Quantitative methods used for monitoring and projecting population trends of species require the estimation of population size. Mark-recapture methods can be used to estimate parameters necessary for determining population size. As part of the Murray River resnagging monitoring program, which is examining the change in population size in response to a resnagging intervention, we undertook an electrofishing detection experiment. In this experiment, we tagged approximately 100 fish per annum with radio-transmitters in a two kilometre study site on the Murray River at Yarrawonga, over a period of seven years. This site was then surveyed non-invasively using radio-telemetry to determine which of these fish were within the site, and then electrofished to determine the proportion of available fish that were captured. The electrofishing surveys were replicated up to three times over a ten day period. Electrofishing detection varied between 8.6 % and 45.7 % from the twelve detection events undertaken thus far. The detection average in 2007 was 14 %; 19 % in 2008; 21 % in 2009 16 % in 2010, 7 % in 2011, 16 % in 2012 and 12 % in 2013. Our results show that variation in detection rates will likely have a significant impact on estimates of population size and vital rates. Furthermore, our approach may offer a cost-effective means for reducing the error in parameter estimates derived from standard mark-recapture experiments.