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Transactions of the American Fisheries Society 120:500-508, 1991 © Copyright by the American Fisheries Society 1991 Experimental Analysis of Prey Selection by Largemouth Bass: Role of Predator Mouth Width and Prey Body Depth K. DAVID HAMBRIGHT Section of Ecology and Systematic and the Ecosystems Research Center Cornell University, Ithaca, New York 14853, USA Abstract.—Piscivorous fish are size-selective predators. Although sizes of prey selectively ingested by piscivores traditionally have been measured in terms of prey length relative to predator length, the relationship between prey body depth (measured dorsoventrally) and piscivore mouth gape may be a more appropriate measure of prey size selection. In 2-d feeding trials with three sizes of largemouth bass Micropterus salmoides, I offered various sizes of shallow-bodied fathead minnows Pimephales promelas and deep-bodied pumpkinseeds Lepomis gibbosus in assemblages of one or both species. All sizes of predators preferred pumpkinseeds with body depths well below the maximum size ingestible. Small predators also preferred fathead minnows with body depths below the maximum size ingestible, whereas intermediate and large predators selectively ingested the largest fathead minnows offered. Largemouth bass never ingested prey of body depth greater than their own external mouth width. Although lengths of selectively ingested fathead minnows and pumpkinseeds differed, largemouth bass showed highest preferences for prey of similar body depths regardless of taxonomic identity. These results suggest that, in addition to setting constraints on maximum sizes of prey that can be ingested by piscivores, the relationship between prey body depth and piscivore mouth gape may also be important in selection of prey within the range of ingestible sizes. Therefore, body depth may be more useful than the traditional measure of prey length as a common measure for examining prey selection by gape-limited piscivores over a wide array of prey species. Piscivorous fish are gape-limited predators, consuming only prey they can swallow whole. Because prey are generally swallowed head- or tailfirst, their body depth (measured dorsoventrally) relative to the size of a piscivore's mouth determines whether they can be ingested (Swingle 195Q; Lawrence 1958; Werner 1977; Tonn and Paszkowski 1986). Thus, in any particular habitat prey with boidy depths greater than the largest piscivore gape are invulnerable to ingestion. The vulnerability of prey within the range of ingestible sizes is determined by other factors such as size distributions of piscivores and prey, prey encounter rates with piscivores, and predator-avoidance behaviors of prey (Wahl and Stein 1988; Hambright et al., in press). Prey-selection behavior of piscivores also influences the vulnerability of prey. Optimal foraging theory postulates that predators maximize the ratio between the benefits gained and the costs incurred in obtaining prey. Obviously, the benefits gained increase as a function of prey size, but cost, in particular that due to handling time, also increases rapidly with prey size (Werner 1974). Hoyle and Keast (1987,1988) demonstrated that, for two piscivores (largemouth bass Micropterus salmoides and grass pickerel Esox americanus), the weight-adjusted handling time for prey of equivalent lengths varied
TABLE 3.—Predicted and actual maximum body depths (standard lengths in parentheses) of fathead minnows and pumpkinseeds ingested by small, intermediate, and large largemouth bass. Predicted values are based on the mean external mouth widths of the predators. Prey standard lengths were calculated with equations (1) and (2) in the text by substituting the mean predator mouth width for prey body depth. Asterisks indicate prey sizes exceeded the naturally occurring size ranges listed in Carlander (1969, 1977).
Largemouth bass group
Small --- s below Intermediate --- i below
Large ---- l below
Mean external mouth width (mm) 14.6 - s 25.6 - i 34.2 - l
Maximum size of fathead minnow (mm)
Predicted 14.6 --- s (59.8) 25.6 --- i (99.2*) 34.2 ----l (130.0*)
Ingested 13.3 -- s (55) 13.3 -- i (55) 13.3 --- l (55)
Maximum size of pumpkinseed (mm)
Predicted 14.6 --- s (39.6) 25.6 --- i (62.3) 34.2 --- l (80.9)
Ingested 13.0 --- s (35) 22.2 --- i (55) 31.5 --- l (75)
Last edited by ewest; 03/31/16 10:36 AM.
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