Following the work of Dr. Joe Schmitt (click here for details), we are currently exploring the population dynamics of blue catfish in the James, Rappahannock and York river systems. Population dynamics is the study of how population sizes and structure change over time and why those changes occur. As such, we are interested in how the number of blue catfish in Virginia tidal rivers is changing through time and how the age and size structure is shifting, as well. This work will bring together multiple sources of information, such as growth, commercial catch and three long-term fish surveys, to give us the best picture of long-term population trends.
The goal of this study is to understand blue catfish population dynamics from the early 1990s to present and use that information to simulate populations into the future with various management strategies. The results will help management agencies balance tradeoffs and generate informed management plans.
We began by examining how blue catfish growth rates changed from 2002-2016. Based on theory from other studies with non-native species, we anticipated growth would decline over time in river systems with differences among rivers due to differences in time since introduction. Preliminary results from this work show that blue catfish growth has declined in the James, Mattaponi, Pamunkey and Rappahannock rivers. Further, trends in average length-at-age were correlated with blue catfish density. We found large variability in length-at-age with 12-year old James River fish from the same cohort differing by as much 20" in length.
To understand how populations are changing over time, we are developing a spatially-explicit population dynamics model. The model will be spatially-explicit to examine each river system (James, Rappahannock, York) individually, as the systems appear to differ with respect to population characteristics (growth, size structure, maximum age). The model will estimate the population sizes in the three rivers of interest, as well as size structure, and fishing and natural mortality rates. The benefit of this modeling approach is that it uses multiple data sources, including commercial harvest, catch from three different long-term fish surveys, length, weight, age and survey species composition data, to generate estimates of population sizes with uncertainty.
Based on the results of the population dynamics model, we will run simulations to examine future populations under various fishing regulations and harvest rates. We will also estimate consumption of native species based on population estimates and Dr. Joe Schmitt's diet and consumption rate data. We will develop management objectives from discussions with Virginia Department of Game and Inland Fisheries and Virginia Marine Resource Commission. Management objectives will be specific targets for management and will have an associated performance measure. We will summarize performance measures for all management objectives for each prospective suite of regulations.
Finally, I will conduct an economic impact analysis to provide helpful information for policy makers. While blue catfish are a non-native species, they provide some economic benefits to the region. Policy decisions should consider both ecological and economic aspects of blue catfish management. I plan to explore the direct economic contributions of blue catfish fisheries both currently and project economic outcomes of various management strategies examined during simulations. The ultimate results of this study will provide policy makers and regulatory agencies with estimates of population sizes, population structure, native species consumption and economic outcomes under various regulation schemes.