First-year cohort dynamics and overwinter mortality of juvenile largemouth bass

Relative year-class size of largemouth bass Micropterus salmoides following recruitment through the first winter can depend upon a variety of life history events. Shoreline electrofishing was used to assess juvenile largemouth bass cohort dynamics from school dispersal through the yearling stage in Jordan Lake, North Carolina, 1987-1995. Measures of environmental conditions and dynamics of juvenile shad Dorosoma spp. were collected concurrently to assess their potential effects on cohort dynamics of juvenile largemouth bass. Abundance of age-0 largemouth bass at the time cohort mean lengths reached 50 mm varied significantly among years, as did growth rates and mortality from July through October. No environmental or shad effects on largemouth bass growth or mortality were detected. Overwinter mortality rates did not vary among years, despite a 50-mm difference among years in largemouth bass mean lengths going into late fall, and little evidence was found to indicate significant size-selective overwinter mortality. Despite significant differences in first-year growth rates and cohort mean lengths entering the first winter, mortality rates of young largemouth bass from school dispersal through the yearling stage did not differ among years, suggesting that variations in cohort dynamics did not act to override initial differences in relative year-class size.