Abundance of Sport Fish in the North Raven River, Alberta, 2005
Mike Rodtka and Rocklyn Konynenbelt
The North Raven River (NRR), also know as Stauffer Creek, has long been regarded as one of Alberta’s finest trout streams. Located in west-central Alberta, Canada, the primary land use in the area is agricultural. By the 1960s it had become apparent that the effects of agricultural and rural development were negatively impacting the stream. The “Stauffer Creek Habitat Improvement Program”, developed by the Alberta Fish and Wildlife Division in 1973, outlined a plan to initiate habitat protection and improvement, while monitoring changes in the NRR’s physical characteristics and fish populations. This monitoring included trout abundance estimation in 1973, 1985, 1995 and 2005 at four study sections along the NRR using electrofishing gear. Our report presents data on trout abundance from the 2005 study, and summarizes past survey data for comparison.
We estimated trout abundance via mark-recapture surveys at the four study sections previously surveyed using methods comparable to those used in past surveys. Mean brook trout abundance in the upper NRR fluctuated from an estimated low of 37 fish/km (95% CL = 28 – 51) in 1995 to a high of 193 fish/km (95% CL = 159 – 242) in 1985. Brown trout abundance fluctuated from a low of 124 fish/km (95% CL = 103 – 151) in 1973 to a high of 744 fish/km (95% CL = 693 – 806) in 1985. Since 1995, brook trout abundance in the NRR appears to have increased, whereas brown trout abundance has decreased. Mean biomass of trout in the study area ranged from an estimated low of 25.4 kg/km (95% CL = 20.8 – 31.4) in 1973 to a high of 86.0 kg/km (95% CL = 75.8 – 99.0) in 1995. Brook trout biomass in the upper NRR ranged from an estimated low of 2.2 kg/km (95% CL = 1.5 – 3.3) in 1995 to a high of 7.2 kg/km (95% CL = 5.7 – 9.5) in 1985. Brown trout biomass was lowest in 1973 at 20.7 kg/km (95% CL = 16.1 – 26.7) and peaked at 83.8 kg/km (95% CL = 73.7 – 97.0) in 1995. Since 1995, brook trout biomass appears to have increased, whereas brown trout biomass appears to have decreased.
The size (length) structure of brook trout and brown trout catches varied among survey years. The brook trout catch in 2005 was composed of significantly larger fish than in 1995 (P < 0.0001), although condition did not differ between years. The brown trout catch in 2005 was composed of significantly smaller fish than in 1995 (P < 0.0001); approximately 50% of the 2005 catch was of fish ≤ 100 mm fork length. No clear trend in condition of brown trout was apparent since 1995. Generally, fish condition tended to decrease with increasing abundance among survey years, and tended to decrease with increasing fish length for both species across years. The most striking trend in the dataset was the general increase in trout biomass which has occurred since riparian habitat conservation and stream improvement initiatives began in the early 1970s.
The 2005 survey results indicated that trout abundance and biomass in the upper NRR declined since 1995, but were still well above the lows in abundance and biomass documented in 1973. It appears the most recent decline in trout abundance and biomass has been driven by a reduction in brown trout abundance and biomass, which accounts for the greatest proportion of trout biomass in the stream. Evaluation of longterm changes in abundance and biomass of trout in the study area are complicated by a lack of consecutive-year trend data. It remains unclear whether changes in abundance and biomass observed in the most recent survey were indicative of a long-term trend or were an artifact from limited sampling of a dynamic system.