Stocks are Chameleons: Plasticity in Gill Rakers of Coregonid Fishes

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Abstract
The characters that apparently render a stock of fish distinctive may in reality be a reflection of the presence or absence of other fish species with which the stock shares its environment. Over the northern hemisphere, the whitefish genera Coregonus, "Leucichthys," and Prosopium offer striking examples of morphological characters being displaced because of coinhabiting species. Whenever one of the groups is missing, another tends to shift its gill rakers so as to fill in the vacant part of the spectrum in gill-raker counts. Plankton-feeding lake whitefish (Coregonus clupeaformis) having many long gill rakers exist in several lakes sympatrically with bottom-feeding lake whitefish having fewer and shorter gill rakers, but only if the even more specialized plankton-feeding species of ciscoes are absent. Lake whitefish in Yukon Territory have on the average fewer rakers in those lakes that contain ciscoes than in those that do not. Artificial transplants demonstrate that gill-raker characteristics can change quickly in a new environment. Gill-raker count is less subject to phenotypically induced variation from environmental influences than are most morphological characters, but it is not immune. Partial hybridization, where bimodal populations are maintained by a balance between gene flow and selection, also poses special problems for the stock concept. When employing even relatively stable characters such as gill-raker counts, one should beware that the "stocks" may be chameleons which shift their appearance in response to the biological color of their surroundings.Key words: coregonids, whitefish, gill rakers, phenotypic variation, hybridization, selection, transplants, stocks

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