Coastal Cutthroat Trout as Sentinels of Lower Mainland Watershed ...

38Populations of juvenile cutthroat trout and coho salmon decreased 81 and 69 %respectively, requiring a year to recover. Similar results have been recently obtained atMarble and Big Ditch Creeks which are agriculturally modified drainages located nearthe township of Abbotsford (data collected by S. Barrett, Ministry of Environment).There, rock weirs that increased water depths in the dredged ditch were partiallymitigative (Figure 10).In drainage ditches with little or no groundwater flows, there was evidence that “thalwegremoval” of very dense vegetation did improve dissolved oxygen concentrations to levelsthat could potentially support salmonids by decreasing biological oxygen demand (BOD),but further controlled monitoring is needed (Slaney and Northcote 2003). Clearly, theultimate solution is to terminate the annual practice of cutting ditch-bank vegetation andthen promote growth of a tree canopy to shade the channel, thereby eliminating excessivein-channel vegetation. However, under existing drainage management practices, it isevident from extensive dissolved oxygen monitoring that vegetative succession andanoxia occurs rapidly in non-groundwater ditches without periodic removal of excessivethalweg vegetation (Slaney and Northcote 2003).Riparian management guidelines for agricultural areas are available as part of the BritishColumbia Environmental Farm Plan (Hallmann and Trotter 2004). Over time, this shouldresult in more environmentally sound drainage practices as is now evident in countriessuch as Denmark.Figure 10. Minnow trap catches (per unit of effort) of juvenile cutthroat and coho ingroundwater-influenced agricultural drainages before and after invasive dredging (fromRosenau and Angelo 2005; data collected by S. Barrett)One of the most important conservation actions recommended for agricultural cutthroatstreams is the purchase of small key pieces of marginal farm lands, particularly onalluvial fans where key nursery streams for both cutthroat and salmon are located. Agood example is in upper McCallum Ditch in the Mountain Slough system where