Abstract
This paper extends theory of shoaling presented by Landa (1998), which uses the economic theory of clubs (Buchanan 1965). The findings include that non-patchy feeding shoals formed for defense increase in size with increased predation and decline in size with increased food concentration. There is strong evidence for the former and a piece of evidence for the latter. The size of shoals formed to find patches of food is unaffected by food availability. When defense is also involved, increasing predation increases shoal size and increasing food availability decreases shoal size. The optimum size of migrating schools—synchronized and polarized swimming shoals—is often the whole population migrating, explaining the mammoth size of some schools. Further, mergers of schools are forecast along the migration route with larger schools at the end of the route than at the beginning. When there is no defense motive for shoaling, non-patchy feeders may form large schools under low food density and be solitary under high food density. Small schools might not be found. When there is also a defense motivation for shoaling, small schools are possible. For schools formed among patchy feeders, increased food availability decreases school size. The same finding holds for schooling among patchy feeders where an added shoal benefit is defense. Also under these circumstances, increased predation increases shoal size.
Published Version
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