Spatial Ecology and Density-Dependent Processes in Smallmouth Bass: The Juvenile Transition Hypothesis

Abstract

Previous research from our long-term study of smallmouth bass Micropterus dolomieu ecology in Lake Opeongo revealed a negative density-dependent relationship between the number of adult males in the population that appeared mature and the number of males nesting in any given year. The availability of nesting habitat is not the limiting resource behind this observation. We present three lines of evidence to argue that the process of forming an adult home range is the key factor in determining what fish join the breeding population. First, the dispersal of age-0 fish from their nest site is limited in their first summer and, assuming that this reflects the location of age-l juveniles, indicates that juveniles can be located relatively close to their nest of origin. Second, juvenile growth is density-dependent with the strength of this relationship increasing during the mid-juvenile period (ages 2, 3, and 4) and weakening at the end of the juvenile period (ages 5 and 6). Third, adult home ranges are large (200-400 ha), traveled extensively each day (1-9 km), established quickly after reproduction, rarely incorporate the nest site and overlap in location from one year to the next. The home range analysis of adults and dispersal of age-0 juveniles provides evidence of a strong spatial structure in smallmouth bass populations. We propose that density-dependent growth during the mid- to late juvenile period reflects a process of movement away from the natal area to locations where adult home ranges will be established. We further propose that this movement is a density- dependent process based on competitive abilities of individuals and possibly reflects social foraging that is observed to increase in juvenile bass after their first year. The spatial spread of juveniles presumably leads to adults establishing home ranges in various locations in the lake and, based on our observations of their behavior and fidelity in the home range, points to a spatial learning process that may be functioning to locate home ranges at the end of the juvenile period. The 95 percent kernel home range is larger for females than for males indicating that there may be differences between males and females operating within this spatial spread process. Acquiring a home range and successfully foraging in what may be social groups of adults and sub-adults may be a prerequisite for maturation and nesting. In conditions of high population abundance, activity necessary for successful foraging and establishment of home ranges may consume larger proportions of assimilated energy as a result of competition and this in turn could slow the maturation process leading to our earlier observation of a density-dependent relationship between adult abundance and the numbers of nesting males. This is the juvenile transition hypothesis.