Density-Dependent Processes in Structured Fish Populations: Interaction Strengths in Whole-Lake Experiments

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 128.235.251.160 on Sun, Abstract. The goal of this study is to identify the mechanisms and measure the strengths of interactions within and among size classes in experimental populations of rainbow trout, Onchoranchus mivkiss. The metric that we used to assess the density-dependent effects was based on consumptive allometry and predator-prey theory. We demonstrate that the interactions among size classes were asymmetrical, favoring larger-bodied individuals. Descriptions of diet and spatial resource use, measures of prey availability, and risk to intra-specific interactions allowed assessment of the relative contributions of exploitative and interference competitive interactions among size classes.Growth of the larger classes was strongly density-dependent and driven primarily by exploitative competition. Growth of the smallest size class was controlled by a combination of exploitative competition within and among size classes and interference competition with larger-bodied conspecifics. This combination of interactions among size classes within populations resulted in a body-size-based asymmetry favoring the larger size classes. Survival of all size classes was positively related to both body size and growth rate. We speculate that the net result of these processes within size-structured populations is compensatory, leading to stable population dynamics. Key w'ordcls: (ci/aiibalisni,; completitive initercictions; Lon1sumllptive alloinetrv; (lenisitV-(lepenld(lence; lCdilsit'-(le/peliClenlt groYwthl (i/1C! sulirvivil; exploitaltive co)p/)etitiOll, relative co0/1tribiitioi Of io'-iteractioni streiigtlhs; iiiterfe'eice CoI1letitiOll, relative contribiitiOii Of; OncorhynchuLs mykiss, rainbowl trout; whlole-lalke experiments.

Predators select against high growth rates and risk–taking behaviour in domestic trout populations

“…We conducted only a single netting trial for habitat use to minimize killing fishes that had survived, because vulnerability to gillnets (and therefore mortality) increases with body size (Biro et al 2003a). Rainbow trout of the size used in this experiment primarily eat zooplankton, which is most abundant in the open-water, or pelagic, habitat (Post et al 1999). We assumed that the pelagic habitat was the most risky based on previous work showing avoidance of this habitat by trout (Post et al 1999), the absence of any physical structure into which they could escape, and focal animal observations of loons which indicated that loons initiate their foraging dives in the pelagic habitat (C. Beckmann, unpublished data).…”

“…Rainbow trout of the size used in this experiment primarily eat zooplankton, which is most abundant in the open-water, or pelagic, habitat (Post et al 1999). We assumed that the pelagic habitat was the most risky based on previous work showing avoidance of this habitat by trout (Post et al 1999), the absence of any physical structure into which they could escape, and focal animal observations of loons which indicated that loons initiate their foraging dives in the pelagic habitat (C. Beckmann, unpublished data). We surveyed the loon lakes over 6 days immediately after stocking, 5 days during the netting trial in the latter half of June, and 4 days in mid-July.…”

“…We used these values (domestic¼ 0:36 and wild ¼ 0:45, p < 0:05, no size effect, p > 0:05) to correct our total catch and obtain an estimate of the number of survivors (accounting for netting mortality during the summer). It is possible to obtain survivorship values of greater than unity when applying such correction factors to the total catch (Post et al 1999), but this bias will not affect conclusions drawn from estimates among experimental treatments. Instantaneous growth rate was calculated according to Post et al (1999) using the vulnerability-corrected mean mass for each cohort in September.…”

“…It is possible to obtain survivorship values of greater than unity when applying such correction factors to the total catch (Post et al 1999), but this bias will not affect conclusions drawn from estimates among experimental treatments. Instantaneous growth rate was calculated according to Post et al (1999) using the vulnerability-corrected mean mass for each cohort in September.…”

Social-ecological interactions, management panaceas, and the future of wild fish populations

“…Progeny from both pure hatchery and hatchery-wild crosses were assigned to the hatchery subpopulation to account for the often-reported reduced fitness in hatchery fish or hybrids relative to wild fish (30,31). Prestocking early YOY numbers of each subpopulation were determined by a Ricker stock-recruitment function (22), resulting in densitydependent survival regulated by ecological competition, as is typical in nature (7,32). A recruitment deviate added stochasticity that propagated through later life stages, resulting in natural population fluctuation (26).…”