Seasonal changes in thyroid hyperplasia, serum thyroid hormone and lipid concentrations, and pituitary gland structure in Lake Ontario coho salmon, Oncorhynchus kisutch Walbaum and a comparison with coho salmon from Lakes Michigan and Erie

A progressive fall in serum thyroid hormone levels was found in Lake Ontario coho salmon, Oncorhynchus kisutch , between May 1978 and January 1979 concomitant with an increase in goitre frequency and thyroid hyperplasia. Despite marked differences in overt goitre frequency in sexually mature adult salmon collected from Lakes Ontario (43.7%), Erie (93.5%), or Michigan (0%), serum thyroid hormone levels and thyroid histology were similar in fish from the three lakes. Two gonadotrop-like basophil cells ('globular'and'vacuolar') were found in the pituitary, particularly in sexually maturing and mature adults and'jacks'. Whereas in salmon from Lakes Ontario or Michigan, the globular cells were vacuolated in Lake Erie salmon they were not; this observation may be correlated with the absence of obvious secondary sexual characteristics in Lake Erie salmon. Serum free fatty acids serum, triglycerides and cholesterol concentrations in Lake Ontario salmon fell progressively from the condition in fish collected from the Lake in May 1978, to that of fish collected during their potamodrous migration in November 1978 and January 1979.     

Thyroid function, pituitary structure and serum lipids in Great Lakes coho salmon, Oncorhynchus kisutch Walbaum, 'jacks' compared with sexually immature spring salmon

Whereas spring salmon, Oncorhynchus kisutch , from Lake Ontario had no overt thyroid goiters, comparable Lake Erie spring salmon had a 44% goitre incidence. Goitre incidence in sexually precocious 'jacks' from Lakes Ontario and Michigan was 43–45% and 100% in 'jacks' from Lake Erie.
All fish displayed thyroid hyperplasia. The degree of hyperplasia (as indicated by thyroid follicle colloid content, thyroid follicle size and shape and thyroid follicle epithelial cell size) in Lake Ontario fish tended to be greater in the 'jacks' than in the spring fish; the reverse was true in Lake Erie fish. Serum total L-thyroxine (T4) concentrations in Lake Ontario spring fish were higher than in any other group. There were no differences in serum T, levels between the groups of 'jacks', nor were serum T, levels in 'jacks' and spring salmon from Lake Ontario significantly different. Serum total triiodo-thyronine (T) concentrations in Lake Ontario spring salmon were significantly higher than in all other groups and that in Lake Erie spring salmon greater than in all groups of 'jacks'. Serum T, levels in Lake Ontario 'jacks' were higher than in Lake Erie 'jacks'. Serum free fatty acid (FFA) and cholesterol levels showed significant interlake differences with lower levels present in Lake Erie salmon. Lake Erie 'jacks' appeared to have fewer and less well vacuolated pituitary gonadotropic basophils than 'jacks' from Lakes Michigan and Ontario. Putative thyrotrop cells appeared to be more numerous and more active in the Lake Erie spring salmon compared with Lake Ontario spring salmon. Prolactin, corticotrop and melanotrop cells appeared less active in spring salmon than in the 'jacks'.     

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.     

The use of chemical cues in the upstream migration of coho salmon, Oncorhynchus kisutch Walbaum

The migratory behaviour of coho salmon was studied in the field. Coho salmon, Oncorhynchus kisutch , exposed to synthetic odours in a hatchery and released into Lake Michigan as juveniles, were recaptured as migrating adults 18 months later. The movements of 43 individuals were followed in experiments examining the role of imprinted chemical cues in the homing migration. Movements in relation to controlled distributions of the imprinting odours revealed that chemical cues are used by the salmon in their upstream migration. Acting with the rheotactic response, fish were guided upstream by imprinting odours simulating their home stream. Coho salmon, in the absence of their imprinting odour, respond negatively to the current. Differential responses to current appear to serve in segregating fish imprinted to odours of different home streams.     

Effect of a commercial trout diet on liver ultrastructure of fed and fasted yearling coho salmon, Oncorhynchus kisutch Walbaum

Yearling coho salmon were fed either a commercial trout (TC) diet or a diet consisting of homogenized adult Pacific Ocean coho salmon (PS), supplemented with vitamins and minerals. The hepatosomatic index of the TC-fed fish killed 12 h after the last feed or after 4 or 8 weeks of fasting were significantly larger ( P 0.05) than comparable PS-fed groups.
'Light' and'dark' hepatocyte cell types were evident in all groups although the dark cells were less numerous in the fasted fish. Hepatocytes of fasted fish contained markedly more endoplasmic reticulum than fed fish. Hepatocytic glycogen content of the TC-fed fish was higher than in PS-fed fish. Whereas in the TC-fed group hepatic glycogen was still evident in fish fasted for 8 weeks, the hepatocytes of the fasted PS-fed group contained little or no glycogen. The suitability of commercial fish diets for salmonids is discussed.     

Growth hormone and seawater adaptation in coho salmon, Oncorhynchus kisutch

1. Effects of growth hormone (GH) were examined on short-term aspects of seawater adaptation in coho salmon smolts. 2. Injection of somatostatin (SRIF) immediately prior to seawater entry suppressed plasma GH levels, but did not have any significant effects at 6 or 12 hr on hematocrits, plasma glucose or plasma Na+ levels. 3. Plasma GH levels increased 250% within 36 hr after seawater exposure. 4. Plasma glucose levels, in contrast, were significantly lower in the seawater fish after 36 hr post-exposure. 5. Plasma Na+ levels increased to 190 mEq/1 by 24 hr but subsequently returned to freshwater levels while hematocrits showed no significant changes over the 72 hr of exposure. 6. The significance of these results is discussed in terms of successful seawater adaptation in coho salmon.     

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.     

Plasma cortisol and sex steroid levels in Great Lakes coho salmon (Oncorhynchus kisutch Walbaum) in relation to fecundity and egg survival

Plasma 11-ketotestosterone and cortisol levels in spawning male and female coho salmon (Oncorhynchus kisutch Walbaum) collected from a Lake Erie (Pennsylvania) stock were significantly lower (P less than 0.01) than in stocks of coho salmon at a comparable stage of sexual development collected from Lakes Ontario (two stocks) or Michigan. Plasma testosterone levels in female coho salmon from Lake Erie were significantly lower (P less than 0.01) than in females from Lake Ontario or Lake Michigan stocks. There were no significant differences in plasma testosterone levels in the males, or 17 beta-estradiol levels in male or female salmon of the four stocks sampled from the three study lakes. This apparent dysfunction in androgen and cortisol secretion in the Lake Erie stock is correlated with and may account for the low fecundity and poor expression of secondary sexual characteristics in that stock.     

Smoltification in the net-pen culture of accelerated coho salmon, Oncorhynchus kisutch Walbaum: prediction of saltwater performance

Prediction of the success of coho salmon, Oncorhynchus kisutch , in making the transition from fresh water to salt water was investigated by the use of statistical comparisons between several freshwater variables and several measures of saltwater performance. Data were collected from two consecutive broodyears of coho salmon that were derived from a nested mating design, grown under enhanced rearing conditions, and raised in marine net-pens. Equations were developed that could successfully predict percentage smolts and saltwater growth from morphological measurements taken in fresh water. However, saltwater survival was not predictable from these measurements. Criteria for predictive equations and the value of these calculations, as well as directions for future research, are discussed.     

Evidence for crypsis in coho salmon, Oncorhynchus kisutch (Walbaum), parr: substrate colour preference and achromatic reflectance

The crypsis hypothesis of salmonid parr coloration and behaviour is evaluated in light of the criteria for protective resemblance. A review of the literature indicates that salmonid parr coloration and behaviour correspond to a cryptic interpretation. Experiments on coho salmon, Oncorhynchus kisutch , parr substrate colour preference indicate that the behavioural correlate of appropriate background choice is satisfied. Absorption spectrophotometry of diapositives of fish and experimental substrates suggests that background matching is achieved through achromatic reflectance and absorption of wavelengths by the silvery sides and parr marks, respectively.     

Changes in intestinal fluid transport associated with smoltification and seawater adaptation in coho salmon, Oncorhynchus kisutch (Walbaum)

The intestinal role in osmoregulatory preadaptation of freshwater-adapted (FW) yearling coho salmon during smoltification was examined. Measurements of intestinal net fluid absorption (J_V) using an in vitro sac preparation showed that J_V of FW parr was significantly less than that of either FW or seawater-adapted (SW) smolts. From two to four weeks following the springtime thyroxin surge, J_V increases in FW coho to a level comparable with that observed for SW smolts. J_V remained elevated in FW smolts throughout most of the summer and then decreased in the autumn. Two months after the thyroxin peak, intestinal wet weight per unit serosal surface area increases. Coho stunts resulting from premature transfer into sea water had significantly higher J_V than that seen in SW smolts. The elevated J_V of stunts is only partially explicable on the basis of differences in body weight between stunts and smolts.
Our observations indicate that an increase in intestinal fluid absorption is a preadaptive change associated with smoltification occurring in concert with alterations in renal and branchial osmoregulatory mechanisms. The timing of the increase in J_V suggests a phase relationship to the thyroxin surge. This temporal relationship may help to explain the efficacy of using changes in plasma thyroxin levels to predict hatchery-fish release dates that may optimize eventual seawater survival.     

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.     

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.     

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.     

Selection on breeding date and body size in colonizing coho salmon,Oncorhynchus kisutch

Selection during the colonization of new habitat is critical to the process of local adaptation, but has rarely been studied. We measured the form, direction, and strength of selection on body size and date of arrival to the breeding grounds over the first three cohorts (2003–2005) of a coho salmon (Oncorhynchus kisutch) population colonizing 33 km of habitat made accessible by modification of Landsburg Diversion Dam, on the Cedar River, Washington, USA. Salmon were sampled as they bypassed the dam, parentage was assigned based on genotypes from 10 microsatellite loci, and standardized selection gradients were calculated using the number of returning adult offspring as the fitness metric. Larger fish in both sexes produced more adult offspring, and the magnitude of the effect increased in subsequent years for males, suggesting that low densities attenuated traditional size‐biased intrasexual competition. For both sexes, directional selection favoured early breeders in 2003, but stabilizing selection on breeding date was observed in 2004 and 2005. Adults that arrived, and presumably bred, early produced stream‐rearing juvenile offspring that were larger at a common date than offspring from later parents, providing a possible mechanism linking breeding date to offspring viability. Comparison to studies employing similar methodology indicated selection during colonization was strong, particularly with respect to reproductive timing. Finally, female mean reproductive success exceeded that needed for replacement in all years so the population expanded in the first generation, demonstrating that salmon can proficiently exploit vacant habitat.     

Diel feeding chronology, gastric evacuation and estimated daily ration of juvenile coho salmon, Oncorhynchus kisutch (Walbaum), in the coastal marine environment

The feeding periodicity of juvenile coho salmon, Oncorhynchus kisutch , was examined from stomach collections taken during five sampling periods in coastal waters off Oregon. Significant differences in stomach content weight were found between several consecutive 3-hour periods, with peak fullness occurring in early morning and around dusk. The rates of gastric evacuation of euphausiid meals were measured in the laboratory at two temperatures. The decline in stomach contents was best described by an exponential model: 90% evacuation of the wet weight contents required an estimated 28.4 and 18.1 h at 11.4 and 13.7° C, respectively. The method of Elliott & Persson was used to estimate a daily ration of 2.4 and 3.7% of the wet body weight at the two temperatures.     

Unilateral ovariectomy increases egg size and reduces follicular atresia in the semelparous coho salmon, Oncorhynchus kisutch

Unilateral ovariectomy (ULO, removal of one ovary) is a powerful technique for studying aspects of reproductive physiology, including follicular recruitment and growth. To examine effects of ULO for the first time in a semelparous species, coho salmon (Oncorhynchus kisutch) were unilaterally ovariectomized during mid-vitellogenesis approximately 3 months before spawning. At termination of the study (79 days post-surgery), single ovaries of ULO fish were gravimetrically equivalent to paired ovaries of sham surgery, control fish. There was no evidence of recruitment of new vitellogenic follicles. Instead, the dramatic increase in ovary mass was attributable to hypertrophy of existing vitellogenic follicles (33% increase in volume) and increased fecundity achieved through a greater than two-fold reduction in follicular atresia. The composition of whole ovaries on a dry weight basis from ULO fish was greater in protein, but lower in lipid than that of control fish. Expressing the data on a per follicle basis, however, showed that follicles of ULO fish contained more protein, ash, water, and lipid. The results indicate that ULO of coho salmon induces compensatory hypertrophy of existing vitellogenic follicles, while maximizing fecundity through reduction of atresia. Thus, 3 months before spawning, coho salmon exhibit the ability to adjust final egg size and number when faced with significant depletion of ovarian follicles. This in vivo system provides a platform for further study of physiological mechanisms regulating follicular growth and atresia, and the trade-off between egg size and egg number. J. Exp. Zool. 309A:468-476, 2008. (c) 2008 Wiley-Liss, Inc.     

Parvicapsula minibicornis in anadromous sockeye (Oncorhynchus nerka) and coho (Oncorhynchus kisutch) salmon from tributaries of the Columbia River

The myxosporean parasite Parvicapsula minibicornis is described from adult sockeye and coho salmon during spawning migrations in tributaries of the Columbia River in Canada and the United States. These observations extend the known distribution of this parasite from the Fraser River drainage basin. The parasite was identified in Columbia River salmonids using polymerase chain reaction (PCR) and by in situ hybridization, but unlike in Fraser River salmon, it was not observed in conventional histological preparations of the kidney. Prevalence of the parasite determined by PCR was higher in spawning sockeye from the Fraser River than in those from the Okanagan River. Our ability to explain the relatively low prevalence and absence of clinical P. minibicornis infections in Columbia River salmon is hampered by our poor understanding of the life cycle of this parasite.