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Diet Differentiation in Polymorphic Arctic Charr in Thingvallavatn, Iceland

H. J. Malmquist, S. S. Snorrason, S. Skúlason, B. Jonsson, O. T. Sandlund and P. M. Jonasson
Journal of Animal Ecology
Vol. 61, No. 1 (Feb., 1992), pp. 21-35
DOI: 10.2307/5505
Stable URL: http://www.jstor.org/stable/5505
Page Count: 15
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Diet Differentiation in Polymorphic Arctic Charr in Thingvallavatn, Iceland
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Abstract

1. The diets of four Arctic charr morphs (Salvelinus alpinus (L.)); small benthic (SB), large benthic (LB), small limnetic (SL) and large limnetic (LL), were analysed on a seasonal and diel basis in different habitats in lake Thingvallavatn, Iceland. All morphs used the littoral habitat (littoral stony zone, 0-10 m depth, and littoral Nitella zone, 10-20 m depth), but limnetics also used the pelagic habitat (0-70 m depth). 2. The morphs exhibited dietary specializations, strongly correlated with distinct differences in trophic morphology. Benthic morphs foraged on zoobenthos, especially the mollusc Lymnaea peregra. Small limnetics fed extensively on open water foods, particularly zooplankton, whereas large limnetics preyed mainly upon threespined stickleback, Gasterosteus aculeatus, less on Arctic charr. Diet of limnetics differed slightly between habitats. 3. Seasonal and diel changes in diet composition were slight, and benthics and limnetics did not overlap in each others specific diet. However, all morphs converged in spring upon insect pupae that are then superabundant. The food segregation between benthics and limnetics was established from age 1 (fork length c. 7 cm) and onwards. 4. The size of snalis eaten increased with increasing size of benthic charr, and the size of charr eaten increased with increasing size of LL charr. This is explained in terms of ontogenetic gape limitations. Larger snails eaten by SB charr relative to those eaten by similarly sized LB charr may be the result of the larger head and more subterminal mouth of SB charr, endowing them with greater ability to engulf larger snails. It is suggested that LL charr are ecomorphs of SL charr which, through ontogenetic shifts, switch from planktivory to piscivory at the size of 22 cm. 5. All morphs, except LL charr, showed peaks in stomach filling and fish caught in gill-nets around twilight in autumn. Depending on the morph, this may be linked with diel variation in availability of food organisms, and risk of predation by LL charr. 6. Strong correlations between ecology and morphology suggest that the benthic and limnetic morphs are adapted to the littoral benthic and pelagic niches respectively of Thingvallavatn. Low fish species diversity, discreteness, and stability of alternate niches in the lake may be important factors in reducing competitive interactions, and thus promoting coexistence by phenotype-specific preferences for resources.

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