Effect of food abundance on aggressiveness and territory size of juvenile rainbow trout, Oncorhynchus mykiss

Territoriality is thought to be an important mechanism of population regulation in stream-dwelling salmonids. Since territory size can limit salmonid abundance, describing the exact relationship between territory size and important environmental variables, such as food abundance, is crucial to understanding the role that territoriality plays in regulating population size. The threshold model of feeding territoriality predicts that: (1) territories will occur between a lower and upper threshold of food abundance, and (2) the frequency of territorial aggression peaks at intermediate levels of food abundance. Optimal territory size models predict that territory size should decrease with an increase in food abundance. While this prediction has now been supported by a limited number of studies in salmonids too few levels of food abundance were used to describe the quantitative relationship between territory size and food abundance. Hence, I manipulated food abundance over a broad range of values (0.62, 1.25, 2.5, 5, 10, and 20% of the fish wet body weight per day) in artificial stream channels (1.92 x 0.77m) and monitored the territorial behaviour of juvenile rainbow trout (Oncorhynchus mykiss). First, I tested the prediction of the threshold model of territoriality: a dome-shaped relationship between territorial aggression and food abundance. Second, I quantified the relationship between territory size and food abundance over a 32-fold increase in food abundance by testing the following predictions: with increasing food abundance, territory size (1) does not change; (2) decreases with a slope of -1.0; or (3) decreases with a slope that is shallower than -1.0. Consistent with the prediction of the threshold model of feeding territoriality, the number of aggressive individuals peaked at the intermediate levels of food abundance. For the fish that were aggressive, there was a dome-shaped relationship between the frequency of territorial aggression and food abundance. As predicted by the models of optimal territory size, the aggressive radius of rainbow trout decreased with increasing food abundance but with a shallower slope than – 1.0, suggesting that territory size changed less than one would expect if fish were maintaining a constant amount food in their territory. The results of my study suggest that an increase in food abundance would have a relatively small effect on territory size and thus on the density of juvenile salmonids

Effect of food abundance on aggressiveness and territory size of juvenile rainbow trout, Oncorhynchus mykiss

How food abundance affects (1) whether or not individuals defend territories and (2) what size of territory is defended are crucial to understanding the role that territoriality plays in regulating the population size of stream-dwelling salmonids. The threshold model of feeding territoriality predicts that territorial defence will be most intense at intermediate levels of food abundance, whereas optimal territory-size models predict that territory size will decrease with increasing food abundance. While the latter prediction has been supported by several studies of stream salmonids, too few levels of food abundance were typically used to describe the exact relationship between territory size and food abundance. Hence, to quantify the relation between the intensity of defence, territory size and food abundance, we manipulated food abundance over a broad range of values (0.62, 1.25, 2.5, 5, 10 and 20% of the fish wet body weight in dry food per day) in artificial stream channels (1.92 × 0.77 m), while monitoring the territorial behaviour of juvenile rainbow trout. As predicted by the threshold model of feeding territoriality, the frequency of territorial aggression was highest at intermediate levels of food abundance, but fish never ceased defending territories entirely, particularly at food densities likely found in the wild. Also as predicted, the aggressive radius of rainbow trout decreased significantly, but only by 30% over a 32-fold increase in food abundance, equivalent to a 51% decrease in territory size. Our results suggest that territory size is relatively insensitive to changes in food abundance and may have a regulatory effect on population density