Shocking results: assessing the injury rates of fishes from pulsed-DC electrofishing

Electrofishing is one of the most widely used methods to sample fishes in freshwater ecosystems. There was an early consensus that AC electrofishing was more injurious to fishes than DC electrofishing, but this was based on a relatively small number of studies. Since the late 1980’s, many studies have documented high injury rates of fishes from DC electrofishing. However, studies of injury from electrofishing are disproportionately skewed towards laboratory work on cool- and coldwater species and further investigation is needed on injury rates to fishes from DC electrofishing conducted in-situ with warm water species. My study assessed injury rates for seven species of fish in the Upper Mississippi River System (UMRS) from PDC boat electrofishing. I examined bluegill Lepomis macrochirus, channel catfish Ictalurus punctatus, freshwater drum Aplodinotus grunniens, largemouth bass Micropterus salmoides, gizzard shad Dorosoma cepedianum, common carp Cyprinus carpio and silver carp Hypophthalmichthys molitrix captured in the Mississippi and Illinois rivers from June 15th to October 31st, 2013 for evidence of spinal injuries from PDC boat electrofishing. Fishes were collected by electrofishing crews conducting routine sampling for two long-term monitoring programs. Of the seven species examined, silver carp and channel catfish were the only fishes that had injuries. Channel catfish had a 26% rate of spinal injury, and silver carp had an injury rate of over 62%. Injury rate for silver carp was significantly greater in the Illinois River (71%) compared with the Mississippi River (32%). Whereas Secchi depth, dissolved oxygen, and temperature were similar between the two rivers, conductivity was significantly higher on the Illinois River (724.82 ±21.14 µS/cm) than on the Mississippi River (422.19 ±30.37 µS/cm). Silver carp injury rate varied significantly with power output and conductivity, whereas channel catfish injury rate did not vary with any of the factors tested. Silver carp between 500 and 549 mm had a greater injury rate than other length groups, whereas channel catfish injury rate did not differ among length groups. Injury rate did not vary with condition factor for either channel catfish or silver carp. I found no significant relationship between any reflex action mortality predictor (RAMP) score and internal injuries of channel catfish caused by electrofishing. To test whether pulse frequency affects injury rates of silver carp, I conducted PDC electrofishing in a side channel of the Illinois River at two pulse frequency setting. Pulse frequency appears to have a substantial effect on injury rate, as reducing pulse frequency to 30Hz reduced injury rate of silver carp to 33.3% and increasing pulse frequency to 120Hz increased injury rate of silver carp to 70%. My research has shown that silver carp and channel catfish in the UMRS are being injured from PDC electrofishing. Injury rate for silver carp varied with size class, conductivity, and power output, but further research needs to be done to establish cause and effect among these highly correlated variables. My results suggest it may be possible to adjust pulse frequency and other electrofishing settings to increase rates of injury and mortality for Asian carp. Managers may want to further explore the potential of using electric fields as a management tool both for impeding the movements of Asian carp and potentially reducing their populations