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

Determinants of multiple central-place territory use in wild young-of-the-year Atlantic salmon (Salmo salar)

Patterns of space use provide key insights into how animals exploit local resources and are linked to both the fitness and distribution of individuals. We studied territory size, mobility, and foraging behavior of young-of-the-year Atlantic salmon Salmo salar in relation to several key environmental factors in Catamaran Brook, New Brunswick, Canada. The 50 study fish were all multiple central-place foragers (i.e., alternated among several sit-and-wait foraging stations) and showed great variability in territory size and the total distance traveled within the territories. Territory size increased with the mean distance traveled between consecutive foraging stations, the number of stations visited, and the mean foraging radius. Fish also varied greatly in how much of the total travel distance was associated with foraging at a station (14.8–91.8%) versus switching among stations (4.6–84.3%). As predicted, fish in slow-flowing waters, where drifting prey were scarce, used larger multiple central-place territories than individuals in faster, more productive waters. Interestingly, however, the most mobile fish did not inhabit slow-running waters as predicted but were found at intermediate (optimal) water current velocities. Hence, our study suggests that among some multiple central-place foragers, increased mobility may not only serve to increase prey encounter rate but may reflect an attempt to patrol territories in favorable habitats. Further studies are needed to determine the generality and the ultimate benefits of multiple central-place space use among stream-dwelling fish and other animals

Courtship rates signal fertility in an externally fertilizing fish.

Sperm limitation is widespread across many animal species. Several mechanisms of sperm allocation have been proposed, including optimal allocation according to clutch size and equal allocation across females. However, considerably less effort has been directed at investigating the behavioural signals associated with sperm limitation in males, which may include mating rate and the intensity of courtship. We investigated whether multiple successive spawnings affect individual male fertilization success, mating rates and courtship rates in Japanese medaka (Oryzias latipes). Across an average of 17 spawning events per male, fertilization success decreased from 83.7 per cent for the first spawning to 40 per cent for the last spawning while courtship rate decreased from 3.4 to 1.5 min(−1). Females appeared to respond to male sperm depletion by reducing clutch size. Our results suggest that male Japanese medaka are sperm-limited, and that courtship rate may be an honest indication of fertilization ability

Dominant convict cichlids (Amatitlania nigrofasciata) grow faster than subordinates when fed an equal ration

Previous studies indicate that dominant fish grow faster than subordinate fish when fed equal rations. It is unclear, however, whether this growth differential is caused by intrinsic differences related to their propensity to become dominant, or by the extrinsic effect of the social stress experienced by subordinates. We first tested whether dominant convict cichlids (Amatitlania nigrofasciata) grew faster than subordinates when fed an equal amount of food. Second, we tested whether the growth advantage of dominants occurred when only visual interactions were allowed between pairs of fish. Third, we randomly assigned social status to the fish to rule out the possibility that intrinsic differences between fish were responsible for both the establishment of dominance and the growth differences. In three separate experiments, dominant fish grew faster than size-matched subordinate convict cichlids, but the growth advantage of dominants was higher when there were direct interactions between fish compared to only visual interactions. Our results provide strong support for the hypothesis that the slower growth rate of subordinate fish was due to the physiological costs of stress

Acute and chronic increases in predation risk affect the territorial behaviour of juvenile Atlantic salmon in the wild

Optimality models predict that territory size will decrease as the costs of defence increase. One poorly understood cost is predation risk, especially the relative influence of short- versus long-term increases in predation risk. Under natural conditions, we quantified the territorial behaviour of juvenile Atlantic salmon, Salmo salar, exposed to either acute or chronic increases in perceived predation risk. The effects of an acute increase in predation risk were assessed by exposing 18 young-of-the-year (YOY) Atlantic salmon to a control of stream water and to an alarm cue (i.e. conspecific skin extract) while monitoring their territorial behaviour. We investigated the effects of a chronic increase in perceived predation risk by quantifying the territorial behaviour of YOY salmon in control versus risky sections of seven sites, where we manipulated the perceived predation risk over a 4-week period by releasing stream water in control sections and alarm cue in risky sections. We found that salmon exposed to the alarm cue decreased the number of switches between foraging stations, but they did not change their territory size or foraging rate. As predicted, YOY salmon in risky sections had smaller territories than in control sections. However, their foraging rates and number of switches between foraging stations did not differ between treatments. Our study suggests that juvenile Atlantic salmon are sensitive to both acute and chronic increases in perceived predation risk under natural conditions, and support the predictions of optimality models that territory size decreases with increasing predation risk

The Influence of Operational Sex Ratio on the Intensity of Competition for Mates

The evolution and maintenance of secondary sexual characteristics and behavior are heavily influenced by the variance in mating success among individuals in a population. The operational sex ratio (OSR) is often used as a predictor of the intensity of competition for mates, as it describes the relative number of males and females who are ready to mate. We investigate changes in aggression, courtship, mate guarding, and sperm release as a function of changes in the OSR using meta-analytic techniques. As the OSR becomes increasingly biased, aggression increases as competitors attempt to defend mates, but this aggression begins to decrease at an OSR of 1.99, presumably due to the increased costs of competition as rivals become more numerous. Sperm release follows a similar but not significant trend. By contrast, courtship rate decreases as the OSR becomes increasingly biased, whereas mate guarding and copulation duration increase. Overall, predictable behavioral changes occur in response to OSR, although the nature of the change is dependent on the type of mating behavior. These results suggest considerable flexibility of mating system structure within species, which can be predicted by OSR and likely results in variation in the strength of sexual selection

Population density and territory size in juvenile rainbow trout, Oncorhynchus mykiss: implications for population regulation.

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. </jats:p