Response of walleye to the addition of spawning substrate in a Nebraska irrigation reservoir

Walleye are a highly sought sportfish in Sherman Reservoir, Nebraska. Natural recruitment of walleye is highly variable which could be due to limited spawning habitat. The Nebraska Game and Parks Commission (NGPC) added 0.17-ha of cobble substrate for walleye spawning habitat during January 2008. This study evaluated a walleye egg sampling gear, the changes in adult abundances and egg density in response to the addition of the cobble substrate along with evaluating existing walleye spawning habitat in the reservoir and estimated walleye egg hatchability from cobble and mud substrates. The walleye egg sampling gear, called egg sampling disks, allowed the density of spawned walleye eggs to be calculated and egg densities up to 390+/-225 walleye eggs per m2 per spawn night were found. The egg recovery efficiency from the egg sampling disks was 58+/-3% using the established field protocol. The abundance of adult walleye, as estimated by electrofishing catch-per-unit-effort (CPUE), increased 15% the first year the cobble was available (2008) but decreased 5% the following year (2009) when compared with adult abundances at the same location before the cobble was added (2007). Gill net CPUE ranged from 0.6+/-0.2 to 1.4+/-0.5 walleye per hour on the cobble. The walleye egg density increased 215% in 2008 when compared with the egg density in 2007 but no increase was observed in 2009 when compared to 2007. The reservoir substrate was found to consist mostly of silt with varying degrees of firmness. Large rip-rap boulders and cobble was only found to exist at areas modified by the NGPC. The percent hatch rate of walleye eggs ranged from 0.7% to 24.9% on the cobble substrate and from 0.1% to 1.2% on the mud substrate. The abundance of adult walleye and egg density only increased during the first year the cobble was available, but it seems the water depth on the cobble may have influenced the results in the second year of availability. Although egg density on the cobble may not have increased both years, the percent hatch rate of the eggs may have increased on the cobble. Based on these results, future additions of cobble substrate in other reservoirs may be successful at increasing walleye adult relative abundance and egg density on the added substrate if the appropriate water depth is present.