Science Advisory Report  2013/032

Recovery potential assessment of Mountain Sucker (Catostomus platyrhynchus), Milk River populations (DU2)

Summary

• The known distribution of Mountain Sucker for DU2 is limited to three drainages areas within the Milk River basin: the North and Milk rivers; a few tributaries of the Frenchman River; and Battle Creek and one of its tributaries, Nine Mile Creek. 
• This species is abundant in the North Milk and Milk rivers in Alberta and its population trajectory appears to have been stable for several decades. Abundance of Mountain Sucker is low to medium in the Frenchman River system and low in the Battle Creek system; population trajectory is unknown in both systems.
• In DU2, Mountain Sucker generally inhabit clear, cool mountain streams ranging from 2 to 40 m in width with predominantly gravel and cobble substrates. Younger fish are usually found in shallower and slower water than adults. Spawning occurs in riffles adjacent to pools of swift to moderate mountain streams usually beginning in late spring to early summer. Little is known about overwintering habitat.
• A population of Mountain Sucker with a persistence probability of about 99% over 100 years and quasi-extinction threshold of 50 adults, experiencing a 15% chance of catastrophe (a one-time decline in abundance of 50% or more) per generation, would require a Minimum Viable Population (MVP) of at least 6,400 adults and at least 3 ha of suitable habitat.
• Modelling results suggest that the Mountain Sucker population in the Milk River system is not in imminent danger of extirpation (i.e., likely exceeds the MVP target of 6,400 adults and has more than the minimum suitable habitat) whereas the two Saskatchewan populations are at risk, although they have probably never had the densities necessary to achieve the MVP of 6,400 adults.
• In the absence of additional harm, recovery efforts or habitat limitations, a MVP of 6,400 adults was predicted to go extinct in 32 years. When survival of all ages was improved by 28%, the population nearly stabilized, and the risk of imminent extinction was eliminated.
• The dynamics of Mountain Sucker populations are very sensitive to perturbations that affect the survival of immature individuals (from hatch to age 2) and to the collective survival of adults (ages 2-6). Harm to these portions of the life cycle should be minimized.
• The greatest threats to the survival and persistence of Mountain Sucker in DU2 are related to cumulative effects of landscape changes causing habitat loss and degradation, especially as a result of flow alteration. Drought and anoxic conditions in combination with water regulation and extraction have the potential to significantly reduce the quantity and quality of sucker habitat.
• There are numerous sources of uncertainty related to Mountain Sucker in DU2, especially in Saskatchewan, including population abundance and trajectory; life history characteristics including rates of survival and population growth; habitat requirements including the distribution and extent of suitable habitat; movement between and among populations; and an understanding of the environmental factors that limit their existence.

This Science Advisory Report is from the January 10-11, 2012 Recovery Potential Assessment of Mountain Sucker, Milk River populations. Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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