Discrete prey availability promotes foraging segregation and early divergence in Arctic charr, Salvelinus alpinus

Many animal species show individual foraging specialisms when potential prey requires prey-specific foraging strategies. Arctic charr are often found as benthic (macroinvertebrate) or pelagic (plankton) foraging specialists. Here, we tested specifically if given a choice of prey with different characteristics individuals would specialise in a single prey type and if individuals would chose prey based on their expressed trophic morphology, in a laboratory experiment and in a field observation. When offered a choice of benthic and pelagic prey most individuals (73%) showed that 100% fidelity to a single foraging source. Na < ve individuals (not previously exposed to natural prey) with more robust head and mouth shape were more likely to forage on a benthic prey source (chironomids). In contrast, individuals with a more fusiform body, larger eye, but more slender head shape were more likely to specialise on pelagic prey (Artemia). Field observations of a natural population of Arctic charr from Loch Doine identified specialists foraging on either plankton or macrobenthos (on the basis of stomach contents) and some generalists. Morphological analysis showed that significant differences in shape reflecting recent foraging history. These results support the hypothesis that the availability of discrete, different prey types results in discrete foraging specialisms which in turn may result in the expression of discrete alternative phenotypes through subsequent plastic ontogenetic process. We conclude that this provides a partial explanation for why ecologically driven evolution processes are particularly prevalent in fishes from post-glacial lake systems.