William P. Dwyer

Atlantic Salmon Growth Efficiency as Affected by Temperature

Abstract A system was designed to measure the effects of temperature on the growth of fish. Water at six different temperatures (4, 7, 10, 13, 16, and 19°C) was delivered to test tanks in triplicate replication. Fifty 1.2-g Atlantic salmon (Salmo salar) were placed into each of the 18 tanks. The fish were weighed every 14 d for eight weighing periods (112 d). At the end of the study, the mean weights of fish held at 4, 7, 10, 13, 16, and 19°C were 1.8, 3.5, 6.1, 10.1, 12.9, and 11.7 g, respectively. Condition factors (weight-to-length ratios) increased as temperature increased. Temperature units required per unit of growth were lowest at the intermediate temperatures and highest at the extremes.

Three-year Hatchery and Field Evaluation of Four Strains of Rainbow Trout

Abstract The performance of four strains of rainbow trout, (Salmo gairdneri) was evaluated under hatchery and field conditions. Growth, conversion, and survival were measured in the hatchery; catchability, growth and longevity data were collected in the field. Fish from each strain were stocked at equal densities into two ponds near Three Forks, Montana, for a replicated field evaluation. The domestic Winthrop and Spring Standard Growth strains grew faster, converted food more efficiently, and were harvested at a faster rate than the wild McConaughy and Fish Lake strains. However, the McConaughy and to some extent the Fish Lake strains remained in the fishery longer—up to 3 years, at which time the project was terminated.

Effect of Backpack Electroshock on Westslope Cutthroat Trout Injury and Growth 110 and 250 Days Posttreatment

Abstract Westslope cutthroat trout Oncorhynchus clarki lewisi that were reared under captive conditions were exposed to 0.75–1.5 V/cm of continuous, 50-Hz pulsed, or variable pulsed (CPS) direct current (DC) for 5 s to determine whether their growth after 110 and 250 d was affected. Individually marked fish served as experimental units. Only one fish, which was shocked in the CPS treatment, died following treatment. After 110 d, fish that underwent CPS and 50-Hz pulsed-DC electroshock treatments had gained significantly (P < 0.05) less weight than control fish, but weight gains by fish exposed to continuous DC were not significantly different from those of control fish. After 250 d, fish exposed to any of the three treatments had gained significantly (P < 0.05) less weight than control fish. At the conclusion of the experiment (250 d), four observers examined the fish to assess whether they had suffered spinal injury. Significantly (P < 0.05) more fish that had undergone electrofishing treatments exhibite...

Effects of Electroshock Voltage, Wave Form, and Pulse Rate on Survival of Cutthroat Trout Eggs

Abstract Eggs of cutthroat trout Oncorhynchus clarki were exposed for 10 s to continuous direct current (DC), pulsed DC (PDC) at 30 or 60 Hz, or the Coffelt Complex Pulse System (CPS®) wave form. Each treatment was conducted at 150 or 225 V. Treatments occurred every other day from fertilization to development of pigmentation in the eye. A different set of eggs (from the same pooled sample) was tested for each voltage-frequency level and day (i.e., eggs were treated only once). Percent mortality at eye-up of eggs exposed to 150 V PDC was no different from that of controls, whereas mortality of eggs exposed to 150 V DC was greater. Eggs exposed to PDC and DC at 225 V exhibited a significant increase in mortality when shocked on days 4–14 postfertilization. At the higher voltages, CPS also caused significant egg mortality. These results suggest that voltage level is more critical to egg survival than either wave form or pulse rate. If it is necessary to shock over redds, low voltages should be used regardle...

Effect of Diet Composition on Performance of Rainbow Trout Brood Stock during a Three-year Period

Abstract Brood stock rainbow trout (Salmo gairdneri) were fed diets low, intermediate, and high in energy and protein over a 3-year period. At the end of the study fish fed diets high in energy and protein weighed more and produced a greater number of larger eggs than did fish fed diets intermediate or low in energy and protein. The average total number of eggs produced by 3-year-old spawners were significantly less than that produced by either 4- or 5-year-old fish; numbers produced by the two older groups of fish did not differ. There was no significant difference in percentage of eyed eggs of 3-, 4-, or 5-year-old fish fed different diets; but the percentage of eggs that reached the eyed stage tended to decrease as age of the fish increased. Number of eggs produced per kilogram of body weight was greatest in 3-year-old fish fed a low-energy, low-protein diet; numbers did not differ between 4- and 5-year-old fish. No significant differences existed in weight of eggs per kilogram of body weight between f...

Catchability of Three Strains of Cutthroat Trout

Abstract In 1983 and 1984 two ponds near Three Forks, Montana, were stocked at equal rates with catchable-size cutthroat trout Oncorhynchus clarki of three strains-Snake River O. clarki subsp., Colorado River O. c. pleuriticus, and the McBride Lake strain of the Yellowstone cutthroat trout O. c. bouvieri. The mean percent harvest for the different strains during a creel census conducted during both stocking years differed significantly: Snake River, 51.9%; McBride Lake, 28.3%; and Colorado River, 11.2%. Thus, the Snake River strain was the most susceptible to angling, and the Colorado River strain the least susceptible. Catch rates among these strains was significantly different and ranged from 0.07 fish/h for the Colorado River cutthroat to 0.3 fish/h for the Snake River cutthroat trout, with the McBride Lake fish being intermediate. Susceptibility to angling appeared directly related to the degree of domestication among the strains.

Management Briefs: Influence of Electroshock on Short-Term Growth of Adult Rainbow Trout and Juvenile Arctic Grayling and Cutthroat Trout

Abstract Rainbow trout Oncorhynchus mykiss (330 mm total length) were exposed to pulsed direct current from a Coffelt BP-6 electroshocker for 10 s. Control fish received the same amount of handling but were not shocked. At termination, 35 d after treatment, the control fish had gained an average of 51.3 g (SD, 18.5) and the treated fish had gained 33.7 g (SD, 17.5). The four fish (8.0%) that died during the experiment were all treated fish. The effects of electroshock on growth of juvenile fluvial Arctic grayling Thymallus arcticus and cutthroat trout Oncorhynchus clarki were also examined. These fish were exposed to the same duration of electroshock as described above, After 28 d, average growth in length and weight of electroshocked Arctic grayling was significantly less than that of the control fish. Both the electroshocked cutthroat trout and the control fish lost weight. However, at the end of the study, the weight of the control cutthroat trout was significantly greater than the electrosocked fish, ...

Management Briefs: Effect of Electroshock on Juvenile Arctic Grayling and Yellowstone Cutthroat Trout Growth, 100 Days after Treatment

Abstract Juvenile Arctic grayling Thymallus arcticus and Yellowstone cutthroat trout Oncorhynchus clarki bouvieri, reared under captive conditions, were exposed to 0.75 or 1.5 V/cm of smooth or 60-Hz pulsed DC for 5 or 10 s (eight treatments) to determine any effects on growth after 100 d. Individually marked fish served as the experimental units. Growth of Arctic grayling was affected by some electroshock treatments; length and weight of fish exposed to pulsed DC was significantly reduced. Arctic grayling in affected treatments gained 9.2–16.5 mm and 17.1–25.7 g less than the corresponding control fish. The 60-Hz pulsed DC reduced growth of Arctic grayling more than smooth DC at the same duration and voltage level. Reduction of Yellowstone cutthroat trout growth was less significant among treatments than reduction of Arctic grayling growth. Sample size of cutthroat trout was smaller, however, at the termination of the study due to tag loss and fish loss. Although only five of the Yellowstone cutthroat tr...

Evaluation of a Low Head Oxygenator at Giant Springs State Fish Hatchery, Great Falls, Montana

Abstract Oxygen content of water is often the primary limiting factor for fish production, as it was at the Giant Springs State Fish Hatchery, Great Falls, Montana. Spring water supplying the 24 outdoor rearing raceways has a dissolved oxygen concentration of only 6.2 ppm and is also supersaturated with gas, primarily nitrogen. The low head oxygenator (LHO) manufactured by Zeigler Brothers, Inc., was tested. The units were evaluated to determine absorption efficiency, degassing ability, and effect on fish performance. The LHO units efficiently increased the DO, reduced nitrogen from 120 to 110% saturation, and reduced total gas from 112 to 108%. Carrying capacity of raceways with the LHO units was at least 2.2 times greater than that of the controls at the time the tests were ended.

Metacercariae of Diplostomum spathaceum in the Eyes of Fishes from Yellowstone Lake, Wyoming

Fish were collected from Yellowstone Lake, Wyoming (USA). Metacercariae of Diplostomum spathaceum was found in the lens of 11 of 12 longnose suckers (Catostomus Catostomus). The mean number of metacercariae per sucker was 59 and the average age of the fish was 11.6 yr. There was no correlation between age and intensity of parasites (r = 0.24). Of 10 cutthroat trout (Salmo clarki) examined, there were metacercariae present in six. The metacercariae were found outside of the lens tissue in the trout; they occurred in the vitreous humor and the retina. These may be a different species from those found in the suckers.