Incubation | CSKT | Division of Fish, Wildlife, Recreation, & Conservation

Bull Trout Incubation

As with spawning, this next stage—incubation—is essentially the same for resident and migratory bull trout. Bull trout eggs and embryos buried in their redds need a constant flow of oxygen-rich cold water—the temperature should be below 46° F—and clean gravel/cobble bottoms. If they have these conditions, the eggs will become young bull trout, or fry as they are called, and emerge from their redds up to 8 months later, when water temperatures are just right.

In the redds, the eggs go through several changes, the most obvious being the eyed stage, when eyes develop and can be seen as black dots in the eggs. The embryos then hatch out of the eggs. They are now called alevins and look like tiny fish with big, round orange bellies. These bellies are the remaining yolks, which continue to provide nutrients to the immature fish as long as they are in the alevin state. Alevins can't really swim, and so remain safe in the gravel until they consume their yolk sacs and become fry.

Because they no longer have yolk sacs, the fry have to leave the protection of the redd to search for food. Their departure from the redd marks the end of the incubation period.

In the Redd

After spawning and the eggs are deposited and buried in October, the eggs are incubated in the redd for several months before hatching in January. The alevins, which can't swim, remain in the gravel until they absorb their yolk sacs in about mid April (200 days after egg deposition). All the time, the current flows through the gravel and cobbles, carrying oxygen to the embryos and washing away wastes. Without that flow of water and the oxygen it carries, the eggs or alevins would suffocate and die.

Oxygen and Eggs: A Matter of Survival

Low oxygen kills salmonid eggs and alevins. That's because just like you and me, the embryos have to breath. Survival rates are directly proportional to the average levels of oxygen the eggs experience during their development. So a constant flow of cold, oxygen-rich water over the eggs and alevins is critical.

From Egg to Alevin

After egg deposition in the fall, relatively warm temperatures facilitate rapid embryonic development. These warmer temperatures, usually 40-50°F, are followed by much colder winter temperatures that approach freezing and subsequently decrease the rate of embryonic development. As winter progresses into spring and temperatures increase, the rate of development is again increased.

Incubation can be divided into four stages:

(3) The alevin stage. The larvae has hatched but has not fully absorbed its yolk sac, has not yet emerged from the gravel of the redd, and cannot swim well. During this stage, the larvae slowly absorbs its yolk sac (its energy source) while its mouth, digestive tract, and excretory organs develop. This stage takes another 65 to 90 days.

(4) The emergence of fry. After the yolk sac is absorbed, the young trout, now about an inch long, is called a fry. It can swim, and it moves up and out of the gravel (emergence). Immediately it darts to the surface and gulps air to fill its air bladder. From this point on it is a free-swimming fish, and it begins to forage for food.

The total incubation time, which depends on water temperature, takes from 200 to 220 days (from October to emergence in mid April to May). The optimal water temperature during this time is from about 36 to 39°F.

CTUs: A Tool for Predicting Early Development

The term “Celsius Temperature Units” (CTU's) sounds complicated but it really isn’t. Here is the formula: A CTU is one degree Celsius for 24 hours. For example, a water temperature of 10° C for one day equals 10 CTUs. Or, 10° C for 12 hours (half a day) is equal to 5 CTUs. Degree days is another term for the same concept.

CTUs are useful because temperature affects bull trout in critical ways— everything from the rate at which eggs develop to the amount of food that fry require and the amount of dissolved oxygen that water will hold. So CTU measurements are used by biologists and in hatcheries to determine the stage of egg development, and to predict the date eggs will hatch. While the method is surprisingly accurate, other factors in the wild, such as oxygen level and water flow, affect development too.

Click on the page 2 button above to see the number of CTUs required for each stage of early bull trout development.

CTUs        Appearance of Developmental Features in Bull Trout Eggs & Alevins

167            Forebrain, midbrain and two areas of hindbrain formed
206            Eyes with a few pigment cells formed
245            Beating heart is now apparent
288            Jaws are forming
332            Blood is now moving through the gill arches
443            Some eggs are hatching, tail fin rays are well defined
503            Hatching is nearly complete
548            Mouth is opening and closing regularly, and swimming can be observed
687            Pelvic fin rays poorly defined
810            Yolk sac is no longer apparent, parr marks appearing

Survival

Female bull trout take great care in selecting the locations for their redds and in redd construction to ensure maximum survival of their young. But during incubation, eggs and alevin are extremely vulnerable and many will die from scouring during high flows, deposition of fine sediments, freezing during low flows, predation or stranding. In a tributary to the North Fork of the Flathead, one study found about 53% of eggs survived to emergence (fry stage). And survival rates can be even lower.

Many bull trout eggs and larvae perish during high flows when the gravels and cobbles of redds are moved by the force of the rushing water, crushing or exposing the buried eggs or alevins. Fortunately, high flows capable of disturbing redds are uncommon in the fall.

Alevins and eggs sometimes freeze in the winter during low flows. Female bull trout try to reduce the risk of this happening by choosing redd sites in areas where groundwater is flowing into the stream. In winter, the groundwater is warmer than the surface water and so can keep the redds from freezing if conditions are not too severe.

Eggs and alevins can also die when fine sediments cover or infiltrate the redds, reduceing the rates of water flow and dissolved oxygen reaching the eggs and alevins. Entombment appears to be the largest mortality factor in inubation studies in the Flathead. Poor land use practices—poor road design for timber sales, driving through streams, overgrazing, and clearing riparian zones, to name a few—can increase killing sediments.

Stranding and dewatering also kills. Redds are described as stranded when flows drop so much that the surface of the redd is above the surface of the water, and as dewatered when the surface of the groundwater drops below the egg pockets. Eggs can tolerate temporary dewatering because oxygen exchange continues as long as the eggs remain moist, but alevins perish quickly.

Although bull trout eggs and alevins in redds are relatively safe from predation, they are not completely so. Invertabrates like oligochaete worms can cause egg mortality, and small sculpins can burrow down into redds, especially in coarse gravel, and there is indirect evidence that they consume eggs of some salmonids. Newly emerged fry that have not yet filled their air bladders are also highly susceptible to predation.

Bull Trout Incubation

In the Redd

After spawning and the eggs are de-posited and buried in October, the eggs are incubated in the redd for several months before hatching in January. The alevins, which can't swim, remain in the gravel until they absorb their yolk sacs in about mid April (200 days after egg deposition). All the time, the current flows through the gravel and cobbles, carrying oxygen to the embryos and washing away wastes. Without that flow of water and the oxygen it carries, the eggs or alevins would suffocate and die.

Oxygen and Eggs: A Matter of Survival

From Egg to Alevin

(1) The pre-eyed stage. During this time the egg is a uniform redish-orange color. Cell division begins as does the development of pre-organ systems. This stage is very sensitive to disturbance.

(2) The eyed stage. Once the eyes become pigmented, the egg is said to be in the eyed stage. The embryo, less sensitive now, begins to develop its organs. Stages one and two combined take from 100 to 145 days.

(3) The alevin stage. The larvae has hatched but has not fully absorbed its yolk sac, has not yet emerged from the gravel of the redd, and cannot swim well. During this stage, the larvae slowly absorbs its yolk sac (its energy source) while its mouth, digestive tract, and excretory organs develop. This stage takes another 65 to 90 days.

(4) The emergence of fry. After the yolk sac is absorbed, the young trout, now about an inch long, is called a fry. It can swim, and it moves up and out of the gravel (emergence). Immediately it darts to the surface and gulps air to fill its air bladder. From this point on it is a free-swimming fish, and it begins to forage for food.

The total incubation time, which depends on water temperature, takes from 200 to 220 days (from October to emergence in mid April to May). The optimal water temperature during this time is from about 36 to 39°F.