Eastslope sculpin COSEWIC assessment and status report: chapter 9

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Population Size and Trends

To date, no studies have provided a quantitative estimate of population size for the "Eastslope" sculpin. However, various studies have measured relative abundance. The "Eastslope" sculpin is considered locally abundant where it is found in Alberta (R.L. & L. 1987; 2002). Surveys in 2000 and 2001 found that the "Eastslope" sculpin had the highest or second highest relative abundance of all fish species encountered in the Milk and St. Mary rivers where they were found (Table 1), although abundance was dependent on season sampled (R.L.& L. 2002). In these surveys of the Milk River, the highest abundance values for sculpins were observed in the North Milk River and decreased downstream to where they were absent in the lowest section of the Milk River mainstem (R.L. & L 2002). A similar pattern was observed in earlier surveys (R.L. & L. 1987), and is likely due to higher abundance of suitable spawning and rearing habitat in the upper reaches (R.L. & L. 1987). In contrast, sculpins were evenly distributed throughout the St. Mary River. In Lee Creek, they were present in the lower section as far upstream as 6 km above Cardston. These assessments of abundance were conducted before the extreme drought conditions experienced, particularly for the Milk River, in fall 2001. However, limited surveys in October 2002 did not indicate a noticeable change in abundance (P&E 2002). These latter surveys were conducted to evaluate fish populations in the Milk River and were concentrated mainly in the furthest downstream section of the Milk River (i.e., from the international border to 57 km upstream), as well as in the lower North Milk River and at the confluence of the Milk River and North Milk River. Similar to previous studies, the results of these surveys found sculpins to be absent in the furthest downstream section of the Milk River to the international border (P&E 2002). However, in the upstream section, the "Eastslope" sculpin was the most abundant fish species encountered.

Trends in population size are difficult to evaluate given the limited information available and the variability in season and location of sampling. Some variation in relative abundance over time is evident, but these changes do not appear to be consistent (Table 2). For example, fall relative abundance values based on catch-per-unit-effort (CPUE) appear to have increased in the upper North Milk River when comparing the results of a survey conducted in 1986 to those of 2000-2001 (R.L. & L. 2002). The most recent fall CPUEs collected in 2002 suggest a slight increase in abundance near the confluence of the North Milk River and in the lower North Milk River (0.5-2.44 fish/min.) compared to sites near the confluence sampled in 1986 (0-0.59 fish/min.) (R.L. & L. 1987; P&E 2002). However, summer 2000 values in the North Milk River are much lower than summer values collected in 1989 by Paetz (1993). Similarly, summer values for the Milk River near the town of Milk River were considerably lower in 2000 compared to 1989 (R.L. & L. 2002). Fall values were similar and low for the lower Milk River in 1986 and 2000 (R.L. & L. 2002). Unfortunately, the most recent surveys in October 2002 did not include sites immediately downstream of the Town of Milk River, where sculpins have been noted previously. In the St. Mary River, summer CPUE values appear to have remained relatively stable from 1989 to 2001 (Table 2). In comparison, summer CPUE values in Lee Creek increased considerably from 1989 to 2001 (Table 2).

Estimates of abundance may be influenced by season, as well as water flow conditions and the ability of survey crews to capture fish. In addition, the population size may fluctuate slightly from year to year depending on migration rates through the St. Mary Canal into the Milk River. It is therefore impossible to determine whether the population in the Milk River is stable, declining or increasing. Given the recent drought conditions, the population may have experienced at least a slight decline in numbers even though the most recent catches in October 2002 (P&E 2002) suggest that the sculpins are still one of the most abundant species in the lower North Milk River. There is no evidence to suggest the population in the St. Mary River mainstem has experienced a significant change in numbers, although a significant increase in Lee Creek was observed (R.L. & L 2002).