Skagit Bull Trout Primer Ch 2: The Spawn

The Skagit bull trout spawning grounds typically define the upper basin limit reached by the various migrating salmonids. In one case the bull trout migrate more than 115 miles from tide water while climbing more than 3,000 feet in elevation to their spawning grounds. As one can imagine, to reach those areas in the Cascade mountains the bull trout likely encounter a variety of potential obstacles, log jams. Long stretches of steep gradients, velocity barriers, waterfalls, and even potentially dewatering of the stream channel. To consistently reach these spawning sites the trout have developed a variety of strategies to deal with potential obstacles. While the bull trout spawns in the fall as the water temperature drops, they begin their upstream migration months in advance taking advantage of the variety of flows available as the summer snow melt runoff begins to decline. This ensures that as the fish reach a barrier they have several strategies to achieve successful passage. If the high flows through a chute form a velocity barriers (the water flowing at a speed faster than the fish can swim) the fish has the time to wait for the falling flow/velocity so they can successfully swim past the barrier. Other times, the fish can take advantage of the higher flows by seeking side overflow channels (a natural fish ladder) that may provide an alternative path around some barriers. Bull trout are also decent leapers and often are able to jump over what initially appears to be a significant barrier. One of the interesting bull trout behaviors occurring at these barriers is the fish rising in the water column lifting their heads out of the water with all the appearance of scanning and assessing the situation. In one eastern Washington basin the fish exhibited this behavior so consistently they nicknamed the falls “peak-a-boo falls”. With this diverse collection of strategies, the region’s bull trout often can access habitats that even the Chinook salmon’s brute strength or steelhead’s acrobatic ability are unable to conquer.


Photo by @Rvrfisher360

Once the bull trout near their likely spawning ground, they seek out secure holding areas that may be "home" for a few weeks to several months. They seek areas such as large pools, log jams, root wad, undercut banks, that can provide cover. These fish are true masters of finding and taking cover, it is amazing how many of these relatively large fish can defy detection in relatively small streams. The fish remain in these holding areas awaiting favorable water temperatures. Water temperatures dropping to 8 degrees Celsius or about 45 degrees Fahrenheit trigger the onset of spawning. If the water later warms above that trigger point during the spawning process all spawning stops. Spawning will resume once the stream temperature drops below that threshold.

The female bull trout begin the spawning process by searching for a suitable spawning site as soon as the temperature drops below that temperature trigger. One can watch those pre-spawn females cruise the gravel shallows (often small pebbles to fist size stones) checking potential sites with a test dig or two which can include arching over the site with the appearance of testing the site for potential upwelling flow with their anal fin, assuring adequate oxygenate water flowing over the eggs. The ultimate redd (spawning nest) site size can vary considerably depending on the type of habitat available in that stream. An ideal site would be in 6 to 12 inches of water with the shoreline or other cover protecting one side of the redd and maybe some overhead cover as well. Once satisfied with a site the female begins the spawning process by facing the current and rolling over on her side where she is uses the lateral motion of her tail to create hydraulic pressure to lift the stones which are moved downstream by the current creating a depression in the gravel. In the company of one or more males, she then positions her vent over that depression to release a portion of her eggs. As she releases her eggs the dominant male will move aside her releasing his milt at the same time to fertilize the eggs. After releasing her eggs, the female moves immediately upstream of that depression to create another depression for the next deposition of eggs and at the same time covering the freshly laid eggs with protective gravel. The female will repeat that process as often as 3 or 4 times in an hour at peak spawning activity with the redd being competed in a day or two. A typically redd maybe only 1 to 2 feet wide and up to 4 feet long with the ultimate redd size influenced by female size, the gravel patch area, and the current over the site. The fecundity (number of eggs) of each female depends on her size; with the small resident fish having as few as couple hundred eggs while the largest females having as may as 10,000. Surprisingly, the egg size in a 30-inch female is not much larger than those of 10-inch fish.


Photo by @Long_Rod_Silvers

Both the female and the males aggressively protect the redd site during the spawning process. The female will actively protect the redd by presenting her side to a potential female competitor for the site and move aggressively towards her forcing her from the from the area. The males exhibit much more aggression in their efforts to protect “spawning rights”. Typically, more than one male (up to 5 or 6) will compete for that right. The dominant male (usually the largest fish) will position near the female (typically on the mid-channel side) ready to move along her side as she positions to release her eggs. Between the egg releases the dominant male chases and at times physically attacks his competitors. Those physical attacks can be quite vicious with the dominant fish grabbing the other male with his toothy jaw to either shake or roll that competitor. The loser of the battle receives a variety of injuries including torn fins, body slashes, and in the most extreme cases, even thrown from the stream. In my experience the most vicious male on male conflicts in the salmonid world occur with our bull trout. Despite the efforts of the dominant male it is not uncommon for one or more the secondary males to sneak in on the spawning. Those “sneakers” position themselves on the offside from the dominant male and attempt to also fertilize the eggs. Amazingly even the smallest of the mature males (those with resident lifecycles) will also successfully spawn with a much larger female usually as a “sneaker”. This interaction on the spawning grounds brings the diversity of life histories back full circle.

As spawning approaches, the maturing males take on vivid colors with enhanced white fin edges, darker side and back color (usually dark green at times approaching black), and an increased contrast in the body spotting. It is common in pre-spawn aggregates for the larger males tended to be the most highly colored. At first it was thought that perhaps since those larger fish had spawned before they might mature earlier. However, in observing actively spawning bull trout it became apparent that the most intensively color male is the dominant male. Remarkably if the dominant male is replaced (leaves the area or a competitor wins the battle) the new dominant fish's colors will quickly intensify (withinin minutes) and the displaced male colors will fade.

The whole active spawning period is fairly short; typically lasting 3 to 6 weeks depending on the weather and corresponding water temperatures. Once a pair completes spawning, the female quickly leaves and the males don’t stick around much longer. At the end of the spawning period the whole migratory population will have vacated the area. Whether in the migrating, pre-spawn, or spawning stages, the fish can experience mortalities from a variety of sources, including stranding and predators (blue herons, eagles, osprey, bears, otters, minks, etc.) and of course anglers. Some years it is estimated 10% or more of the expected spawners (based on snorkel or redd counts) may die before spawning.


Photo by @M_D