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.     

Diel feeding periodicity, daily ration and prey selection of a riverine population of juvenile chinook salmon, Oncorhynchus tshawytscha (Walbaum)

The diel feeding periodicity, daily ration and prey selection of juvenile chinook salmon, Oncorhynchus tshawytscha , were studied in relation to the available prey. Maximum dry weight of food intake occurred about dawn, when mayflies were the major prey, but the greatest number of freshly eaten prey occurred during the afternoon, when chironomids and terrestrial dipterans predominated. Feeding activity at night was low, with smaller mayflies comprising up to 50% of the prey. During the day the young salmon fed selectively on chironomids and the larger mayflies, while trichopterans and terrestrial taxa were under-represented in the diet. Food consumption over the 24-h period averaged 8.3% of the fish dry body weight. Prey abundance in the drift explained about 50% of the composition of the diet. Although the fish selected larger mayflies, size apparently was not a main criterion for selection because chironomids, although smaller than mayflies, were also frequently eaten. Previous dietary experience of the fish and the diel pattern of prey abundance appear to best explain the selective feeding of juvenile chinook 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.     

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.     

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.     

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.     

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.     

Gastric evacuation of single and multiple meals by piscivorous coho salmon,Oncorhynchus kisutch

Synopsis Gastric evacuation of multiple meals of recently emerged sockeye salmon, Oncorhynchus nerka, consumed by juvenile coho salmon, O. kisutch, were compared with that expected from a single meal evacuation rate model developed for the estimation of food consumption by coho in the field. Significant interaction occurred between meals consumed two hours apart (p<0.001). Evacuation of the first of two meals (29.9% of initial weight remaining) was significantly faster (p<0.05) than that of a single meal (36.1% of initial weight remaining) after four hours, whereas evacuation of the second meal (72.2% of initial weight remaining) was significantly slower (p<0.001) than that of single meals (50.6% of initial weight remaining) after two hours. The total weight of the multiple meal remaining after four (0.191 g), six (0.138 g) and eight (0.070 g) hours averaged within 6% of that predicted by the single meal evacuation model (0.186 g, 0.115 g and 0.073 g, respectively). These data suggest that the stomach evacuation model based on single meals is adequate for estimating the evacuation of prey consumed by continuously feeding coho salmon.     

Strontium marking of hatchery reared coho salmon, Oncorhynchus kisutch Walbaum, identification of adults

Hatchery-raised coho salmon were marked with stable strontium prior to their release to sea. Returning marked spawners were correctly identified after 18 months at sea by measuring the Strontium (Sr) concentration of the central region of their scales and vertebrae.     

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.     

Strontium marking of hatchery-reared coho salmon (Oncorhynchm kisutch, Walbaum)

In the management of saimonid populations it is often necessary to distinguish hatchery-reared from wild stocks. This study examines the feasability of marking fish tissue by substituting a biologically rare element, strontium, for calcium.
Sixteen-month-old hatchery-reared coho salmon ( Oncorhynchus kisutch , Walbaum 1792) were fed a diet, to which 10000 ppm stable strontium was added, for 60 days. Prior to their seaward migration in June 1976, the treated coho contained 32 times as much strontium in their vertebrae as did the control coho.
Sixty-eight precocious males'jacks' returned to the hatchery in the fall of 1976 and 1200 adults in the fall of 1977. Strontium-treated 'jacks' contained 1·4 times as much strontium as did control 'jacks' however, treated and untreated fish could not be distinguished as adults. The reduction in the strontium concentration of the treated fish could be accounted for by growth.     

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.     

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.     

Growth hormone binding to tissues of normal and stunted juvenile coho salmon, Oncorhynchus kisutch

The premature transfer of juvenile Coho salmon into seawater results in high mortality and a reduction of growth (stunting) of a significant portion of the population while the remainder of the salmon continue to grow normally. Binding studies were conducted with ^I25I-labelled tilapia ( Sarotherodon mossambicus ) growth hormone (tGH) to determine if differences in specific binding of tGH existed in membrane preparations of tissues of stunted and normal coho salmon. Membrane preparations of both liver and gill of stunted salmon obtained from stocks in California, Washington and British Columbia and kidney membrane preparations obtained from stunted salmon in California exhibited deficiencies in ¹²⁵I-labelled tGH-binding sites compared to corresponding membrane preparations of normal smolts. These results indicate that stunted coho salmon have a decreased ability to bind GH which may be reflected in perturbations in GH-mediated processes.     

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.     

Effect of hunger on the daily pattern of feeding rates in juvenile pink salmon, Oncorhynchus gorbuscha Walbaum

Juvenile pink salmon, Oncorhynchus gorbuscha , in two test groups of differing hunger level fed continuously on live copepods, maintained at high densities, throughout 12 h feeding periods in the laboratory. Analysis of video films showed that mean feeding rates were initially (first 10 min) between 51.5 and 63.8 prey-capture attempts (snaps) fish^-1 10 min^-1, but declined abruptly with increasing satiation to relatively constant levels of about 5.0 and 8.0 snaps fish^-1 10 min^-1, which were maintained during the subsequent 11 h of feeding. The data suggests that after the initial filling of their stomachs with food, juvenile pink salmon keep their stomachs full by feeding at a rate that balances the gastric evacuation rate of 18.6 mg prey h^-1 at 11° C. Less than 15% of the fish's stomach contents need be evacuated apparently for spontaneous feeding to resume or occur. Increased hunger level (by increasing a pre-test food deprivation period from 24 to 72 h) resulted in fish increasing their average feeding rate and thus their ration consumed from 23.6 to 39.8% dry body weight per 12 h.     

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.     

Ontogeny of the olfacto-retinalis projection in coho salmon (Oncorhynchus kisutch)

Filling of nucleus olfacto-retinalis neurons by cobaltous lysine injections into one eye of coho salmon of different ages revealed that the terminal nerve projection to the retina is established when the fish leave their freshwater environment. At this time salmons go through a metamorphosis termed "smoltification." FMRF-amide-like immunoreactive neurons in the nucleus olfacto-retinalis already exist in pre-smolts before the retino-petal projection is established. Thus, for the first time in any vertebrate, a projection of the terminal nerve is shown to develop during an advanced ontogenetic state.