Determination of a Representative Reach to Estimate Brook Trout Biomass in Pennsylvania Streams

The Pennsylvania Fish and Boat Commission (PFBC) classifies trout streams for management purposes according to estimates of biomass (kg/ha) of wild trout.  Biomass is estimated by multiplying abundance estimates by fish weights.  One of the key questions when sampling trout populations is what is an appropriate representative stream reach length to sample when estimating trout biomass?  In this study, we empirically determined the required length of stream that needs to be sampled in order to estimate brook trout biomass in Pennsylvania streams within 25% of the true biomass.  Long stream reaches (600 m) were chosen separate 1^st and 2^nd order streams and these reaches were broken into twelve 50-m sub-reaches.  Each sub-reach was sampled by typical removal electrofishing techniques until either five electrofishing passes were completed or no brook trout were captured.  The total biomass of all brook trout over all 50-m sub-reaches was considered the “true” biomass.  We then performed computer simulations in which various numbers of 50m sub-reaches were randomly selected and catches from each sub-section were summed within the first three electrofishing passes to simulate removal sampling of reach lengths ranging from 50 to 550 m.  Population estimates were made using a removal estimator and biomass was calculated using various stratification schemes based on fish size.  Estimates of biomass were then compared to the “true” biomass to assess the possible range in bias of biomass estimates for a given reach length.  Results from our simulations suggest a 350-m stream reach is adequate to estimate brook trout biomass within 25%.  This information will be valuable to PFBC in developing standardized protocols for assessing trout biomass in small streams and the methodology we used to determine an appropriate reach length may be used by other agencies when developing sampling protocols.