ACKNOWLEDGEMENTS
Partial funding for this project was provided by the Iowa Department of
Natural Resources and US Fish and Wildlife Service. We thank numerous
graduate students (C. Camacho, C. Sullivan, A. Matthews, and N.
Tillotson) and technicians that spent countless hours assisting with
larval fish sample collection and processing.
Abstract: The Upper Mississippi River (UMR) represents one
invasion front to bigheaded carps Hypopthalmichthys spp. in North
America. Agencies often sample larvae to determine the conditions that
bigheaded carp reproduction occurs, but the ability for conventional
ichthyoplankton sampling to detect bigheaded carp reproduction compared
to native fish is unknown. We used occupancy models to estimate larval
bigheaded carp and native fish larvae detection probabilities and
assessed how habitat type, Julian date, river discharge, and water
temperature affect occupancy. We sampled larvae in pools 18-20 of the
UMR every two weeks at the Iowa, Skunk, and Des Moines River confluences
in backwater, side channel, and thalweg habitats. Detection
probabilities increased with water volume filtered and was lower for
larval bigheaded carp than freshwater drum, gizzard shad, and percids.
Freshwater drum and bigheaded carp larvae had higher detection in
thalweg and channel border habitats compared to backwaters. Occupancy of
bigheaded carp peaked on June 19th at 20°C, increased
with discharge, and declined with coefficient of variation (CV) of water
temperature and discharge. Gizzard shad and percids occupancy peaked on
May 24th and increased with CV of water temperature
while occupancy of freshwater drum peaked on July 3rd,
decreased with CV of water temperature, and increased with water
temperature. Our results indicate bigheaded carp are more difficult to
detect than native larvae and identified conditions associated with
larval occupancy that can be used to maximize detection and better
understand when, where, and under what conditions larvae are present
while accounting for imperfect detection.
Key Words: ichthyoplankton, Silver Carp, Bighead Carp, Freshwater Drum,
Gizzard Shad, percids, distribution models, sampling efficiency