatchery-reared smolts. The gill Na⁺K⁺-ATPase activity was also significantly higher in the wild coho smolts at the time of release. Ninety-eight per cent of wild and 80% of hatchery-reared fish survived to the estuary, 8 km downstream of the release site. No difference was found in migration speed, timing or survival between smolts released during daylight and those released after dark. Wild smolts, however, spent less time in the river and estuary, and as a result entered the ocean earlier than hatchery-reared smolts. Average marine swimming speeds for wild smolts were double those of their hatchery-reared counterparts. While hatchery smolts dispersed in both a northward and southward direction upon entering the marine environment, the majority of wild smolts travelled north from the Campbell River estuary. The wild coho salmon smolts were more physiologically fit and ready to enter sea water than the hatchery-reared smolts, and as a result had higher early survival rates and swimming speeds.     

Disease resistance and health parameters of growth-hormone transgenic and wild-type coho salmon,Oncorhynchus kisutch

To extend previous findings regarding fish health and disease susceptibility of growth-enhanced fish, hematological and immunological parameters have been compared between growth hormone (GH) transgenic and wild-type non-transgenic coho salmon (Oncorhynchus kisutch). Compared to non-transgenic coho salmon, transgenic fish had significantly higher hematocrit (Hct), hemoglobin (Hb), mean cellular hemoglobin (MCH), mean cellular volume (MCV), and erythrocyte numbers, and lower white cell numbers. In addition, resistance to the bacterial pathogen Aeromonas salmonicida (causal agent of furunculosis) has been assessed between the strains. Higher susceptibility of transgenic fish to this disease challenge was observed in two separate year classes of fish. The present findings provide fundamental knowledge of the disease resistance on GH enhanced transgenic coho salmon, which is of importance for assessing the fitness of transgenic strains for environmental risk assessments, and for improving our understanding effects of growth modification on basic immune functions.     

Variation in developmental biology and microsatellite DNA in reproductive ecotypes of kokanee, Oncorhynchus nerka: Implications for declining populations in a large British Columbia lake

Kokanee are the nonanadromous (freshwater resident) form of sockeyesalmon (Oncorhynchus nerka) found in lake ecosystems throughoutthe North Pacific region. Kokanee commonly exhibit two reproductiveecotypes; `stream-spawners' that reproduce in streams tributary tolakes, and `beach-spawners' that reproduce on submerged lakeshore gravelbeaches. Okanagan Lake, in the southcentral interior of BritishColumbia, Canada, contains beach- and stream-spawning kokanee and bothecotypes have declined dramatically in abundance over the last 20 years.We examined developmental biology (developmental rate to hatching andemergence) and genetic divergence at eight microsatellite loci toinvestigate phenotypic and genetic differentiation between ecotypes tounderstand selective and demographic factors that might influence therecovery of depressed populations. Beach-spawning female kokanee weresmaller and produced smaller eggs than females from stream-spawningpopulations. There was no striking difference in time to 50%hatching between ecotypes, but beach-spawning kokanee developed fasterfrom hatching to emergence. Microsatellite loci were highly polymorphicin kokanee (between 5 and 23 alleles per locus) and showed significantdifferentiation among populations (average = 0.018). There was,however, no significant variation attributable to spawning ecotype afteraccounting for variation within ecotypes. Simulated population-mixtureanalyses indicated good potential for genetic classification of kokaneeas beach- or stream-spawners; estimated mixture proportions were within11% of actual proportions averaged over 50 replications. Our datasuggest that Okanagan Lake kokanee constitute at least two managementunits within a single watershed; the ecotypes appear adapted to distinctthermal reproductive environments and show modest moleculardifferentiation from one another. Persistence of kokanee within OkanaganLake may depend, in part, on management plans that recognize thedistinctions between the sympatric reproductive ecotypes.     

Outbreaks of phaeohyphomycosis in the chinook salmon (Oncorhynchus tshawytscha) caused by Phoma herbarum

Phoma herbarum has been associated with two outbreaks of systemic mycosis in hatchery-reared chinook salmon (Oncorhynchus tshawytscha) fingerlings. Affected fish exhibited abnormal swimming behavior, exophthalmia, multiple rounded areas of muscle softening, protruded hemorrhagic vents, and abdominal swelling. In all affected fish, swimbladders were filled with whitish creamy viscous fungal mass, surrounded by dark red areas in swimbladder walls, kidneys, and musculature. Clinical and histopathological examinations suggest that the infection may have started primarily in the swimbladder and then spread to the kidneys, gastrointestinal tract, and surrounding musculature. Consistent microscopical findings included broad septate branched fungal hyaline hyphae, 5–12 μm in diameter within the swimbladder, stomach, and often within and adjacent to blood vessels. Profuse growths of woolly brown fungal colonies were obtained from swimbladders and kidneys on Sabouraud medium. On corn meal agar the formation of pycnidia, characteristic of Phoma spp., was detected within 10 days of incubation. Morphological and molecular analyses identified this fungus as Phoma herbarum. This report underscores systemic fungal infections as a threat to raceway-raised salmon.     

A histological description of gonadal development and sex differentiation in the coho salmon (Oncorhynchus kisutch) for both untreated and oestradiol immersed fry

Gonadal development is described in detail for coho salmon ( Oncorhynchus kisutch ) between hatching and 1000 degree-days, post-hatch, to aid sex reversal research. Germ cell morphology and sequence, vascular and reproductive duct development, and gross characteristics of the gonads are presented. Sex differentiation occurs by 380 degree-days, post-hatch (800 degree-days, post-fertilization) and is direct to male and female. Oocytes proliferate rapidly after differentiation while the testes enter a period of quiescence. Fry immersed for short durations in oestradiol (200 μg ⁻¹) are also examined. Hormone immersion advanced sex differentiation by 70 degree-days. The immersions were applied early, at 20 and 90 degree-days, post-hatch, yet still altered the sex ratio and timing of differentiation. Definitive germ cells, which are abundant during this period, may be the type most receptive to steroid treatment.     

Gastric evacuation rates of white perch,Morone americana,determined from laboratory and field data

Synopsis Gastric evacuation rates (R) of white perch,Morone americana, were determined in laboratory experiments and by using field data. The resulting relationship ofR and temperature (T) for white perch wasR = 0.028e^0.106T (r² = 0.98). The high r² of the regression indicates good agreement of the combined laboratory and field data. Our rate compares well with those available for other species; and especially for Eurasian perch, which has a similar thermal existence to white perch in Lake Erie.     

Development of net energy intake models for drift-feeding juvenile coho salmon and steelhead

We developed models to predict the effect of water velocity on prey capture rates and on optimal foraging velocities of two sympatric juvenile salmonids, coho salmon and steelhead. Mean fish size was ~80 mm, the size of age I+ coho and steelhead during their second summer in Southeast Alaska streams, when size overlap suggests that competition might be strongest. We used experimentally determined prey capture probabilities to estimate the effect of water velocity on gross energy intake rates, and we modeled prey capture costs using experimental data for search and handling times and published models of swimming costs. We used the difference between gross energy intake and prey capture costs to predict velocities at which each species maximized net energy intake rate. Predicted prey capture rates for both species declined from ~75 to 30–40 prey/h with a velocity increase from 0.30 to 0.60 m·s⁻¹. We found little difference between coho and steelhead in predicted optimum foraging velocities (0.29 m·s⁻¹ for coho and 0.30 m·s⁻¹ for steelhead). Although prey capture ability appears to be more important than are prey capture costs in determining optimum foraging velocities, capture costs may be important for models that predict fish growth. Because coho are assumed to pay a greater swimming cost due to a less hydrodynamic body form, we also modeled 10 and 25% increases in hydrodynamic drag to assess the effect of increased prey capture costs. This reduced optimum velocity by 0 and 0.01 m∙s⁻¹, respectively. Habitat segregation among equal-sized coho and steelhead does not appear to be related to the effects of water velocity on their respective foraging abilities.     

Experimental infection of coho salmon Oncorhynchus kisutch by exposure of skin, gills and intestine with Piscirickettsia salmonis

Piscirickettsiosis pathogenesis was examined using some tissues as entry portals of Piscirickettsia salmonis in coho salmon. Juvenile fish, weighing approximately 8.4 g, were used in this trial. Inocula were prepared using the strain SLGO-95 of P. salmonis. The micro-organism was cultured in the CHSE-214 cell line as described by Fryer et al. (1990) and doses containing 10(4.7) and 10(3.7) TCID50 were prepared. Each dose was used to infect the fish via skin, gills and intestine. Skin and gills were exposed by calibrated drops, and the intestine by an intubation through the anal opening. Some fish were injected intraperitoneally with the same P. salmonis doses, as positive virulence controls. Sham-inoculated fish for each of the tested routes were also included as negative controls. Piscirickettsiosis was experimentally reproduced with all the inoculation methods. Cumulative mortalities and survival analyses showed that the most effective entry portal was skin followed by intestinal intubation and finally by gill infection.