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By Guillermo Giannico Fisheries biologists use a wide variety of tools and methods to estimate fish abundance. In the case of salmon, both spring trapping of juvenile fish - as they migrate from their freshwater habitat to coastal waters - and fall counts of adult fish returning to their natural streams to spawn, are good examples of methods used to obtain data in fisheries research. In general terms, this type of work is carried out in watersheds with large wild salmon runs (i.e., without hatchery enhancement) and, when possible, adequate historical records. Such rivers or streams are considered "index" systems and are used as indicators of regional trends in juvenile salmon production and spawned returns by the Department of Fisheries and Oceans (DFO). The Salmon River, in Langley, has been designated and "index" stream for Coho salmon by the DFO because this watershed supports one of the largest wild runs of Coho salmon in the Lower Fraser Valley. In the past, seasonal juvenile Coho salmon trapping in the Salmon River was carried out at Williams Park using temporary wooden traps, and estimation of spawned numbers was done by trapping and marking some adult fish near the mouth of the river and subsequently counting the proportion of marked vs. unmarked carcasses that were found in the spawning grounds. Although, the methodology used for estimating fish abundance was appropriate, the fish fences and trap boxes that were used as well as their location in the watershed were far from ideal. The relatively small and weak wooden fences and traps were not only rendered useless some years during peak water flows but their location, in the middle reaches of the river, prevented all juvenile salmon downstream from Williams Park from being included in the population census. It was obvious that a permanent counting structure was needed in the lower reaches of the Salmon River to ensure that more accurate data were collected. Thus, the new Salmon River fish counting fence was built near the corner of Glover Road and Rawlison Crescent. The land adjacent to the fence is part of a family farm and a lease agreement was reached with its owners which allows the DFO to use the site in return for providing bridge access for cattle to hay fields across the river. The basic fence is an adjustable aluminum "rake-like" weir that reaches the bottom and spans the width of the river. When used to capture returning adult Coho salmon, the rake fingers are half immersed and direct fish through a "funnel" and into a box trap. Afterwards, those fish are measured, counted and some of them are tagged (this is done to evaluate the reliability of the marking methods used estimate fish abundance). For capture of downstream migrating juveniles in spring, the weir is fully immersed and covered with �" plastic mesh. The mesh keeps the small fish from passing between the fingers of the weir and helps direct them into a box trap. A 56� long steel bridge provides the anchor points for 6 electric winches that raise and lower the weir to adjust its height to match the river level. The bridge represents an excellent example of how a decommissioned railway flat car can be recycled to be of use again and serve an entirely different purpose. If you are in the vicinity of Fort Langley during November and early December, stop by the fence and you will be able to witness how adult fish (mostly Coho salmon) are handled and marked in their upstream spawning migration by the DFO�s stock assessment staff. If you visit the fence during May or early June you will have the opportunity to observe and learn how juvenile Coho salmon are marked and counted. Because at that time the fence becomes a temporary barrier for the downstream migration of recently spawned adult Steelhead trout (note: unlike Pacific salmon, Steelhead trout do not die after spawning and may live to spawn two or three times), you may also be able to meet some volunteers from the Salmon River Enhancement Society who help move the trout past the fence. This �rescue program� represents a temporary solution that needs to be replaced by a better alternative because the stress those trout suffer as a result of it is likely to have a negative impact on their survival. |