Browsing by Author "Joseph Hightower, Committee Member"

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Age-Dependent Tag Return Models for Estimating Fishing Mortality, Natural Mortality and Selectivity
(2005-09-09) Jiang, Honghua; Kenneth Pollock, Committee Chair; Cavell Brownie, Committee Co-Chair; David Dickey, Committee Member; Joseph Hightower, Committee Member
We extend the instantaneous rates formulation of fisheries tag return models to allow for age-dependence of fishing mortality rates in Chapter 1. This is important in many applications where tagged fish vary over a large range of ages (and sizes). We focus on a model based on assuming selectivity by age is constant over years and that above a certain age selectivity is fixed at 1. We show that it is possible to allow natural mortality, M, to vary by age and year. We allow for incomplete mixing of tagged fish and for fisheries to be pulse, continuous or continuous over part of the year. We focus on the case where all or most age classes are tagged each year. We investigate model identifiability and how well parameters can be estimated using analytic and simulation methods. Results show that some models with the tag reporting rate estimated are singular or near-singular. The age-length key method commonly used for age specification may produce substantial errors in converting size to age, especially for the older fish. To reduce such errors, in Chapter 2 we propose two alternative sampling designs to the standard one of tagging all age classes: one where only age 1 fish are tagged, another where both age 1 and age 2 fish are tagged. Catch-and-release fisheries have become very important to the management of overexploited recreational fish stocks. Tag return studies where the tag is removed regardless of fish disposition have been used to assess the effectiveness of restoration efforts for these catch-and-release fisheries. In Chapter 3, we extend the instantaneous rate formulation of tag return models introduced in Chapter 1 to catch-and-release tagging studies. We illustrate the methods using multiple age class tag return data on striped bass (Morone saxatilis) from the Maryland Department of Natural Resources (MDNR). We found evidence that M is age dependent and that M has increased since 1999 possibly due to an outbreak of the disease (mycobacteriosis) in striped bass in the Chesapeake Bay.
The Disinfection and Protection of Microorganisms in Complex Water Systems.
(2010-06-11) Berecz, Michael; Michael Stoskopf, Committee Chair; Thomas Losordo, Committee Member; Jay Levine, Committee Member; Joseph Hightower, Committee Member; Phillip Russell, Committee Member; Michael Stoskopf, Committee Member
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.
The Influence of Temperature and Forage Availability on Growth and Habitat Selection of a Pelagic Piscivore
(2006-12-08) Thompson, Jessica Suzanne; Montserrat Fuentes, Committee Member; James Rice, Committee Chair; Joseph Hightower, Committee Member; James Gilliam, Committee Member
Habitat characteristics influencing growth of fishes often affect habitat selection because behaviors leading to rapid growth are under selective pressure. For pelagic piscivores such as striped bass Morone saxatilis stocked into southern reservoirs, temperature and prey density will likely be the most important factors influencing growth and spatial distribution. The traditional paradigm used to understand reservoir striped bass has focused on unsuitable physical conditions that may develop during summer stratification when hypolimnetic hypoxia forces fish into warm epilimnetic water. This study investigates how forage availability modifies the effects of physical conditions on this species. Striped bass in Badin Lake, NC, tagged with temperature-sensing transmitters tolerated temperatures above 27 degrees Celcius for two months to avoid hypolimnetic hypoxia but still displayed rapid growth. Bioenergetics models showed that high consumption rates allowed these fish to allocate energy to growth even during the summer and to grow rapidly during the fall as temperatures cooled. In contrast, striped bass in Lake Norman, NC, had slower growth despite experiencing warm summer temperatures for two to four weeks less. Lake Norman striped bass had lower consumption rates, and bioenergetics model simulations in which habitat conditions were exchanged between the reservoirs indicated that differences in forage availability had a greater relative effect on growth of striped bass than differences in thermal regime. These results suggest that criteria for determining the suitability of reservoirs for striped bass should incorporate a measure of prey availability. As with growth, habitat selection of Badin Lake striped bass was not solely dependent on temperature. Growth rate potential (the rate of growth of a predator occupying a particular location characterized by temperature and forage fish density) provided a better explanation of the spatial distribution of striped bass than did temperature or forage fish density alone. These results suggest growth rate potential can help us understand how fish integrate information on temperature and forage fish density, but it is important to note that growth rate potential was not a perfect predictor of the spatial distribution of striped bass so we cannot assume fish will optimize their patch choice with respect to this variable under all circumstances.