Why do Arctic Grayling ( Thymallus arcticus) Get Bigger as You Go Upstream?

• Published in: Canadian Journal of Fisheries and Aquatic Sciences
• Main Authors: Hughes, Nicholas F., Reynolds, James B.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1994
• Subjects: Arctic grayling; Thymallus arcticus
• Online Access: http://dx.doi.org/10.1139/f94-216; http://www.nrcresearchpress.com/doi/pdf/10.1139/f94-216

During the summer Arctic grayling in interior Alaskan streams get bigger as you go upstream. We performed a fish removal experiment in a small stream to test two hypotheses about the mechanism that produces this size gradient. The first hypothesis was that all sizes of Arctic grayling prefer positions in the headwaters, but that large fish defend these, forcing smaller ones to occupy positions further downstream (the competition for ranked positions hypothesis). The second hypothesis was that small Arctic grayling actually prefer positions in downstream reaches, and that as they grow their habitat preferences change, and they move to positions further upstream (the size-dependent habitat preference hypothesis). Under this hypothesis, exclusion by other individuals plays no part in producing the whole-stream size gradient. Using the distribution pattern of fish immediately before the removal experiment we developed two sets of rules, one for each hypothesis, to predict how a smaller post removal population, with a different size structure, should be distributed. By comparing these predictions with the actual distribution pattern of the population two years after the removal, we found that the competition for ranked positions hypothesis made much more accurate predictions than the alternative.