Asian Carp reproductive ecology along the Upper Mississippi River invasion front
Clay L. Pierce
Introduced in the 1960s and 1970s, Grass Carp (Ctenopharyngodon idella), Silver Carp (Hypophthalmichthys molitrix), and Bighead Carp (H. nobilis), collectively called Asian Carp in the US, have invaded the Mississippi River Basin and are expanding their range, threatening ecosystem integrity. Impounded sections on the Upper Mississippi River associated with lock and dams may be poor habitats for reproduction and recruitment due to their lentic flow characteristics and perceived lack of adequate spawning habitat. However, tributaries connected to the impounded sections possess several requirements needed for successful spawning and observations of adults are becoming more prevalent. Unfortunately, little is known regarding the reproductive status of Asian Carp populations in these tributary systems. Models have been created to determine the thermal and hydrologic suitability of rivers for reproduction. However, these models focus on the general feasibility of reproduction in a river and do not attempt to pinpoint river locations best suited for adult spawning. Furthermore, efforts to confirm areas of current establishment have largely relied upon the detection of early life stages. However, discrepancies and lack of information on morphological egg characteristics of Asian Carp and native species has made traditional visual identification of fish eggs difficult and unreliable. Therefore, the overarching goal of this thesis was to provide a set of tools to predict, identify, and analyze Asian Carp reproduction. The three main objectives to achieve this goal were to develop an egg identification tool that could identify Asian Carp eggs preserved in ethanol, to determine reproductive dynamics in three tributary rivers connected to the mainstem Upper Mississippi River, and to develop a GIS model to predict the most likely locations for Asian Carp reproduction in the Des Moines River. Results demonstrate that a combination of morphometric measurements from genetically identified ethanol preserved eggs in a random forests classification model was a good alternative for Asian Carp (Bighead, Grass and Silver carps combined) egg identification. Back calculated spawn dates of Asian Carp eggs and larvae collected in the Des Moines, Iowa and Skunk rivers suggest that peak spawning occurs during late May and June, coinciding with a substantial decrease in adult GSI and increase in post spawn females; however, spawning occurred as late as August. Egg and larval spawn dates were within the spawning optimum when water temperatures were 18 to 30 Ã Â°C and channel velocities were 0.7 m/s or higher. Spawning occurred on rising and falling limbs of the hydrograph at 24 and 48 h intervals, indicating a rising hydrograph was not necessary for spawning. Densities of eggs and larvae were higher in the downriver section compared to the upriver section within each tributary during both years. Densities among tributaries were similar in 2014 and but were significantly higher in the Des Moines River than the Iowa and Skunk rivers during 2015. Densities among the three confluence sites were similar for the Iowa and Skunk during each year. However, densities among sites associated with the Des Moines River confluence were higher in the mainstem Upper Mississippi River in 2014 and higher in the tributary site near the mouth in 2015. Back-calculated drift distances of eggs indicated reproductive GIS models were able to predict reproduction areas in the upper portions of the lower Des Moines River but were less successful at identifying areas where reproduction was documented in the lower portions. Collectively, this thesis documents the first observations of Asian Carp spawning in Upper Mississippi River tributaries in Iowa. Back calculated drift distances of eggs, spatiotemporal variation in egg and larval densities, and adult reproductive assessments suggest that Asian Carp reproduction may be ubiquitous throughout some tributaries, such as the Des Moines River; however, reproductive concentration varies annually. Asian Carp reproduction and subsequent establishment in the Upper Mississippi River tributaries could provide sources of recruitment for the impounded sections of the Upper Mississippi River and other areas of poor reproduction, further expanding Asian Carp distribution in the US.