Interactions between Brook Charr, Salvelinus fontinalis, and Nonsalmonid Species: Ecological Shift, Morphological Shift, and Their Impact on Zooplankton Communities

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Abstract
A quantitative survey of 26 oligotrophic Quebec lakes indicated that the presence of creek chub (Semotilus atromaculatus) or white sucker (Catostomus commersoni) had an impact on brook charr (Salvelinus fontinalis) populations. Evidence for competitive interactions among these species was provided by the observation that (1) the mean annual yield (kilograms per hectare) of charr was significantly reduced when they lived sympatrically with these species, (2) charr shifted their food habits from benthic organisms to Zooplankton in the presence of creek chub and (or) white sucker, and (3) the length of the gill rakers of charr was significantly higher in lakes containing chubs and suckers. As the length of the gill rakers is reported to be correlated with the extent of plankton feeding, this latter result was interpreted as a morphological change associated with increased exploitation of Zooplankton in sympatry. When occurring with creek chub, the niche shift of brook charr resulted in a significant change in Zooplankton community structure. The proportion of large Zooplankton organisms (>1.1 mm) declined whereas that of the smaller ones (<0.6 mm) increased. Also, the mean length of Holopedium, the larger cladocerans, was significantly reduced in sympatric creek chub–charr lakes compared with lakes where charr lived allopatrically. A similar impact on Zooplankton was not as clear when charr occurred with white sucker. When living with white sucker, brook charr populations appeared to be sufficiently reduced as to have little or no effect on Zooplankton communities.