Silde show - Bull Trout Spawning

  • In the fall, bull trout begin moving into tributaries of Chester Morse Lake to spawn. The peak of spawning activity occurs during the last week of October and first week of November in the Cedar River Municipal Watershed.
  • Bull trout are sometimes observed holding in pools immediately upstream of the reservoir before moving farther up river to spawn.
  • Bull trout excavate a pit for eggs by slapping their tails on the gravel. The digging process cleans fine sediment and sand out of the gravels, creating good conditions for egg development. After laying eggs, the fish covers the pit containing the eggs with cleaned gravel. The final product is a redd, as seen in the lighter colored gravel here marked with the white bag.
  • The Cedar River is the largest tributary of Chester Morse Lake, and generally 75 percent of all redds occur in this river. Because Chester Morse Lake is a managed as a reservoir for water supply, the water level varies depending on rainfall and water demand. Sometimes bull trout must navigate an extensive length of river where no spawning gravels are found before reaching spawning beds.
  • Long slow riffle/glide habitat are favored locations for redd sites in this system. SPU staff walk reaches of the river in the fall counting and finding locations for bull trout redds. These data provide one method to track changes in the bull trout population through time.
  • Often numerous bull trout redds are found in the same patch of river.
  • PIT (passive integrated transponder) tag antennas track the movement of tagged adult bull trout during the spawning season providing date and time stamps on entry and exit of the river system.
  • Many bull trout utilize spawning habitat immediately upstream of Chester Morse Lake where redds may become "inundated" as the reservoir refills in early spring. As water levels rise, the river begins to back up and a fine layer of sediment is deposited over the redd. The effects of this refill process are being investigated.
  • While many adfluvial bull trout observed in the river during the spawning season appear to be quite large (~450 mm), we frequently observe smaller individuals (~200mm) exhibiting bright spawning coloration.
  • Several sections of river are favored by bull trout and as evidenced by the white markers on each redd in this photo. Superimposition of redds is common in these reaches through the spawning season.
  • Finding bull trout redds early in the season while algae is present in the rivers is quite easy as the cleaning of gravel by fish creates bright "clean" patches in the riverbed.
  • Predators such as river otters are common during the fall months in reaches favored by spawning bull trout. Most spawning activity occurs at night.
  • Our understanding of spawning behavior by individual fish has greatly increased through an acoustic tagging project. One large male in this photo (with orange floy tag near dorsal fin) was detected making 21 separate trips between the reservoir and river system. On this particular trip, the fish moved into a small tributary, Cabin Creek, near a large wood debris (LWD) enhancement site to spawn.
  • Most spawning is complete by early December when snow begins to fall around Chester Morse Lake.
  • Upon completing spawning, adfluvial bull trout return to Chester Morse Lake.
  • Several redd sites have been monitored to determine emergence timing of fry during the spring in the Cedar River. These redds are covered with a "redd cap". As young fry swim up from the gravels, they drift back into the tail end of the net where we collect them to determine timing of emergence.
  • Young bull trout fry emerge from gravels at the earliest by the last week of February and continue to emerge through spring months.