Chaetodon capistratus were found to show preferences, determined by an electivity index, for species of food that were not correlated with the amounts of food available in the field. G. flabellum (with G. ventalina) and Zoanthus species were preferred over
some more abundant species, notably M. annularis. The number of nips taken was not related to the size distribution of the food units, which decreased in number from small to large units. The fish showed a preference for larger units and, in fact, chose preferred species that had
more of the larger sizes available. In the laboratory, a general preference was shown for clumped configurations rather than for dispersed or solitary units whereas, in the field, no distinction was made merely between forms, whether massive, branching or flattened. The size of a fish
did not confer advantages in feeding or in aggressive encounters. The fish were, however, significantly more aggressive in food than in non-food situations in the laboratory and on preferred species in the field. Aggression was also correlated with the less preferred sizes in the field as
a result of the increased visibility of competitors.
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