The typical territory defended by juvenile brook charr (Salvelinus fontinalis) in a laboratory stream tank was cordate in shape, with the resident stationed at the downstream end facing into the current. Responses by the resident were less aggressive toward intruders at increasing distance from the resident's station. Chases predominated immediately around the station, charges were more frequent further away, and lateral displays were most frequent at the territory perimeter. Defense of the anterior (upstream) area tended to be more aggressive, with equal numbers of charge and chase interactions, while laterally less aggressive and presumably less costly (energetically) charges predominated. These details of territory structure and defense agree with the predictions of an hypothesis of the territory as an economically defensible resource.

SynopsisDirect observations of young-of-the-year brook charr, Salvelinus fontinalis, in a second-order woodland stream indicated that most of their feeding effort was directed toward sub-surface, drifting prey (83% of feeding time) . Feeding from the substrate and water surface was much less frequent (17% of feeding time) . Comparisons of gut contents to drift net and substrate fauna samples corroborated that the most commonly consumed prey (chironomid and trichopteran larvae, ostracods, and ephemeropteran nymphs) were captured primarily from sub-surface, invertebrate drift . The disproportionate numbers of some prey species in the guts of several fish indicate that some prey selection occurred . Territories appeared to be cardioidshaped, and were often contiguous, with dominance hierarchies evident among the residents . Agonistic interactions were frequent . Charges and chases predominated (91% of interactions) while lateral displays were infrequent (9% of interactions) . Overall, these fish spent most of the daylight hours station-holding (77%) and feeding (18%) . While only 3% of total time was spent in aggression, this amounted to 14% of the time a fish spent away from its station . There was some indication that territories were defended at a cost of feeding time .

SynopsisJuvenile brook chart-, Salvelinus fontinalis, defending territories in a laboratory stream channel changed their defensive tactics when fish numbers or water current velocity were increased. The associated increase in energy costs of territorial defense were apparently reduced by using relatively more lateral displays and fewer chases to repel intruders, and by decreasing territory size. Territory size tended to increase with food ration, although levels of aggression were not affected. These results emphasize the importance of energetic considerations in territoral defense in these fish, and are discussed in terms of current hypotheses regarding the resource base for territoriality.

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Basic theory predicts that two-dimensional territories should be circular, but results from stream-dwelling juvenile salmonid fishes appear to deviate consistently from such a shape. We present a model for cardioidal territories based on data from juvenile brook charr (Salvelinus fontinalis) and show that circles can be considered as a limiting case for cardioids. Since they are regular geometric shapes directly related to circles, but with one concave side, cardioids may be useful for some analyses of territorial data.