Charles D. Ziebell

Acclimation of Rainbow Trout to High pH to Prevent Stocking Mortality in Summer

Abstract Rainbow trout, Salmo gairdneri, were exposed to gradual and rapid increases in pH to determine if they would acclimate to values above 9.0. The trout became acclimated to a pH of 9.8 when they were exposed to gradual increases over a period of 5 days. When trout were exposed to an increase of pH to 9.5 in 6 hours, marked stress and 50% mortality occurred. However, when the pH increase was to 9.3 in 6 hours, the trout only had a temporary loss of appetite.

Food and Habitat Use by Different Sizes of Largemouth Bass (Micropterus salmoides) in Alamo Lake, Arizona

ABSTRACT Each of three size groups of largemouth bass (Micropterus salmoides) differed in foraging behavior and habitat use in an Arizona reservoir. Small bass ( 38 cm TL) were generally found as solitary individuals near submerged structures suitable for ambush feeding. The allometric growth of these fish may have rendered them too bulky to effectively forage in the limnetic zone. Differences in habitat use and foraging behavior for different sizes of largemouth bass in the reservoir appear to be the result of the interaction of the behavioral flexibility of the species, site-specific resources and ecomorphological constraints.

Impact of Threadfin Shad on Macrocrustacean Zooplankton in Two Arizona Lakes

ABSTRACT Analysis of data on macrocrustacean zooplankton in two Arizona lakes provided field evidence that large populations of threadfin shad (Dorosoma petenense) are capable not only of causing species shifts in zooplankton, but also of virtually eliminating zooplankton communities for extended periods as long as 2 years. Our analysis suggests that the long-term negative effects of stocking threadfin shad may greatly outweigh the short-term positive benefits to piscivores.

Vulnerability of Redbelly Tilapia Fry to Bluegill Predation with Changes in Cover Availability

Abstract The successful use of redbelly tilapia (Tilapia zilli) to control aquatic plants in ponds with game fish depends on high survival of tilapia young. We tested the vulnerability of redbelly tilapia fry under parental care to predation by bluegill. Experiments were conducted for a 3-day period in 5.5-m diameter pools at three levels of artificial plant cover (25, 50, and 75%) and at two densities of predators (15 and 20 per pool). Predation rates on 14-day-old, unprotected fry also were determined. Fry survival was lowest with 25% cover (mean 40%) and highest with 75% cover (mean 72.5%). The survival of fry at 50% cover (mean 68.2%) was significantly higher than survival at 25% cover, but there was no significant difference in fry survival between 50 and 75% cover. Fry survival was poor without parental care, ranging from 6 to 34%. Extrapolation of the survival data indicated that too few Tilapia zilli, when stocked with predatory game fish, would survive to the size where they could adequately cont...

Diet Shifts to Benthic Feeding by Threadfin Shad

Abstract Threadfin shad Dorosoma petenense were tested to determine if they would eat benthic organisms in both the presence and absence of zooplankton. They deliberately fed on chironomids in a sand substrate when no other food was available, but when offered a choice, selected Daphnia pulex over the chironomids. When the daphnids were all consumed, the fish resumed benthic foraging. Thus, threadfin shad can shift their diet to take advantage of other available foods when their primary food is lacking. Received September 27, 1982 Accepted April 22, 1983

Evaluation of pH Shock on Hatchery-reared Rainbow Trout

Abstract The effect of transferring hatchery-reared rainbow trout (Salmo gairdneri)from water with a pH of 7.2 to water with pHs ranging from 8.5 to 10.0 was evaluated in 48-h tests. All fish survived in the control (pH 7.2) and at pH 8.5. Survival was 88% at pH 9.0, 68% at pH 9.5, and O at pH 10.0. After the 48-h exposure, the remaining test fish were fed their usual pelleted food. Trout in the control and those held at pH 8.5 fed well. Only a few of the fish held at pH 9.0, and none of those held at pH 9.5, fed. These data indicate that it is not advisable to stock rainbow trout reared in nearly neutral hatchery water into lakes with a pH of 9.0 or higher.

Effects of Dietary Nonpreferred Aquatic Plants on the Growth of Redbelly Tilapia (Tilapia zilli)

Abstract Feeding experiments were designed to compare the growth rate of redbelly tilapia (Tilapia zilli) fed four aquatic plants: Najas marina, Egeria densa, Myriophyllum exalbescens, and Chara sp. Fish ranging in size from 9 to 11 cm total length were divided into five groups, four groups each receiving a plant and one group receiving no food. Redbelly tilapias consumed more Chara sp. than any of the other plants. The consumption rate was 79% body weight/d. Najas marina was second to Chara sp., being consumed at a rate of 67% body weight/d. Fish growth on diets containing Chara sp. or N. marina did not differ greatly. Fish fed E. densa nearly maintained their initial weight although they consumed very little of this plant. Fish fed M. exalbescens lost weight, as did the unfed group. The results indicate that control of Chara sp. and N. marina in small lakes and ponds by redbelly tilapia may be possible.