Asymmetries in Competitive and Predatory Interactions in Fish Populations

Abstract

A fundamental feature of fish and many other taxa is their potential for individuals to grow during most of their life. While increasing in size, fish undergo size-specific changes in niche, generally involving an increase in the mean size of prey eaten. Considering foraging efficiency alone, these ontogenetic niche shifts can be related to morphological changes and increasing energetic costs. The former is a result of an increased capacity to capture larger prey and a loss in the ability to capture and retain smaller prey, while the latter relates to the necessity to switch to increasingly larger prey to compensate for increased energetic demands with increasing body size (Kerr 1971; Mittelbach 1981; Persson 1987 a). Since individual growth rates in fish are indeterminant and dependent on resource supply, resource competition will directly affect ontogenetic niche shifts by depressing growth rates. In most cases, however, competition and predation interact and ontogenetic niche shifts in fishes are therefore often a result of both competitive and predatory processes (Gilliam 1982; Werner and Gilliam 1984; Werner 1986; Mittelbach and Chesson 1987; Gilliam and Fraser this Vol.). An illustrative example of how competitive interactions interact with predation is competition-induced reduction in growth rate, which will prolong the time the fish is susceptible to gap-limited predators (Tonn et al. 1986). The strength of competitive and predatory interactions respectively may also change ontogenetically such that while competitive interactions are prevailing between certain size (age) classes, predatory interactions are prevailing between others.