Each spring, smolts leave the Miramichi River to journey to the ocean, where they will grow to become adults and eventually return to the river. During the spring run, the Miramichi Salmon Association conducts smolt population estimates on the Miramichi River using a mark-recapture system. Through this program, we are able to calculate an estimated number of Atlantic Salmon smolts (age 2-3 years) as they leave specific rivers in the Miramichi to continue their journey into the Atlantic Ocean. This valuable information helps conservation groups and regulators make important decisions about the management of the Miramichi watershed.
The MSA puts rotary screw traps, known more commonly as smolt wheels, on the Miramichi River to capture fish for smolt estimates. These smolt wheels remain on the water from the beginning of the smolt run until the end.
A smolt wheel looks like a large funnel. The big end of the wheel faces upstream and the river current makes the wheel rotate. Fish swimming upstream are gently directed by the wheel’s rotation into a holding well. Smolt wheels can capture anywhere between a few to several hundred smolts per day, depending on weather conditions and the time of year.
Smolts captured in the smolt wheel are brought to shore to be counted and each fish is measured. Scale samples are taken to determine the smolt’s specific age. The Miramichi Salmon Association then applies a “streamer tag” (a type of tag that falls off naturally in the water after a period of time) to each smolt. Regardless of how many smolts are captured each day, field workers only tag up to 100 smolts per day to limit the number of fish being handled. Any untagged smolts are released where they were captured. Tagged smolts are released further upstream, about 5km from the site where they were captured. It takes tagged smolts about a day to reach the smolt wheel again where they will be recaptured. This method of capturing, tagging, and recapturing smolts is called the “Mark-Recapture” strategy.
The MSA uses three key pieces of information to figure out how many smolts left the river during the spring run:
1) The total number of fish captured in the smolt wheels,
2) The total number of fish that were tagged,
3) The total number of fish that were captured a second time.
These three numbers are put into a system that uses information about smolt runs and mortality to calculate a smolt population estimate. The Miramichi River needs approximately 1.8 million smolts (1.2 million from the SW branch and 600,000 from the NW branch) migrating to the ocean annually to support a healthy population.