Spawning Brown Trout by Wayne Sheridan
Brown trout spawn from the last week in October until the middle of December, with peak spawning occurs in early-mid November. After reducing their food intake in August due to low warm water temperatures, September (usually cooler and wetter) sees a dramatic increase in food consumption as they prepare their long spawning runs.They have to increase their food intake in order to develop the eggs and milt (sperm) for spawning, and for making their long and arduous runs up the rivers to their spawning beds.
Once the water temperatures drop to about 12C and the photoperiod (ratio of sunlight to night) and water levels are right, they start their migration runs. Their homing ability to their natal streams is absolutely amazing. In one experiment they took 50 female and 50 male brown trout from their natal stream (after they had run up to spawn), brought them across the lake and placed them up on spawning beds on another river. All the brown trout returned to their original stream, except for one male brown trout (guess he met a real cute female along the way, nature like real life). Their amazing homing ability will return them to exactly the same location from which they were hatched.
Once the female reaches the spawning grounds, she starts looking for the proper habitat to lay her eggs. She is very selective on the location of her redd (nest). She prefers a water depth of 24-45cm, but will spawn as shallow as 12-18cm. Water velocity also has a major determination on spawning site, water velocity optimal range 40-70cm/sec. Potential spawning sites are characterized by upwelling water through gravel or by water currents flowing downward into the gravel.
Water hydraulics are extremely important in allowing water to flow through the gravel to the eggs. As you can see the completed redd picture on the left (pictures courtesy of Andy McKee M.N.R.) on how the water flows upwards through the redd’s gravel (flowing left to right). They prefer gravel sized from 3-15mm, but will spawn in what’s available. Gravel at 8-12mm will allow excellent flow of water through them while trapping sand and silt in the upper 10 mm of the redd (forms a sand/silt barrier), but allowed water to pass through the redd to the eggs.
Now that she has selected her spawning site, she will start to make her redd (nest) and select her mate. She will dig out a hollow in the gravel, with her tail, using a fanning motion. This will help loosen and remove sand/silt and debris from the redd. The male brown trout will chase off competing males for the right to spawn with her. Once water temperatures drop to 7-9C, and photoperiod and water levels are right, spawning will occur. The female swims into the hollowed redd she has made and the male swims beside her. As she releases her eggs into the redd, the male releases his milt into the water in order to fertilize the eggs.
Sometimes a smaller male jack will sneak in on the other side of the female in order to spawn. This is actually beneficial to diversify the genetics of the offspring that he fertilizes. Once she has released some of her eggs, she will gently cover the eggs with gravel, using her tail. This action also cleans the gravel of sand/silt and debris, allowing better flow of water to the eggs.
As you can see from the picture, she cleans a lot of sand/silt out of the redd. She may place all her eggs in one large redd, or lay some of her eggs and select a different location for the rest of her eggs. This process is extremely stressful on both the male and female brown trout. The highest mortality rates usually occur shortly after spawning, especially for the female. If she doesn’t release all her eggs, they can become infected in her abdominal wall.
The eggs have may challenges to overcome before being hatched. Even though she lays many eggs (1,000-2,000), not very many eggs will survive to hatch and emerge into the river. Some eggs are not fertilized and other eggs are damaged when she places the gravel over them.
Sometimes another trout will pick the same location to spawn after she has, and destroys the redd. During a severely cold winter, some of the eggs will freeze, especially those closest to the surface. Floods into spawning grounds will damage and/or wash away the redd. Fungus can also infect the redd, spreading from egg to egg until the whole redd is destroyed. Insects can also damage some of the eggs. Excessive sand and silt (due to soil erosion) will smother and kill the eggs. Soil erosion is one of the major contributing factors in reduced spawning habitat.
The eggs slowly develop over the winter, surviving on their yolk sack. The water temperature determines their development. The higher the temperature, the faster the eggs develop, with hatching usually occurring from the end of April until mid May. This is timed so that there will be food available for the fry. If all conditions remain optimal and the egg hatches, the fry still has to work its way through the gravel and break through the sand/silt layer in the gravel until it is free. Its not surprising that only about 10% of the eggs laid actually emerge as fry, under ideal conditions.