An Assessment of the Relationship Between Electrofishing Catch-Per-Unit-Effort Data and Fish Abundance

We evaluated the catchability relationship for multiple fish
species sampled with boat electrofishing as conducted by the Long Term
Resource Monitoring Program (Upper Mississippi River Restoration -
Environmental Management Program) in backwater lakes of the Mississippi and
Illinois rivers by testing for a relationship between CPUE and abundance
estimates derived from block net samples.  Of the seventeen species examined,
nine had significant catchability relationships for density and eight species
had significant catchability relationships for wet mass.  Smallmouth buffalo,
common carp, bullhead minnow, freshwater drum, silver carp, channel catfish,
and western mosquitofish all had significant catchability relationships for
both density and wet mass.  Gizzard shad and bluegill had significant
catchability relationships for density but not for wet mass, and
orangespotted sunfish had a significant catchability relationship for wet
mass but not for density.  Electrofishing proved to be consistently biased
toward larger individuals within a species, with mean length for 13 of the 17
species examined greater in electrofishing samples relative to block net
samples.  We also found significant catchability relationships for species
richness and for two groupings of fish species:  forage fishes, and
commercially harvested fishes.  The catchability relationship for
recreationally harvested fishes was not significant, likely because several
of the species within this grouping had non-significant catchability
relationships.  Fish community structure was not significantly different
between block nets and electrofishing for density, but community structure
based on wet mass differed significantly between block nets and
electrofishing, likely reflecting a compounding of errors from the size bias
of electrofishing.  Our study suggests that boat electrofishing as conducted
by the Long Term Resource Monitoring Program (LTRMP) is effective for several
species and species groupings of fish in backwater lakes of the Mississippi
and Illinois rivers.  Nevertheless, limitations do exist for this method.
Trend analyses using multiple gears may be an effective way to strengthen the
inferences that can be drawn from LTRMP data given the uncertainty regarding
how catchability varies among gears, through time, and through space.