Blue Catfish were introduced to tributaries of the Chesapeake Bay in the 1970s and 1980s to establish new fisheries, but their presence has generated concern for native species. Much of that concern stems from incredible abundances reported by scientists and fishers, contributing to the belief that a large proportion of the biomass in these tidal rivers is now Blue Catfish. Thus, scientists wanted to know how many Blue Catfish are out there and how can we control their population sizes?
Scientists from Virginia Tech try to net the large numbers of James River Blue Catfish surfacing during an electrofishing survey in 2015. Photo Jason Emmel.
Among the tidal river systems in Virginia, the James River appears to support the “healthiest” population of Blue Catfish. The species is quite abundant in the James River and fish can exceed 100 lbs. Consequently, managing the species in the James River is possibly more complicated than in other systems, due to the popularity for recreational fisheries (see blog post) and sizes conducive to predation on at-risk native fishes (see blog post). Thus, understanding the population size in the James River became a priority for scientists and managers. In 2018, two separate studies were published aiming to determine how many Blue Catfish exist in the tidal portions of the James River. One study was conducted by Virginia Department of Game and Inland Fisheries scientists (hereafter VDGIF study) and another was conducted largely by scientists from Virginia Institute of Marine Science (hereafter VIMS study). Both studies used mark-recapture techniques to estimate abundances, but the study designs, study areas, and analyses differed. Those interested in the details of these studies should see the original works (listed below in references).
A small Blue Catfish from the James River collected near Henrico, VA using boat electrofishing.
Mark-recapture studies are based on marking animals (e.g. using a fin clip or a tag), releasing them, and later resampling to examine the proportion of tagged animals in the population (click here for description). We can estimate population sizes based on these proportions using mathematical and statistical models that vary in complexity due to characteristics of the sampling scheme, objectives of the study, and biology of the system. Ideally, we could collect and count every animal in a population, but this would be extremely time consuming and certainly impossible in large river systems. There are a number of assumptions associated with estimating abundance using mark-recapture techniques that I won't discuss. Those interested in learning more should consult an ecology textbook where mark-recapture methods are presented. The characteristics of a population of animals and the sampling scheme may require complicated analyses to generate a reliable estimate of abundance.
The VDGIF study estimated abundance in the years 2007 and 2014 in Powell Creek, a tidal creek flowing to the James River. This study examined 2.98 miles of Powell Creek (104 acres) via electrofishing surveys over a three day period in July of 2007 and 2014. VDGIF staff collected a total of 16,494 Blue Catfish over the two years and estimated the Blue Catfish abundance in Powell Creek, as well as density (fish per acre) and biomass (weight of Blue Catfish per acre). The VDGIF study estimated there were 29,745 Blue Catfish in Powell Creek in 2007, but estimated abundance declined to 10,041 by 2014. In terms of density this equates to 286 Blue Catfish per acre in 2007 and 97 Blue Catfish per acre in 2014. When considered as Blue Catfish biomass this equates to 237 lbs per acre in 2007 and 165 lbs per acre in 2014.
Study areas (red) for two studies estimating the population size for James River Blue Catfish in relation to Charles City, Virginia (black star). Solid red indicates Virginia Department of Game and Inland Fisheries study area (Powell Creek), whereas hatched pattern indicates Virginia Institute of Marine Science study area (James River between Upper Chippokes Creek and Chickahominy River).
The VIMS study obtained fish from a commercial fisher fishing with baited hoop nets. The VIMS study used a longer time frame for recaptures for their study, as tagging took place in July-August of 2012 and 2013 and fish were scanned for tags until December 2014 . A total of 35,252 fish were tagged with a total of 1,456 recoveries. Authors estimated the abundance of Blue Catfish in 2013 as 1.6 Million fish from the 7.46 mile river segment examined. The authors reported their findings were applicable for fish 9.4-18.1 inches fork length, as this size range described fish sizes vulnerable to hoop nets. This equates to 220 Blue Catfish per acre within the size range and spatial extent mentioned above.
These studies provide the first estimates of Blue Catfish density within the Chesapeake Bay watershed and support that Blue Catfish populations are extremely large in the James River. VDGIF staff found a decline in abundance among years examined, which is consistent with trends in catch rates from two separate electrofishing surveys. Although these studies provide important information on the abundance of this invasive species, further research is needed to understand how abundance differs from the fall line to the most downstream extent of their range in Chesapeake Bay tributaries. Authors from the VDGIF study suspected main channel habitats examined in the VIMS study may support larger numbers of Blue Catfish than smaller tributaries. A better understanding abundance over time and space would help managers decide how to tackle this complicated fisheries management problem.
Bunch, A. J., Greenlee, R. S. & Brittle, E. M. (2018) Blue catfish density and biomass in a tidal tributary in coastal Virginia. Northeastern Naturalist 25, 333–340. (Request full-text here)
Fabrizio, M. C., Tuckey, T. D., Latour, R. J., White, G. C. & Norris, A. J. (2018) Tidal habitats support large numbers of invasive blue catfish in a Chesapeake Bay subestuary. Estuaries and Coasts 41, 827–840. (Obtain full-text here)