, Volume 154, Issue 2, pp 423–433 | Cite as

Behavioral and energetic costs of group membership in a coral reef fish

Behavioral Ecology


Animals in social aggregations often spend more time foraging than solitary conspecifics. This may be a product of the relative safety afforded by aggregations: group members can devote more time to foraging and less time to antipredator behaviors than solitary animals (the “risk reduction” effect). All else being equal, risk reduction should result in higher food intake for grouped animals. However, intragroup competition may force group members to spend more time foraging in order to obtain the same food ration as solitary individuals (the “resource competition” effect). We compared these opposing explanations of foraging time allocation in a coral reef fish, bluehead wrasse (Thalassoma bifasciatum). Aggregations of juvenile bluehead wrasse experience safety-in-numbers, and preliminary observations suggested that juveniles in aggregations spent more time foraging for copepods in the water column than solitary juveniles. However, the risk reduction and resource competition hypotheses are indistinguishable on the basis of behavioral observations alone. Therefore, we collected behavioral, dietary, and growth data (using otolith growth rings) for bluehead wrasse at multiple reefs around a Caribbean island. Despite spending more time foraging in the water column, grouped fish did not capture more prey items and had slower growth rates than solitary fish. Thus, the increased foraging time of grouped fish appears to reflect resource competition, not risk reduction. This competition may limit the size and frequency of aggregations among juvenile bluehead wrasse, which have been shown to experience reduced mortality rates in larger groups. Bluehead wrasse recruits also spent less time foraging but grew faster at sites where planktonic copepod prey were more abundant. This suggests the possibility that large-scale spatiotemporal variability in the abundance of planktonic copepods over coral reefs may produce corresponding variability in the dynamics of reef fish populations.


Foraging behavior Group-size effect Growth rate Resource competition Risk reduction 



We thank J. Barr, S. Brander, J. M. Ecker, and C. Grigsby for expert assistance in the field. J. Godwin, E. Marsh, P. Munday, and St Croix Ultimate Bluewater Adventures provided invaluable logistical support. S. Hamilton and J. Samhouri kindly provided instruction in otolith analysis and, with S. Sandin, plankton tube traps. We are grateful to K. Klose and S. Cooper for providing equipment and assistance with plankton and benthos analysis. Ideas and comments from D. Booth, S. Brander, L. Dill, S. Hamilton, D. Kramer, the Sih lab group, and especially S. Gaines and S. Holbrook greatly improved this work and the manuscript. J. W. W. was supported by National Science Foundation Predoctoral and University of California Regents fellowships. This is contribution 260 from the Partnership for Interdisciplinary Studies of Coastal Oceans, funded primarily by the Gordon and Betty Moore Foundation and the David and Lucile Packard Foundation. This work complied with the current laws of the United States Virgin Islands (USVI); all fieldwork was performed in accordance with University of California Santa Barbara Institutional Animal Care and Use Committee permit no. 4-04-403 and USVI Department of Planning and Natural Resources permit nos. STX-024-04 and STX-025-05.

Supplementary material

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  1. 1.Department of Ecology, Evolution, and Marine BiologyUniversity of California, Santa BarbaraSanta BarbaraUSA
  2. 2.Department of Wildlife, Fish and Conservation BiologyUniversity of California, DavisDavisUSA

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