The Yellowstone Lake Trout

Set in the midst of the world's first National Park, Yellowstone Lake lies at 7,731 feet and covers an area of almost 140 square miles. The prolonged winters at this altitude keep the lake frozen over for virtually half of the year, from about November to May. The surrounding countryside, a mosaic of pine and fir forest, alpine meadows and rocky bluffs, is blanketed through winter with up to five feet of level snow and drifts that are far deeper. In spring the snow is quick to melt: high summer temperatures develop even at this altitude.

The streams that enter Yellowstone Lake carry little in the way of mineral salts. So the lake produces relatively small populations of aquatic invertebrates. Midge larvae and pupae are the most abundant; on a warm summer evening clouds of adult midges that have hatched from the water form mating columns resembling plumes of smoke drifting along the lake shore. In the shallow lake margins freshwater shrimps or scuds are amongst the most numerous large invertebrates, while in the surface layers of the deeper water, clouds of tiny zooplankton (crustaceans such as Daphnia) graze the summer bloom of algal phytoplankton.

Long winters and a relatively low level of food production mean that the wild cutthroat trout in Yellowstone Lake do not grow big. Most scale less than a pound; a two pounder is a large fish; a trout of more than three pounds is a giant. It is worth noting, however, that Yellowstone cutthroat trout have grown up to 16 lbs. weight when introduced to other lakes, such as Red Eagle Lake in Montana, that formerly held no trout.

The native cutthroat wait until the May melt before they run from the lake to spawn in the feeder streams. The spawning run is very rapid, as the fish have ripe ovaries and testes when they set off on the journey upstream. Within a month of leaving the lake they have paired, cut their redds in the riverbed gravel, laid and fertilized their eggs, and returned downstream to the lake. It has been estimated that up to 20 per cent die during this mating period; but those that survive quickly regain their condition through the summer and early autumn, and most are ready to make a second spawning run by the following spring.

In the warm summer temperatures the eggs develop rapidly. After about five weeks the fry swim up through the gravel, and most head straight downstream to the lake. A small proportion remain in the spawning streams, however: a reserve population that could survive a catastrophe destroying the lake's stock. The young fish that remain in the river usually move down to the lake the following spring, though a few remain in the streams for up to two years.

In the lake the fry and smaller trout move to the deeper water where they feed almost entirely on plankton or food that has drifted out from the shallow margins. The larger and older fish concentrate in the shallower water around the shore, feeding on more substantial invertebrates such as the abundant midge larvae and pupae and shrimps, or on insects blown on to the water from the surrounding forest and scrub. In many lakes throughout the world the larger trout would prey very heavily on their smaller relatives; but the Yellowstone cutthroat is neither a cannibal nor a fish-eater.

The Yellowstone trout is the most thriving wild form of cutthroat. No other variety has ever been introduced into the lake, so the Yellowstone trout has neither suffered from competition nor had its genetic structure contaminated through hybridization. The lake also has been spared from commercial exploitation by net fishing. The survival of the Yellowstone cutthroat almost certainly owes much to the harsh conditions that prevail through the long winter, and to the protection provided by the National Park.

Yellowstone Park is famous for its hot springs and geysers. In the late 19th and early years of the 20th century, anglers liked to fish from the shore close to one of the hot springs. When a trout was hooked it was lifted from the water without a net and swung directly into the spring, where it died immediately. After a couple of minutes the rod was again, lifted, this time with the trout ready for eating. This practice was made illegal in 1912. But elsewhere in the world wild trout stocks were being over-exploited and allowed to dwindle. It is a pity that other lakes that once held their own special strain of wild trout were not given-similar protection.