Role of satiation in the functional response of a piscivore

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Role of satiation in the functional response of
a piscivore, largemouth bass (Micropterus
salmoides)
Timothy E. Essington, James R. Hodgson, and James F. Kitchell
Abstract: We evaluated whether satiation regulates the predation rates of a piscivore, largemouth bass (Micropterus
salmoides), in natural settings. A functional response model indicated that predation rates can be reduced by satiation
when mean prey density is high or when prey encounters are highly patchy. We then used bioenergetics modeling to
estimate the predation rates of individual bass in four lakes during a 16-year period and used stomach content mass in
diet samples to evaluate the variability in daily predation rates. Predation rates, expressed as the proportion (p) of the
maximum daily consumption rate, were low (mode = 0.3, mean = 0.4). Stomach fullness (s), expressed as the proportion of the stomach fullness associated with the maximum sustainable consumption rate, was highly variable, and 13%
of all bass diets had s > 1, indicating that bass could opportunistically forage at rates exceeding their maximum sustainable rate. The low predation rates and the ability to consume prey at rates exceeding the maximum sustainable rate
make it unlikely that satiation was an important constraint on bass predation rates. Thus, satiation effects widely represented in modeling studies may be a rare component in piscivore–prey interactions, while prey behavior may be a more
important component governing predation rates.
Résumé : Nous avons évalué si la satiété régulait les taux de prédation d’un piscivore, l’achigan à grande bouche (Micropterus salmoides), en milieu naturel. Un modèle de réponse fonctionnelle a indiqué que les taux de prédation peuvent être réduits par la satiété quand la densité moyenne de proies est élevée ou quand les rencontres de proies sont
très intermittentes. Nous avons ensuite utilisé la modélisation bioénergétique pour estimer les taux de prédation individuels d’achigans dans quatre lacs durant 16 ans et mesuré la masse d’échantillons de contenus stomacaux pour évaluer
la variabilité des taux quotidiens de prédation. Les taux de prédation, exprimés comme la proportion (p) des prises par
rapport au taux maximum de consommation quotidienne, étaient faibles (mode = 0,3, moyenne = 0,4). Le degré de
remplissage de l’estomac (s), exprimé comme la proportion du contenu stomacal par rapport au degré de remplissage
de l’estomac associé au taux de consommation soutenable maximum, était très variable, et 13% de tous les contenus
stomacaux donnaient des valeurs de s supérieures à 1, ce qui indique que les achigans peuvent de façon opportuniste
consommer des proies à des taux excédant leur taux soutenable maximum. Les faibles taux de prédation et la capacité
des achigans de consommer des proies à des taux excédant leur taux soutenable maximum font qu’il est peu probable
que la satiété influe de façon importante sur les taux de prédation des achigans. Ainsi, les effets liés à la satiété largement signalés dans les études de modélisation pourraient n’intervenir que rarement dans les interactions entre piscivores
et proies, tandis que le comportement de ces dernières pourrait influer davantage sur les taux de prédation.
[Traduit par la Rédaction]
Essington et al.
556
Introduction
The functional response describes the relationship between the density of prey and the rate at which individual
predators consume them (Solomon 1949; Holling 1959). Although the exact form of this relationship varies, nearly all
forms include prey density-dependent predation rates over
some range of prey densities and prey density-independent
predation rates at high prey densities. The shape of the funcReceived June 29, 1999. Accepted November 3, 1999.
J15221
T.E. Essington1 and J.F. Kitchell. Center for Limnology,
University of Wisconsin, 680 N. Park St., Madison,
WI 53706, U.S.A.
J.R. Hodgson. Department of Biological Sciences,
St. Norbert College, DePere, WI 54115, U.S.A.
1
Author to whom all correspondence should be addressed.
e-mail: [email protected]
Can. J. Fish. Aquat. Sci. 57: 548–556 (2000)
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tional response curve has strong effects on predator–prey
dynamics because predation mortality may be either density
dependent, inversely density dependent, or density independent (Murdoch and Oaten 1975; Hassell 1978). Further,
nonlinear functional responses coupled with dome-shaped
recruitment functions can lead to multiple stable states in
predator and prey abundances (May 1977).
Predator–prey models with nonlinear functional responses
often predict oscillatory or unstable population dynamics,
yet data from natural systems often fail to match these predictions (e.g., Murdoch 1994). One way to resolve these discrepancies is to determine whether predation rates in natural
settings are in a range where constraints associated with the
density-independent portion of the curve are important. In
other words, how often are predation rates on the asymptotic
portion of the functional response?
This question is typically addressed by fitting a functional
response curve to observed predation rates over a wide range
of prey densities. However, quantifying the functional re© 2000 NRC Canada