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Mechanisms of food resource partitioning and the foraging strategies of rainbow trout (Salmo gairdneri) and kokanee (Oncorhynchus nerka) in Marion Lake, British Columbia Hyatt, Kim D.

Abstract

This study was conducted to satisfy three objectives. The first was to provide a detailed description of the differences between the prey contents of rainbow trout (Salmo qairdneri) and kokanee (Oncorhynchus nerka) compared either to each other or to the prey contents of the natural environment. The second was to determine how elements of anatomy, physiology and behaviour interact to promote the acquisition of species specific diets by trout and kokanee. The third was to precisely identify the foraging strategies of the two predators by assessing how different anatomical and behavioural characteristics serve as interrelated adaptations that suit each species to effectively use a specific habitat-prey complex. Matched samples of trout and kokanee from Marion Lake exhibit only modest A dietary overlap (mean of C λ = .462, range.136 to .881). Although kokanee appear to track the environmental abundance of prey more closely than trout, both predators exhibit pronounced patterns of "density independent" acquisition of prey from the total complex of prey that is apparently available in the lake. To test hypotheses about the factors that control these dietary patterns, I conducted a series of studies concerning where trout and kokanee choose to forage, when they choose to forage, how they search for prey, how they attack prey, and how experience in encountering various prey alters the predator's foraging behaviour. Temporal segregation of trout and kokanee foraging activities is not well-developed under field conditions and appears unlikely to promote strong patterns of food-resource partitioning. By contrast, spatial segregation is well-developed and clearly plays a major role in promoting the acquisition of relatively large numbers of nearshore benthic prey ( eg. planorbid snails or odonates ) by trout and of relatively large numbers of offshore, water-column prey ( eg. chironomid pupae ) by kokanee. Close inspection of the details of predator and prey distributions indicates that many aspects of food-resource partitioning are not logical outcomes of spatial segregation. Differences in predator search-techniques do not determine the presence or absence of various prey types in predator diets, however, differences in predator search behaviours do bias them to obtain different quantities of particular classes of prey. Kokanee search procedures allow them to detect prey in both exposed and concealed locations while trout detect only exposed prey. While searching for benthic or lake-surface prey, kokanee maintain search positions which allow them to detect prey of smaller sizes than trout. This clearly favours the trend for kokanee to include greater quantities of small prey (eg. Hyalella sp.) in their diets than trout. Differences in search procedures do not explain why kokanee obtain a greater proportion of their diet than trout from small zooplankton (≤ 1 mm ) in late summer or why kokanee seldom exploit any of the relatively large ( > 4 mm ), armoured prey that are common in the diet of similar sized trout. Differences in both behavioural and morphological characteristics involved in the attack phase of foraging by trout and kokanee serve as the basis for explanations of a number of differences between the diets of free-ranging predators. These differences include: the greater utilization of aerial prey by trout, the inclusion of large numbers of copepods in the diet of kokanee but not of trout, the generally greater utilization of zooplankton by kokanee compared to trout, and the relative-scarcity of large ( > 4 mm ), armoured prey, in the diet of kokanee. A series of laboratory experiments was used to examine the extent to which short term experience might influence food-resource partitioning by trout and kokanee. These experiments offered convincing evidence that differential effects of experience will amplify the trends in resource partitioning already set in motion by differences in habitat selection, search procedures, and attack procedures. I argue that the morphological and behavioural traits that control food "selection" by trout and kokanee in Marion Lake are a consequence of the evolution of mutually exclusive foraging strategies. Trout are portrayed as D-strategists that concentrate on relatively large, dispersed prey for the bulk of their energy requirements. Adaptations which enable trout to differentially exploit large prey include: procedures for area-extensive search; a predisposition to attack relatively large, armoured-prey; large mouth-size; and persistent responses to opportunities to attack large prey. An inability to sustain high attack rates on small prey ( < 1 mm ) at high density ( 35 per liter ) and a tendency to ignore or reject such prey suggest that trout are not well-adapted to exploit relatively small, morphologically-uniform,prey. Kokanee are portrayed as C-strategists which concentrate on relatively small, contagiously-distributed prey for the bulk of their energy requirements. Adaptations which enable kokanee to differentially exploit small prey include: procedures for area-intensive search; a predisposition to attack relatively small, morphologically-uniform prey; small mouth-size; well-developed gill-rakers; and an ability to sustain high attack rates on small planktonic prey. Low ingestion success with a variety of large benthic prey and a tendency to ignore or reject such prey under laboratory conditions where they serve as the sole source of food are evidence that kokanee are not well adapted to exploit large, armoured-prey. Adaptations associated with the search, capture, manipulation and ingestion phases of the foraging cycle appear in each instance to be evolutionary responses to specific features of a given habitat-prey complex. C-selected or D-selected foraging strategies appear to be mutually exclusive evolutionary avenues down which trout and kokanee have been directed by the fundamental nature of a given habitat-prey complex.

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