Yearto Year Climate Variability and Grayling Growth

As seen in figure 5.6, there is a large amount of annual variability in streamflow and temperature in the Kuparuk River as a direct result of summer precipitation and air temperatures. The growth of the arctic grayling, the only species of fish in this river and in a nearby smaller stream called Oksrukuyik Creek, is strongly affected by the flow rate and by the temperature as a result of its life cycle.

The arctic grayling (Thymallus arcticus) is found in North America from Minnesota north to northern Canada and northern Alaska. Adults reach approximately 35 cm in length and live as long as 20 years. Grayling adults live in pools within streams and exclusively feed on drifting insects, such as immature stages of mayflies, stoneflies, black flies, and other stream insects. Spawning takes place in the spring as soon as the adults return from lakes where they have overwintered to their summer territories in the pools. The young live in shallow water along the edge of the streams where they feed on tiny insect larvae and other invertebrates.

Deegan et al. (1999) measured the growth of young-of-the-year and adult fish for a number of years (figure 5.9), and they have used the variability of the flow and water temperatures to make correlations with physical factors controlling growth. The young were measured in field samples collected throughout the summer, and their average weight in grams at the end of the summer was taken as the amount of growth. At the beginning of each summer, adults were caught on a barbless hook, weighed, tagged (in fact, most of the population is already tagged), and released. At the end of the summer the same fish are recaught, weighed, and released. In this way, the growth for each summer can be calculated as the grams added or lost per day. Measurements were made in control sections of the river as well as in sections where the primary productivity was increased by daily fertilization with phosphorus (details in Peterson et al. 1993).

The young-of-the-year fish grew best in warm and relatively dry summers when the stream discharge was low. This makes ecological sense because the young fish would find suitable habitat in small pools and the warm temperatures would encourage the growth of their tiny prey. In high-flow years the small pools are absent, and the young fish have to live in the stream itself.

In contrast, the adult fish grew best in cool summers with high flow rates. Under these conditions, a greater number of drifting insects move downstream compared with the number in low-flow years. In addition, the low temperatures result in a lower rate of metabolism for the fish that could lead to improved growth for the same amount of food eaten. In two relatively warm summers, the adults in the Ok-srukuyik Creek actually lost weight over the summer.

The adults of this population of grayling appear to grow reasonably well most sum-

Kuparuk River

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Oksrukuyik Creek

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Average Discharge (m3/s)

Figure 5.9 Interannual variability of adult and age 0 arctic grayling is related to river discharge and nutrient availability.

mers. The young survive and grow well only during exceptionally warm and dry summers with low flow. Thus, a part of the population does well no matter what the environmental conditions are during that year. However, the survival of the populations depends on the exceptional year classes that occur infrequently, every 6 or 7 years. The population as a whole survives only because the fish are so long lived—they can live for up to 20 years. It is likely that the great amount of climate variability at this site would not allow fish that only lived 6 or 7 years to survive as a population.

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