JEFF BOXRUCKER, Oklahoma Fishery Research Laboratory, 500 E. Constellation, Norman, Oklahoma 73072, USA

The severity of the winter of 1995-96 resulted in winterkills of threadfin shad Dorosoma petenense in most Oklahoma reservoirs. Mid water trawls were used to monitor juvenile shad abundance in Hugo Reservoir April through September, 1995 and 1996. Weekly samples were collected with a 500-micron net April through June. Bi-weekly samples were collected in July and August with a 3-mm net. Bi-weekly samples were collected in September using a 6-mm net. Juvenile shad abundance was dramatically reduced in 1996. Peak abundance of shad was 1200/1000 m³ and 300/1000 m³ in 1995 and 1996, respectively. The abundance of gizzard shad D. cepedianum remained unchanged following the winterkill with peak densities reaching 300/1000 m³ both years. Threadfin shad abundance declined precipitously following the winterkill; peak abundance was 800/1000 m³ and 75/1000 m³ in 1995 and 1996, respectively. Threadfin shad abundance peaked in late September, 1996 compared to mid-July in 1995. Mean lengths of shad in the trawl samples was similar through early July both years. However, the size of shad in the 1996 samples exceeded those in the 1995 samples beginning in mid-July and by September shad averaged 10 mm longer in 1996 than in 1995. Historical rotenone data from Lake Texoma indicated that gizzard shad recruitment increased following threadfin shad winterkills. Trawl data from Hugo indicated that this was not the case in 1996.