Top Down Influences of Larval Shad on Zooplankton Abundance at Hugo Reservoir, Oklahoma

Jeff Boxrucker
Oklahoma Fishery Research Laboratory, Oklahoma Dept of Wildlife Conservation, 500 E. Constellation, Norman OK 73072 (405) 325-7288 jboxrucker@odwc.state.ok.us

The data presented are part of an evaluation of an experimental water level manipulation plan intended to increase recruitment of largemouth bass Micropterus salmoides , white crappie, Pomoxis annularis, and shad Dorosoma spp. The objective of this study was to determine if increased shad abundance had a negative effect on zooplankton abundance which, in turn, may influence survival of age-0 sport fish. Weekly trawl and zooplankton samples were collected from April through September, 1995-98. Crustacean zooplankton and copepod abundance was negatively correlated with shad abundance in 1995, 1997, and 1998. Copepod nauplii and cladocerans were negatively correlated with shad abundance in 1995 and 1997. Cyclopoid copepod, Daphnia, and rotifer abundances were not related to shad densities. A winter-kill of threadfin shad D. Petenense in 1995-96 decreased shad catches in the 1996 trawl samples by >10 fold. Zooplankton abundance in 1996 was double the levels of the other years studied, exceeding 100/L throughout the sampling period. However, increased zooplankton abundance in 1996 did not improve survival of age-0 crappie and largemouth bass as measured in fall trap-net and electrofishing samples.