First-Year Growth, Condition, and Size-Selective Winter Mortality of Freshwater Drum in the Lower Missouri River

Abstract

We compared first-year growth and relative condition (K_n ) of the 1997 and 1998 year-classes of freshwater drum Aplodinotus grunniens among three sites in a 235-km reach of the channelized Missouri River and tested for the occurrence of size-selective overwinter mortality during the first winter. Prewinter mean length was 15 mm greater, mean weight was 8 g greater, and mean K_n was 5% greater at the upstream site than at the downstream site. The prewinter mean length of age-0 freshwater drum was significantly greater in 1997 (115 mm) than in 1998 (109 mm), but K_n was significantly greater in 1998 (107) than in 1997 (102). There was no evidence that density-dependent interactions influenced prewinter growth and K_n. Size-selective overwinter mortality of the smallest size-classes of freshwater drum occurred at two of three sites during the 1997–1998 winter, and K_n decreased 9–15%. Size-selective overwinter mortality of the 1998 cohort of freshwater drum did not occur during the 1998–1999 winter, and K_n declined 0–10%. A prolonged growing season (through early December 1998), in conjunction with less severe winter water temperature conditions, apparently minimized the incidence of size-selective overwinter mortality for the 1998 cohort of freshwater drum. We conclude that size-selective overwinter mortality of age-0 freshwater drum occurs in the lower channelized Missouri River but depends on the length of the prewinter growing season, winter duration, and the severity of winter water temperatures.