Abstract:
During the summer months Arctic grayling in interior Alaskan streams get bigger as you travel from downstream reaches to the headwaters. On a smaller scale, within individual pools, the largest fish holds position in the middle of the current, near the deepest part of the pool, and smaller fish hold positions progressively further downstream or to the side of the pool. The results of this study support the hypothesis that a single process--competition for profitable feeding positions--produces both the whole-stream and within-pool distribution pattern. Field experiments showed that competition for desirable positions is responsible for the distribution patterns adopted by groups of fish sharing a pool, and for the size-gradient of fish over the length of the stream. In both cases large fish excluded smaller ones from the most desirable positions. Modeling work suggested that Arctic grayling locate and rank positions on the basis of profitability. Within pools this conclusion was supported by a close fit between the positions predicted by a foraging model and the positions actually selected by Arctic grayling. Over the length of the whole stream this conclusion was supported by the model's prediction that feeding positions become more profitable as you go upstream.
