A multidisciplinary approach to evaluating population status and disease dynamics of smallmouth bass
Open Access
- Author:
- Schall, Megan Victoria
- Graduate Program:
- Ecology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 30, 2017
- Committee Members:
- Tyler Wagner, Dissertation Advisor/Co-Advisor
Tyler Wagner, Committee Chair/Co-Chair
Vicki S. Blazer, Committee Member
W. David Walter, Committee Member
Val Richard Beasley, Outside Member - Keywords:
- smallmouth bass
fish ecology
fish health - Abstract:
- Fish and wildlife populations are impacted by various stressors including overexploitation, habitat destruction, introduced species, pathogens, contaminants, and climate change. These stressors can have effects at multiple levels of biological organization, from cellular to ecosystem-level impacts. Smallmouth bass (Micropterus dolomieu) are an important socioeconomic and ecological species of conservation interest throughout the United States including in the Susquehanna River Basin, Pennsylvania. Within the past 10-15 years, there have been concerns about the overall health of smallmouth bass in the Susquehanna River Basin, which has manifested itself in declines in the abundance of adult fish, mortality events of young fish, evidence of endocrine disruption, and clinical signs of disease. Potential contributing factors include bacterial and viral pathogens, parasites, degraded water quality conditions, and contaminants. Elucidating which potential contributing factors are important is complicated by the fact that the Susquehanna River Basin is comprised of river-tributary networks that vary in habitat type, habitat quality, and therefore the potential for exposure to different stressors. In riverine systems, there is a large amount of variability in how much a smallmouth bass might move over its lifespan, with some evidence for long range movements. Therefore, understanding the movement ecology and gene flow of smallmouth bass will directly help efforts to identify and understand ecological drivers of smallmouth bass populations, and is necessary to help inform management decisions (e.g., fishing regulations), the design of ecological field studies, and conservation of management units. In addition, despite the fact that declines in smallmouth bass catch rates have been noted, trends in relative abundance have not been quantified or put within a regional context to see if observed patterns in the Susquehanna River differ from other large river systems in the mid-Atlantic region that also support smallmouth bass fisheries. I evaluated population trends, individual fish movement, and population genetic structure of smallmouth bass in Chapters 1-3. Chapter 4 looks specifically at a potential stressor of smallmouth bass by focusing on the disease dynamics of a myxozoan parasite infecting juvenile smallmouth bass. This parasite has been found across the Susquehanna River Basin, but how its prevalence varies across the landscape and is influenced by anthropogenic land use practices is not understood. In Chapter 1 of my dissertation, I evaluated trends in adult smallmouth bass catch per effort data (CPE) from the Susquehanna River Basin and compared trends to out of basin sites both within and outside the state of Pennsylvania. Dynamic linear models, as opposed to traditional linear models, permitted investigation of short term non-monotonic changes in fish CPE, which is important in order to detect short-term perturbations and nonlinear responses of populations to stressors. When compared with other rivers, the Susquehanna River Basin had the largest magnitude and probability of decline in smallmouth bass CPE. The declines began in the late 1990s prior to the first documentation of disease, but the general patterns of decline and trend towards recovery at the end of the study was not consistent across the Susquehanna River Basin. This chapter provided a foundational level of understanding on smallmouth bass CPE temporal dynamics in the study system over important time periods relevant to ongoing disease and mortality events. Chapters 2 and 3 examine population genetics and spatial movement ecology of adult smallmouth bass in the system. Both chapters were largely motivated by observations of adult smallmouth bass using both river and tributary habitats across the Susquehanna River Basin. This led to questions regarding whether fish utilizing tributaries and rivers were separate ecological and genetic groups. In Chapter 2, population genetics structure of smallmouth bass was evaluated across 24 sites (both river and tributary sites) in the Susquehanna River Basin and compared to one out of basin site. Using twelve polymorphic microsatellite loci, limited genetic differentiation (overall mean pairwise FST = 0.012) and lack of population structure (k = 3 admixed population clusters) was found across the Susquehanna River Basin. Chapter 2 provided important information for the future conservation of smallmouth bass including the fact that there was little genetic support for the use of separate management units within this system. Chapter 2 quantified gene flow across a large spatial extent; however, there was still uncertainty about finer-scale seasonal movement dynamics of smallmouth bass. Understanding how individual smallmouth bass were using rivers and tributaries could help inform the mechanisms contributing to genetic similarity across the basin. Knowledge of the movement of smallmouth bass in the system could also be important for understanding the range of stressors fish could be exposed to, especially if river and tributary habitats are both utilized. To evaluate the spatial movement ecology of smallmouth bass in the Susquehanna River Basin, I completed a radio-telemetry study in connected river-tributary habitat in the Susquehanna River Basin. I evaluated movement dynamics and river-tributary usage as well as investigated environmental drivers (i.e., temperature and flow) of fish movement. In general, fish movement was variable with some fish moving large distances (n = 76 fish, average = 27.2 ± 25.9 km, range = 0.2 to 118 km). Movement between rivers and tributaries was common. Using a generalized additive mixed model, I found varied seasonal effects of temperature and flow on fish movement. These findings indicate the importance for maintaining fish movement corridors for smallmouth bass, including connectivity between river and tributaries. Additionally, long distance movement supports the possibility for gene flow and lack of genetic differentiation demonstrated in Chapter 2. Chapter 4 focuses on the prevalence of a myxozoan parasite infecting juvenile smallmouth bass. This research investigated temporal and spatial factors, including land use, that were hypothesized to influence parasite prevalence. For this chapter, I used a hierarchical logistic regression model to evaluate spatial and temporal variability in myxozoan parasite prevalence across four years and at 31 sites. There was little temporal variability in myxozoan prevalence (posterior mean annual prevalence ranged from 0.42-0.55); however, prevalence varied substantially among sites (posterior mean site-level prevalence ranged from 0.08-0.82). Myxozoan prevalence had a negative relationship with urban land use and a positive relationship with agricultural land use. The effects of both urban and agricultural land use were stronger at the local scale when compared to the accumulated catchment scale. The relationship between prevalence and land use indicates that land use practices could be mediating parasite-host relationship, including invertebrate host density and parasite abundance, and contributing to infection rates in smallmouth bass. This chapter provided insight into land use relationships that may be important for myxozoan parasites in the Susquehanna River Basin. My dissertation research crosses multiple sub-disciplines of ecology to provide valuable information that can be used to help understand smallmouth bass ecology, health, and can inform management of this important fishery. For example, fishing regulations, which extend a half mile into tributaries in sections of the Susquehanna River Basin, may need to consider larger distances of protection into tributaries given smallmouth bass movement in the system. Additionally, smallmouth bass movement across a large area could indicate exposure to stressors across a larger gradient. Land use impacts may differ at local and watershed scales as demonstrated for the myxozoan parasite investigated. Thus, in the context of fish health, future research efforts should integrate finer-scale information with large-scale habitat conditions and processes and consider the potential for cross-scale interactions. In a larger context, it is important to consider why we as humans care about smallmouth bass health. The fish health issues observed in the Susquehanna River is more than a smallmouth bass problem. Smallmouth bass may be more sensitive to specific stressors than other organisms, but there could also be larger human health and ecosystem implications. The Susquehanna River provides drinking water to millions of inhabitants located along the river and also major cities outside of the basin. The Susquehanna River Basin is also part of the larger Chesapeake Bay Watershed and provides around half of the freshwater to the bay. The Susquehanna River also provides an important role in ecosystem functioning and a source for recreating across the region. Recent impairment of portions of the river for coliform bacteria and fish consumption advisories by Pennsylvania Department of Environmental Protection have led to concerns not only for the health of smallmouth bass, but also for the overall impact the health of the river could have on humans. Understanding smallmouth bass ecology, an important indicator of ecosystem health, could provide a better understanding of the health of the river and therefore the health of the many people and economies that depend on this valuable resource.