Browsing by Author "James A. Rice, Committee Chair"

Now showing 1 - 3 of 3

A Bioenergetics Assessment of Temperature and Food Consumption Effects on Growth of Reservoir Striped Bass.
(2006-08-07) Davias, Lori Anna; Kevin Gross, Committee Member; Jeffrey A. Buckel, Committee Member; James A. Rice, Committee Chair
I examined the influence of temperature and food availability on growth of striped bass Morone saxatilis in seven North Carolina reservoirs. Reduced condition and summer mortality events of stocked striped bass in some southern reservoirs have been attributed to the habitat 'squeeze' of high summer temperatures and low dissolved oxygen (DO). However, in a recent study of Lakes Badin and Norman, NC, Thompson et al. (2005) intensively studied striped bass thermal selection, diet, mortality, and energetics, and concluded that food consumption was more important than temperature in explaining the large differences in growth between the systems. Following on this study, I analyzed striped bass growth, diet, predator and prey energy densities, and temperature experience in each of an additional five reservoirs in 2003-2005. Striped bass growth and thermal experience varied widely across all seven reservoirs. Fish growth to age-7 varied from less than 2kg to nearly 6 kg, and fish spent between 0 and 87 days in very warm (≥27°C) water during summer stratification. Striped bass from lakes that experienced the most severe summer temperature also exhibited relatively fast growth. Simulations of a Wisconsin bioenergetics model parameterized for striped bass estimated that striped bass annual consumption varied between 3,144g for age-2 fish in Lake Gaston and 24,616g for age-5 fish in Jordan Lake. I also used the bioenergetics model to test for the relative effects of water temperature and food consumption on growth by conducting a series of "habitat exchange simulations." In this approach I simulated how much the growth of a particular size fish in one reservoir might change if it had experienced the temperature or food consumption of a similarsized fish in another reservoir. The difference in growth predicted when exchanging consumption was greater than that resulting from exchanging temperature for every pair of lakes compared except for those including Lake Rhodhiss. This lake had an extended period over the summer where striped bass could remain in preferred 20°C habitat, and simulations where these temperatures were combined with high food consumption resulted in the greatest annual growth for striped bass. However, my results stress that regardless of temperature, low food consumption will only yield moderate to slow growth of striped bass. On the other hand, striped bass subjected to very warm temperatures can maintain good growth but only if adequate forage exists. Knowledge of availability and abundance of clupeid prey, in addition to information on thermal conditions, is thus especially important for management of striped bass in southern reservoirs.
Hypoxia Tolerance in Two Juvenile Estuary-Dependent Fishes
(2004-01-14) Shimps, Elizabeth; James A. Rice, Committee Chair; Peter Rand, Committee Member; Jason Osborne, Committee Member
Hypoxia events, or low dissolved oxygen (DO) conditions, occur frequently in North Carolina estuaries during the summer. These events may have harmful effects on important fish stocks, including spot (Leiostomus xanthurus) and Atlantic menhaden (Brevoortia tyrannus), but their consequences are not well understood. As part of a larger study examining effects of hypoxia on juvenile estuary-dependent fishes, I investigated direct mortality due to hypoxia in juvenile spot and Atlantic menhaden. The objectives of these experiments were to determine how the extent of mortality varies with the severity of hypoxia and the duration of exposure, and to explore how vulnerability to hypoxia changes across species, temperature, and fish size. Atlantic menhaden and spot were tested at two temperatures, 25° and 30°C, and three dissolved oxygen concentrations, 0.6, 0.9, and 1.2 ppm. Survival analyses were performed on the data relating survival rate of each species to dissolved oxygen concentration, duration of exposure, temperature, and fish size. The data were also analyzed using an LC₅₀ approach for comparative purposes, and 12-hour LC₅₀ estimates (concentrations causing 50% mortality) ranged from 0.9-1.1 ppm O₂. Spot and menhaden exposed to 1.2 ppm O₂ showed no mortality in 24 hrs at 25°C, and only 30-40% mortality at 30°C. In contrast, both species experienced 100% mortality in 2-6 hrs at 0.6 ppm O₂. There was a modest effect of size on hypoxia tolerance, with small spot being less tolerant than large spot, while the converse size effect was observed for menhaden. Spot were consistently less tolerant to hypoxia than menhaden and both species were less tolerant to hypoxia at 30°C than at 25°C. Preliminary experiments showed that a 24-hour acclimation to sublethal levels of hypoxia caused significantly reduced mortality upon subsequent exposure to lethal hypoxia concentrations. This study is part of a larger effort integrating lab experiments and field observations in a spatially-explicit, individual-based model to quantify changes in fish survival, growth and distribution in response to water quality changes. Results from this study indicate that while direct mortality due to hypoxia will vary with species, size, and temperature, mortality will likely only be substantial when these species are exposed to oxygen concentrations less than about 1 ppm O2. Given the severity of hypoxia necessary to cause mortality and the ability of fish to behaviorally avoid hypoxia, direct mortality due to hypoxia may not occur on a large scale. Therefore, the greatest impacts due to hypoxia may be indirect, due to density-dependent effects on growth and survival as fish avoid hypoxic areas, or via mechanisms caused by stress imposed by sublethal hypoxic conditions alone or in concert with other stressors.
Hypoxia-induced growth rate reduction in two juvenile estuary-dependent fishes.
(2002-04-24) McNatt, Regan Allison; James A. Rice, Committee Chair; Joseph Hightower, Committee Member; William Swallow, Committee Member
As eutrophication of coastal waters increases, water quality issues such as hypoxia have come to the forefront of environmental concerns in North Carolina. Many fisheries in North Carolina are dependent on estuaries for essential nursery habitat. Chronic hypoxia during the summer has become a common occurrence in North Carolina estuaries, increasing the exposure of juvenile fish to hypoxic conditions. I conducted a laboratory study to investigate how various degrees of hypoxia affect growth rates of juvenile spot (Leiostomus xanthurus) and Atlantic menhaden (Brevoortia tyrannus) at two different temperatures. For a two-week period I exposed fish to one of four constant dissolved oxygen (DO) levels (6.0, 4.0, 2.0, or 1.5 mg O2 l-1), at one of two temperatures (25°C or 30°C). A fifth DO treatment, included for spot at 30°C, allowed DO to fluctuate from 10.0 mg O2 l-1 during the day to 2.0 mg O2 l-1 at night. This diel fluctuation follows the natural DO cycle in tidal estuarine creeks. Growth measurements were recorded at the beginning, middle, and end of experiments. In all trials, growth rates at 1.5 mg O2 l-1 were significantly lower than growth rates in the 6.0, 4.0, 2.0 mg O2 l-1, and fluctuating treatments. The reduction in growth rate ranged from 31% to 89% among trials. Hypoxia is often associated with fish kills, which attract attention in the media and research. However, sublethal levels of hypoxia typically are more prevalent and may affect fish on a more frequent basis. Hypoxia does not need to be lethal to have detrimental effects on juvenile estuary-dependent fishes.