A temporally stable spatial pattern in the spawner density of a freshwater fish: evidence for an ideal despotic distribution

Abstract

We explore the causal basis for a temporally stable spatial pattern in the density of spawning individuals of a freshwater fish. Based on a comparatively long-term set of data spanning one decade, reproductively active brook trout (Salvelinus fontinalis) inhabiting a near-pristine river on Cape Race, southeastern Newfoundland, occupy areas of either high or low density. Compared with their low-density counterparts, high-density aggregations are typically more than 30 times denser despite occupying only 4% of the available habitat. High-density areas are characterized by slower flow and suspected groundwater or bog-water seepage, attributes likely to increase the probability of offspring survival in Freshwater River. Disparity in density between high- and low-density aggregations declined as total population size increased, a pattern consistent with the predictions of the ideal free distribution. The larger body sizes of trout in the high-density aggregations may prevent others from occupying the most preferred spawning habitat. This spatial pattern in spawner density is consistent with that predicted by an ideal despotic distribution, although we cannot discount the influence that Allee effects might have on the distributional patterns of spawning individuals at low population sizes.