Bow River Sport Fish Population Monitoring, 2003 and 2005
Trevor Council and Travis Ripley
The lower Bow River within and below the city of Calgary provides a quality sportfishery for rainbow trout, brown trout and mountain whitefish. In an effort to maintain the lower Bow River as a quality sport fishery and destination point for anglers, monitoring of sport fish populations is a high priority. Current data was required to identify population trends and to provide updated information for a review of the sport fish regulations. As a result, a 4-km index site downstream of Calgary was sampled in 2003 and 2005 to monitor sport fish populations in the Bow River. These data collections were then compared to previous sampling events.
Population sizes of brown trout, rainbow trout and mountain whitefish remain within the range of long-term population fluctuations. For all species, population size based on all size classes of fish was lower in 2005 than in 2000 and 2003. It is suspected that large flow volumes associated with a flood event in June 2005 resulted in the temporary displacement of fish that may have had implications on population sizes evaluated post-flood in 2005. This was most evident for mountain whitefish, which exhibited a population decrease of over 75%.
Relative species abundance remained unchanged between 2003 and 2005, with rainbow trout constituting approximately 50% of the catch. This result differed from surveys in 1999 and 2000 where mountain whitefish was the most numerous species captured at 54.3% and 47.6% of the catch, respectively. In all years, brown trout made up approximately 20 - 30% of the catch.
There was little change in the size distribution of brown trout between 2000, 2003 and 2005. Increasing numbers of brown trout were typically found in the 150 - 300 mm fork length (FL) range. Catch rate in 2003 was the highest for most size classes. Similar to brown trout, the size distribution for rainbow trout showed little change among the three study years. A higher number of rainbow trout in the 320 - 400 mm FL range was captured in 2005 compared to the earlier study years, likely due to displacement of smaller fish from the June flooding event. Catch rates for larger rainbow trout (> 400 mm) were also much higher in 2003 and 2005. Mountain whitefish catches were more variable, but tended to be evenly distributed across all size classes. The 2005 catch represented the lowest mountain whitefish catch rate for almost all size classes among the study years.
Mean relative weights of brown trout, rainbow trout and mountain whitefish indicated healthy individuals in all size categories. Individual fish were often heavier than average for their respective length. Age range of fish remained unchanged between study years for all species, with catches typically including ages-1 to 7. Growth rates of brown trout and rainbow trout showed little change across study years, whereas growth rate of mountain whitefish increased in 2005. Rainbow trout growth rate remained high, exceeding similar populations in Alberta and the U.S.