Diet differentiation in polymorphic Arctic charr in Thingvallavatn, Iceland

In diets of four Salvelinus alpinus morphs - small benthic (SB), large benthic (LB), small limnetic (SL) and large limnetic (LL) - were analysed on a seasonal and diel basis. 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). 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. 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 c7 cm) and onwards. Size of snails eaten increased with increasing size of benthic charr. 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. LL charr may be ecomorphs of SL charr which, through ontogenetic shifts, switch from planktivory to piscivory at the size of 22 cm. All morphs except LL charr showed peaks in stomach filling and fish caught in gill-nets around the twilight in autumn. This may be linked with diel variation in availability of food organisms, and risk of predation by LL charr. 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. -from Authors