A Modified Mark-Recapture Study to Determine Electrofishing Gear Bias

The Maryland DNR Alosine Restoration Project uses electrofishing techniques to develop indices of relative abundance.  There is now interest in developing measures of absolute abundance.  To that end, a modified mark-recapture study was conducted to develop a measure of gear bias - since a recapture probability of 10% is the probability that a fish will be observed and caught, fish caught in an electrofishing “run” under the same sampling conditions represent 10% of fish in that section of the river.  Four electrofishing “runs” were conducted during the 2013 spawning run on the Choptank River.  All hickory shad netted received a fin clip unique to that “run” and were released from the back of the boat.  The boat then returned to the starting point.  For each “run”, the number of fish unmarked or with a previous mark were noted.  Program MARK was used for the analysis.  The Cormick-Jolly-Seber Live Recaptures model produced a capture probability of 0.1034 at AIC of 100.46.  The Huggin’s Closed Captures model yielded p = 0.3897 at AIC of 290.09.  Because this capture probability reflects water clarity and other sampling conditions, the MDNR will repeat the study to determine the stability of the estimate.