Eastslope sculpin COSEWIC assessment and status report: chapter 2

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COSEWIC
Executive Summary

"Eastslope" Sculpin
St. Mary and Milk River Populations
Cottus Sp.

Species Information

The “Eastslope” sculpin is a small freshwater sculpin found in Canada only in the St. Mary and Milk River systems of Alberta, and perhaps the Flathead River of British Columbia. The taxonomy of this fish has been in dispute, but efforts to resolve the situation are underway. The most recent genetic and morphological findings suggest that the “Eastslope” sculpin is a new species, which is in the process of being formally described. The St. Mary and Milk River populations comprise a designatable unit and the Flathead population may comprise another, though the latter is not assessed in this report.

 

Distribution

“Eastslope” sculpin are found in the Milk and St. Mary rivers in Alberta and Montana, as well as in the Flathead River in B.C. Regardless of taxonomy, the “Eastslope” sculpin has an extremely limited distribution in Alberta. Its presence in the St. Mary River above the St. Mary Reservoir and in the Milk River appears to be limited in part by its preference for cooler water temperatures and clean rocky substrates, and within these systems, it is the only sculpin species present.

 

Habitat

“Eastslope” sculpins are found in moderately cool streams with riffle habitat, rocky or gravel substrate and slow to fast currents, and were usually absent from pools where bottoms were entirely sand or clay. The greatest alterations to sculpin habitat in the St. Mary and Milk rivers are related to water diversions, reservoirs and water removal for irrigation. These factors, in combination with the frequent droughts experienced in southern Alberta, seriously affect the availability of sculpin habitat. Little of the area occupied is under public control, and protection measures would depend on legislation and regulation designed for habitat protection.

 

Biology

Life history information for the “Eastslope” sculpin is extremely limited, and much of the information available is based on a limited number of studies of Cottus populations from other western systems. The only study to specifically describe the life history of this sculpin in Alberta noted that all Cottus species in Alberta, including the “Eastslope” sculpin, spawned during the late spring. The fecundity of sculpin specimens collected from the Milk and St. Mary rivers generally ranged from 100 to 250 eggs; eggs likely hatch within 2 to 3 weeks, and young of the year reach 30-40 mm in total length by the end of their first summer. Both sexes are believed to be sexually mature at the age of 23 months. Aquatic insect larvae appear to make up the majority of the diet, but molluscs, fish, and even sculpin eggs may also contribute. Neither juvenile nor adult “Eastslope” sculpin appear to undergo extensive migrations.

 

Population Sizes and Trends

The “Eastslope” sculpin appears to be locally abundant where it is present, but its distribution has changed in the Milk River system since it was first observed in the 1960s. It appears to have expanded downstream in the Milk River over time, although it remains absent in the furthest downstream sections. It has apparently been extirpated in the upper Milk River since it was documented there in the mid-1980s. No changes have been observed in its distribution in the St. Mary River, where it is currently found only above the St. Mary Reservoir.

 

Limiting Factors and Threats

Water removal, diversions and reservoirs associated with irrigation, in combination with the frequent droughts of southern Alberta, have likely had the greatest impact on population size and distribution over time and will continue to be the greatest threats to the existence of the species in Alberta.

 

Special Significance of the Species

This genetically distinct sculpin represents an important component of the genetic diversity found in the western sculpin complex and deserves a high level of protection.

 

Existing Protection or Other Status Designations

The “Eastslope” sculpin has been approved for listing as threatened (as of June 2004) under the Alberta Wildlife Act. In view of its extremely limited distribution in Alberta, a provincial management plan was developed in the 1990s to aid in protecting existing populations. More recently, the Fish and Wildlife Division of Alberta Sustainable Resource Development commissioned surveys in the Milk River (2000 to 2002) to help determine the status of several non-game fish species, including the “Eastslope” sculpin, and to provide recommendations with regards to protection.

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