Applying edge-core theory of intraspecific life-history variation to the control of invasive silver carp (hypophthalmichthys molitrix)

As a species expands its range, individuals along the leading edge of the expansion will face different selective pressures than individuals at the core of the population. The result is that ‘edge’ individuals will display different phenotypes than ‘core’ individuals. Specifically, animals on the edge of a range will generally have larger body sizes, grow faster, have higher reproductive investment, and shorter longevity. This phenomenon, called the “edge-core effect,” is commonly studied in invasive species, as range expansion is one of the phases of a biological invasion. Phenotypic plasticity that allows individuals within a species to vary in life-history traits such as growth, age-at-maturity, fecundity, and longevity will aid an invasive species in adapting to a novel range. Silver carp (Hypophthalmichthys molitrix) are an invasive cyprinid fish that were introduced to North America in the 1970s and have since spread throughout the Mississippi River Basin. During the 2000s, silver carp invaded the Illinois River and began advancing towards the Laurentian Great Lakes, making them a species of high concern. While there are several reports that silver carp in North America differ in growth rate and reproductive traits when compared to silver carp in their native range in Asia, little is known about how silver carp vary within a range. Specifically, it is unknown if an edge-core effect is causing differences in life-history traits in silver carp within the same river system. In this dissertation, I examine silver carp from ‘edge’ and ‘core’ populations in the Illinois River to assess if an edge-core effect exists for reproduction, growth, and health. Gonadosomatic index, reproductive hormones, and gametogenesis were analyzed in male and female silver carp collected from the La Grange and Starved Rock reaches between April-October 2018, and gametogenesis was analyzed in silver carp from the La Grange and Marseilles reaches between August-October 2017. To determine if growth rates differed between ‘edge’ and ‘core’ populations, I used scales to back-calculate length-at-age and determine von Bertalanffy growth curves for silver carp from the Marseilles, Starved Rock, and La Grange Reaches in 2017 and 2018. Plasma proteins, blood smears, hematocrit, hepatosomatic index, head kidney melanomacrophages, and health assessment index scores were compared between the Starved Rock and La Grange Reaches in 2018. Results from this study show that silver carp from ‘edge’ and ‘core’ reaches do not differ significantly in spawning activity, but large-scale environmental conditions do play a role. Additionally, I show that female silver carp are batch spawners with indeterminate, asynchronous oocyte recruitment while male silver carp produce a determinate number of spermatozoa at the beginning of the spawning season. Comparison of growth rates revealed that silver carp from the Marseilles Reach grow significantly faster than silver carp from the La Grange Reach, though no difference was detected between the Starved Rock and La Grange Reaches. Finally, while silver carp from ‘edge’ and ‘core’ reaches differed in body condition, lymphocyte proportions, and hematocrit values, no other health parameters differed between sampling sites. Livers and kidneys frequently received high scores on the health assessment index, and large leukocytes were predominant, which could indicate infection. Overall, results indicate that silver carp differ between ‘edge’ and ‘core’ populations in growth rates, but not in reproduction or health. This project also included a study to examine the effects of sociability on avoidance behavior, specifically in relation to carbon dioxide and temperature using bluegill sunfish (Lepomis macrochirus) as a model. Bluegill were exposed to either increasing carbon dioxide or increasing temperature first in isolation, then as part of a group. Each bluegill also had their social personality quantified using a social network score. While social personality did not affect avoidance behavior, bluegill exhibited significantly higher avoidance of both carbon dioxide and temperature when they were in a group compared to when they were in isolation. Results from this study have implications for the use of carbon dioxide as a non-deterrent barrier system to prevent the movement of silver carp and other invasive fishes, as shoaling fish might react to the barrier differently than fish in isolation. The results presented in this dissertation can be used to inform the management and control of silver carp and other species that exhibit edge-core effects on life-history traits.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste