Blackfly

Blackflies are both a component of stream life and a measure of ecological recovery in the account of pesticide effects on salmon. Their larvae were among the aquatic insects killed by DDT; when blackflies returned relatively quickly, they helped feed newly hatched salmon, though their return could not restore the larger insects on which older young salmon depended.

Life in the Stream

The larvae live along stones beneath riffles and where water spills over steeply slanting rocks. Before spraying, they formed part of a rich community of aquatic life that also included caddisfly larvae in protective cases and stonefly nymphs clinging to rocks. These insects were food for salmon and trout, linking the small organisms of the stream to its larger fish.

DDT and the Break in the Food Web

After DDT spraying, the stream insects were dead, including blackfly larvae. The loss reached beyond the insects themselves: young salmon were left without the aquatic food on which their growth depended. The disturbance was not quickly undone. Smaller species, including midges and blackflies, could become reestablished relatively rapidly, but the larger aquatic insects required years rather than months to recover in numbers sufficient to support a normal salmon population.

Return and Ecological Consequences

In the spring of 1955, salmon fry in the Northwest Miramichi encountered a changed food web. Midges and blackflies had returned in numbers and provided their normal food. With abundant food and fewer older young salmon competing for it—the older fish had been killed by the previous year's spraying—the fry grew quickly and survived in large numbers. The blackfly's return was therefore a limited recovery within a much wider disruption: it benefited the youngest salmon, while the slower recovery of larger insects left later stages of salmon growth without the same assurance of food.