Overwinter Growth and Survival of Largemouth Bass: Interactions among Size, Food, Origin, and Winter Severity

—Winter severity (temperature, duration, and photocycle), geographic origin, food availability, and initial body size likely influence growth, survival, and, therefore, recruitment of age0 largemouth bass Micropterus salmoides. We collected age-0 largemouth bass (70–160 mm total length) from low (338N), intermediate (408N), and high (458N) latitudes throughout their natural range (origin), and we subjected all three groups of fish to three experimental winters that mimicked these latitudes (N 5 9 largemouth bass per treatment). Within each winter and origin, one-half of the largemouth bass were fed fish prey, whereas the remaining one-half were starved. Winter strongly influenced survival; overall survival rates in the high-, intermediate-, and low-latitude winters were 34.9, 59.4, and 61.1%, respectively (x2 test, P , 0.05). Largemouth bass from 338N suffered high mortality in the high-latitude winter. Across all winters, more fed fish (64.5%) survived than did starved fish (38.1%) (x2 test, P , 0.05). Pooling fish into small (,100 mm) and large ($100 mm) size classes revealed that more small fish died than did large fish in the lowand high-latitude winters, but this was not the case in the middle-latitude winter. Wet weights (g) of fed largemouth bass increased, remained constant, and declined in the low-, intermediate-, and high-latitude winters, respectively. Wet weights and total energy content (kJ) of fed individuals were consistently higher than those of their starved counterparts in all winters. However, energy density (kJ/g) of fed individuals often declined to levels similar to those of starved largemouth bass. Winter temperature combined with duration likely dictate the northern limit of largemouth bass by reducing growth, even when food is abundant. Because survival of individuals from the low latitude was poor in higher latitude winters, stocking southern largemouth bass in northern systems may translate to high mortality and perhaps to degradation of physiological tolerances of local populations through hybridization. Overwinter survival during the first year of life may drive the recruitment and, thus, the yearclass strength of many fishes (Oliver et al. 1979; Toneys and Coble 1979; Post and Evans 1989). Poor first-summer growth resulting in small fall body size is often implicated as a primary factor influencing winter survival (Gutreuter and Anderson 1985). In addition, winter survival may be influenced by local factors, including food availability and local adaptation, and geographic processes, such as winter severity (defined as duration, photoperiod, and temperature regime). In order to understand what factors govern variability in first-winter survival across the range of * Corresponding author: [email protected] 1 Present address: North Carolina Wildlife Resources Commission, 102 West Whitaker Mill Road, Raleigh, North Carolina 27608, USA. 2 Division of Biology, Kansas State University, Manhattan, Kansas 66506, USA. 3 Department of Fisheries and Allied Aquacultures, Auburn University, Auburn, Alabama 36849, USA. Received September 16, 1998; accepted March 2, 1999 a species, we must determine the relative importance and the potential interactions of these factors. Herein, we explore how these factors influence the first-winter growth and survival of largemouth bass Micropterus salmoides, a widely sought sport fish (Hall 1996). For age-0 largemouth bass, size-selective mortality should be greatest at the highest latitudes, where winters are most severe (Post and Evans 1989; Schultz and Conover 1997). This expectation is based on several assumptions, including cessation of feeding (Johnson and Charlton 1960; Coutant 1975; Oliver et al. 1979; Miranda and Hubbard 1994a, 1994b), during which energy for metabolic needs is provided by body stores (Sullivan 1986). Owing to their higher weight-specific metabolic rates (Brett and Groves 1979; Rice et al. 1983; Jobling 1993), small fish may be more susceptible to starvation than large fish (Oliver et al. 1979; Sullivan 1986; Smith and Griffith 1994; Griffiths and Kirkwood 1995; Cargnelli and Gross 1996, 1997; Meyer and Griffith 1997). Hence, exhaustion of energy reserves is often implicated as