Population processes responsible for larger-fish-upstream distribution patterns of Arctic grayling (Thymallus arcticus) in interior Alaskan runoff rivers

During the summer months, Arctic grayling (Thymallus arcticus) in Alaskan s treams adopt a larger-older-fish-upstream distribution pattern. In this pap er, I analyse data from two large interior Alaskan rivers to determine how population processes maintain this size and age gradient. These analyses su pport the hypothesis that age-phased recruitment and growth-dependent movem ent are primarily responsible for this distribution pattern. Age-phased rec ruitment describes the way that the mean age of fish recruiting to a reach increases upstream, from ages 0-1 in the lower river to ages 3-7 in the hea dwaters. This process begins with the concentration of spawning fish, and t he resultant fry, in the lower reaches of the river. Downstream movement du ring the first year of life further concentrates young fish in the lower ri ver. Over time, the distribution of this cohort broadens steadily as indivi duals move further upstream, so that fish recruiting to headwater reaches a re 3-7 years old. This process contributes to both size and age gradients. Growth-dependent movement magnifies the size gradient by sorting fast-growi ng fish into the upper river and slow-growing fish into the lower river. Th is sorting results from the fact that individuals making long-distance upst ream movements tend to have grown particularly rapidly that year, while ind ividuals making long-distance downstream movements tend to have grown espec ially slowly that year. I rejected the hypothesis that age and size gradien ts are the result of whole-stream gradients in growth or mortality acting o n a sedentary population. However, there was some evidence that fish did gr ow more slowly in the lowest 40 km of one river, although this made only a minor contribution to the size gradient and growth rates were remarkably co nstant for the next 120 km. There was no suggestion that spatial variation in mortality rate contributes towards the size or age gradient, but natural and sampling variability could have obscured small but significant differe nces between reaches.