By Liz Jones
This year, the Lilwat Fisheries Commission in Mt. Currie and Fisheries and Oceans Canada are collaborating on the operation of a sockeye salmon enumeration fence on the Birkenhead River.
The fence, an upstream pointing "V" shape, spans the Birkenhead River and is 200 feet long. It was installed over a three-day period and became operational on Saturday, Aug. 30. To count sockeye salmon, the fence has an adjustable width opening where fish are allowed to pass through on their migration upstream to the spawning grounds. The salmon are not stopped in their upstream migration; however the fence is occasionally closed for safety reasons or emergency situations.
To enable accurate nighttime counting, lights are secured to a wooden counting booth where the crew is situated. Every hour, the fence is cleared of any leaf or wood debris that may float downstream and impinge water flow through the fence.
The fish migrate 24 hours a day, with peak migration numbers observed moving through the fence early in the morning. For instance, between 7 a.m. and 11 a.m. on Monday (Sept. 15), 11,196 fish swam through the fence.
As of midnight on Sept. 14, a total of 147,173 sockeye have passed through the fence.
The project is scheduled to run as late as Oct. 13, at which time all of the fence material will be totally removed from the site.
This week the fisheries crew started the sockeye biological sampling program. Sockeye salmon die after spawning, and the carcasses are examined and sampled for biological information such as sex, length, age, and success of spawning. The technicians will sample a specified number of spawned sockeye until the end of the run.
Other species of salmon are also enumerated. In addition to the above, we have been using a helicopter for over-flights of the Birkenhead River to count chinook salmon from August until September. Chinook carcasses are also recovered and sampled for biological information.
Regular surveys of spawning areas for coho salmon will begin in October and run until the end of January.
For more information, contact the Lilwat Fisheries Commission office at 604-894-1212.
The history of sockeye salmon enumeration on the Birkenhead River is one of the longest of any salmon population in the Fraser River system. The first estimates were made by enumeration fence from 1905 to 1935, though not all years had complete counts as heavy late summer and fall rainfall often caused the fence to wash out. In 1939, the first mark-recapture technique of enumeration of a Fraser River salmon stock was applied to Birkenhead River sockeye, and this continued almost every year up to 1999. The early work here established the basis of later studies not only in the Birkenhead River, but all other Fraser River areas.
Since 2000, various other enumeration methods have been attempted with mixed success. 2003 marks the first attempt at constructing and maintaining an enumeration fence across the river since the early part of the century.
Historical abundances of sockeye salmon returning to the Birkenhead River range from as low as 15,000 in 1939 to as high as 350,000 in 1986, and have averaged 90,000 fish. The 2003 return is turning out to be one of the largest on record.