We manipulated population density of juvenile rainbow trout (Oncorhynchus mykiss) across a range of realistic densities in artificial stream channels, while controlling food abundance in two
different ways: in Experiment 1, the total amount of food was held constant over a threefold increase in density, whereas in Experiment 2, the per capita amount of food was held constant over an eightfold increase in density. We tested the contrasting predictions that territory size
(i) is not affected by population density; (ii) decreases with population density as 1/n, where n = the local population size; or (iii) decreases with population density but towards an asymptotic minimum size. In Experiment 1, territory
size decreased with increasing population density. With the broader range of densities used in Experiment 2, territory size initially decreased with density and then leveled off at a minimum territory radius of 20–30 cm. Our results suggest an asymptotic minimum size of about
0.2 m2 for a 5 cm rainbow trout, similar to what is observed for high-density conditions in the wild. This minimum territory size could potentially set an upper limit on local population density and help regulate the population size of stream salmonids.
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