Differential growth rates and mortality rates between gizzard shad and threadfin shad at Hugo Reservoir Oklahoma, 1997-1998

Barbara J. Adams*
500 E. Constellation Norman, OK 73072, University of Oklahoma, (405) 325-7288, fax (405) 325-7631, bjadams@ou.edu

Jeff Boxrucker
500 E. Constellation, Norman, OK 73072, Oklahoma Fisheries Research Laboratory (405) 325-7288, fax (405) 325-7631, jboxrucker@odwc.state.ok.us

Young -of-year shad (Dorosoma spp.) collections for Hugo Reservoir, Oklahoma in 1995 and 1996 suggested differential morality rates between gizzard shad (D. cepedianum) and threadfin shad (D. petenense) larvae and juveniles. The objectives of this study were to determine the instantaneous mortality rates and growth rates between weekly cohorts of gizzard shad and threadfin shad and to investigate relationships between mortality and growth to zooplankton abundance. Trawl samples from 1997 and 1998 were collected from Hugo Reservoir with paired 1-m² -towed nets starting in April and ending in September. Otoliths were removed from 15 shad of each species per 5-mm length group per sample. Daily growth rings were counted, hatch dates calculated and fish assigned to respective weekly cohorts. Catch rates (no./m³) for each cohort were calculated from each sample. Twenty consecutive weekly cohorts of shad spp. were identified for 1997. The aging process of the 1998 data is ongoing and will be reported at the meeting. For both years, two separate spawns of threadfin shad were found; a spring spawn (April through June) and a late July through early August spawn. Mortality rate comparisons of the two threadfin shad spawns will be made. In 1997, age-0 gizzard shad dominated the samples through May and threadfin shad dominated the samples from June through September. Mortality and growth rates will be reported at the meeting.