Examining Interpopulation Variation in Bluegill Growth Rates and Size Structure: Effects of Harvest, Maturation, and Environmental Variables

—Understanding how life history variation and the environment affect growth and population size structure is an important theme of fisheries ecology. However, the interactions among important biotic and abiotic factors and their relative importance in the context of population-specific variation are seldom considered. We used an information theoretic approach to investigate how environmental variables interact with important life history parameters to influence population size structure of bluegills Lepomis macrochirus. We sampled bluegill populations in 23 Illinois reservoirs to examine the effects of temperature, water transparency, and prey availability and bluegill density, harvest, and size at maturation on bluegill growth and population size structure. Good growth and large body size were associated with warm, clear lakes containing abundant prey resources. An exception to this pattern was early growth (length at age 2): larger body size at age 2 was associated with relatively low Secchi depth (i.e., turbid water). In addition, size at maturation was positively related to size of adult male bluegills. Interestingly, bluegill density and harvest mortality—factors that are often considered important within populations—did not contribute much explanatory power to our among-system models. Whereas harvest and density-dependent growth have been shown to influence individual bluegill populations, their effects across a range of populations appear to be mitigated by temperature, water transparency, and prey availability. Although growth of individuals can be influenced by a variety of factors, our investigation suggests that fairly straightforward mechanisms at broader spatial scales are responsible for explaining variation among populations. Quantifying and understanding variation in individual growth rates, body size, and population size structure is an important theme of fisheries ecology and management. Body size is a function of an organism’s life history strategy, a determinant of its ecology, and an important practical response variable in natural resource management (Werner and Gilliam 1984; Aday et al. 2006). The challenge for any comprehensive investigation of variation in growth among individuals within and among populations is to account for the number of intrinsic and extrinsic factors that influence body size. The ultimate size that individual fish achieve (and, by extension, population size structure) is often a reflection of growth rate, and there are numerous papers describing the relationship between growth and specific biotic and abiotic parameters. Among fishes, temperature and prey availability are among the most studied variables affecting growth and ultimate size (e.g., McCauley and Kilgour 1990; Belk and Houston 2002; Hoxmeier et al. 2006). However, because individuals must balance competing energetic demands related to survival and reproduction, growth rate is also influenced by life history parameters, such as longevity and timing of maturation (e.g., Stearns 1976; Roff 1984). A complete understanding of variation in population size structure therefore requires consideration of more than just temperature and prey. Mortality, in particular, can be a strong selective force, and recent efforts to quantify the effect of angling on fish populations have demonstrated that the artificial, size-selective nature of harvest can have a significant influence on individual life histories. Fishing influences population size structure through more than just the mechanical removal of large fish. Alterations in growth rates (e.g., Conover and Munch 2002), maturation schedules (Diana 1983; Drake et al. 1997; Olsen et al. 2004), fecundity (Healey 1978; Baccante and Reid 1988), larval survival (Walsh et al. 2006), and other characteristics have been reported in response to angling, and rapid evolution in response to strong selective pressures is now widely accepted (e.g., Stockwell et al. 2003; Conover et al. 2006). Although they provide evidence of the relationship between artificial mortality and population size structure, the * Corresponding author: [email protected] 1 Present address: Minnesota Department of Natural Resources, 1801 South Oak Street, Lake City, Minnesota 55041, USA. 2 Present address: Department of Biology, North Carolina State University, 127 David Clark Labs, Raleigh, North Carolina 27695-7617, USA. Received April 25, 2008; accepted November 7, 2008 Published online March 19, 2009 423 Transactions of the American Fisheries Society 138:423–432, 2009 Copyright by the American Fisheries Society 2009 DOI: 10.1577/T08-076.1 [Article]