Dr. Schmitt's Research
Dr. Joe Schmitt (Ph.D, Virginia Tech, 2018) looked at the diet of blue catfish in the James, Pamunkey, Mattaponi, and Rappahannock Rivers. His crew collected stomach contents from 17,762 catfish (16,110 blue catfish), making this one of the largest diet studies ever conducted, and the largest diet study for any catfish species. Additionally, they opportunistically sampled diet contents from flathead catfish on the James and Pamunkey Rivers. Dr. Schmitt also explored diet overlap between blue catfish and white catfish in tidal creeks.
The goal was to determine how diet changed with season and location on each river, and to better understand how non-native catfish may be affecting important native species such as American shad, alewife, blueback herring, blue crab, American eel, and menhaden.
Catfish were gathered using low-frequency electrofishing. This temporarily stuns fish and allowed us to scoop them up and hold them in a livewell. Fish quickly recovered and stomach contents were removed using a couple different methods. Diet contents are extracted from large fish using "pulsed gastric lavage", which is basically pumping their stomachs with water forcing food to travel back up the esophagus and out the mouth (Click here for video). Small fish were sacrificed to remove stomach contents to reduce the amount of time needed to collect samples from large numbers of fish. The picture below depicts the average size of catfish we shock: there is no shortage of small (10-12") blue catfish in our tidal rivers.
Catfish prey items were placed in zipper-seal plastic bags, placed on ice, and transported to the lab for analysis. Stomach contents were identified to species, weighed, counted, and entered into our database. Fish prey were often highly digested, so we utilized advanced molecular techniques to identify these prey items (DNA barcoding, click here for video).
Dr. Schmitt also estimated how much a blue catfish can eat. He used a combination of lab and field studies to estimate "maximum daily consumption", or the maximum amount of food a blue catfish can consume in a 24 hour period. Dr. Schmitt calculated consumption rates using three methods: 1) field methods, 2) lab methods, 3) empirical model based on water temperature, growth, and activity level of fish.
The field methods included sampling fish every three hours over a 24 hour period. Dr. Schmitt and his crew looked at how stomach fullness changed between intervals (for more detail read Heroux and Magnan 1996).
The team estimated blue catfish maximum consumption in tanks, as it varies with water temperature, prey type, and predator size. We calculated consumption rates at three temperatures (approximately 40, 60, 80 degrees F) using two prey types (gizzard shad and blue crab) and blue catfish ranging from 12" to 36" long. We will use these studies to identify how consumption changes seasonally, with prey type, and with catfish size. This information can be then be linked with population size estimates and diet composition information to estimate population level consumption (kg/year) of species such as American eel, blue crab, or American shad (we are using lab experiments similar to Bourret et al. 2008).
Finally, we used an equation based on growth rates, mortality rates , average annual water temperature, and activity level (presumed from tail shape) to calculate annual consumption of the population per pound of blue catfish (see Palomares and Pauly 1998).