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Behavioural ecology of the longnose dace, Rhinishthys cataractae (Pisces:Cyprinidae) : significance of dace social organization Bartnik, Victor George

Abstract

The social behaviour of a stream population of longnose dace, Rhinichthys cataractae dulcis, is described. Both males and females occupy the same riffle areas and defend territories (approx. 10 cm in diameter) during the breeding phase. In addition, male dace are evidently attracted to and interact agonistically with other males resulting in the formation of clusters of male territories. During the night, females leave their territories to court and spawn with territorial males. After spawning, males show strong nest site attachment and are inhibited from eating eggs. Parental males remain directly over the nest site and frequently probe the substrate with their snouts. They defend the area against all fish with the exception of receptive females with which they may spawn. Comparison with a stream population of the sub-species, R. cataractae cataractae, reveals differences in phenotypic appearance, diurnal rhythm of breeding activity, and female territoriality. Unlike R.c. dulcis, R.c. cataractae are reproductively active during daylight hours and males display bright nuptial coloration. Breeding coloration in males of R.c. dulcis is faint or absent. Unlike R.c. dulcis, males and females of R.c. cataractae are segregated into different habitats and females remain non-territorial. Experimental analyses of the physical and biotic factors involved in the causation of territorial behaviour were conducted in a laboratory stream tank. The major physical factor involved in the selection of territories by male dace is the presence of coarse gravel suitable for spawning. Female dace select and defend only enclosed areas which provide both overhead cover and shelter from current. Females prefer shelters which contain food and thus the presence of food may be a proximate factor involved in the selection of territories by female dace. Laboratory tests indicate that female territorial behaviour is elicited by interaction with breeding males. Outside the breeding phase, when territoriality is relaxed, male and female dace differ little in the areas they choose to occupy. The experimental evidence suggests that the functions of male territory are: i) provision of space in which males can court and spawn with females with minimal interference from other males, and ii) protection of eggs from intraspecific predation. For females, territoriality apparently functions to reduce conflicts with males (i.e., reduce courtship harassment and attacks from males). Changes in the behaviour of males occur after spawning and coincide with periods of greatest egg vulnerability. These post spawning behavioural activities of males protect freshly deposited eggs while making them less accessible to predators. The evidence also suggests that a major function of territory in both sexes is the provision of shelter from current. Dace not defending a shelter may be forced to make frequent movements in the strong current of the riffle habitat. Dace swimming against strong currents for even short periods (i.e., 5 min) become fatigued and lose their ability for coordinated locomotion. Such stressed individuals may be vulnerable to predation. The data also suggest that dace territorial behaviour may act as a dispersing mechanism, thereby limiting the density of breeding fish in a localized area. Territorial males of clustered groupings remain behaviourally synchronized. They display strong site attachment and interfere little with the reproductive activities of neighbouring males. Consequently, these social groupings function to further reduce interference from conspecifics. Assemblages of more widely spaced territorial males, created in the laboratory, experienced greater intraspecific interference and egg predation than did males of clustered territorial groupings. Thus it seems most probable that intraspecific interference and egg predation have provided a major selection pressure favouring both male territoriality and territory clustering in longnose dace.

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