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COSEWIC Assessment and Update Status Report on the Deepwater Sculpin (Western and Great Lakes-Western St. Lawrence Populations) in Canada

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COSEWIC
Executive Summary

Deepwater sculpin
Myoxocephalus Thompsonii

Great Lakes-Western St. Lawrence Populations
Western Populations

Species Information

The deepwater sculpin, Myoxocephalus thompsonii, is a lake-dwelling sculpin in North America. Much confusion and misinformation exists due to the lack of recognition of differences between three closely related taxa: deepwater sculpin, freshwater forms of fourhorn sculpin (Myoxocephalus quadricornis), and marine fourhorn sculpin. This has resulted in misidentifications and muddled taxonomy. However, the deepwater sculpinhas been shown to be specifically distinct from both marine and freshwater fourhorn sculpin. It has an elongate body, lacks scales, and can be separated from other cottids based on the absence of cephalic horns, a gill membrane that is free from the isthmus and distinct separation between the two dorsal fins.

 

Distribution

The deepwater sculpin is almost entirely restricted to Canada. The speciesoccurs throughout formerly glaciated regions from the Gatineau region of southwestern Quebec through the Laurentian Great Lakes, northwest through Manitoba, Saskatchewan, and northward to Great Bear and Great Slave lakes. An additional isolated population is also known from Upper Waterton Lake of southwestern Alberta. Their distribution within this widespread range is disjunct, due to the patchy distribution of lakes with suitable environmental conditions that also occur in areas with former glacial lake connections. However, information gaps about the species are also due, in part, to the remote locations and associated logistic challenges of sampling ecologically suitable lakes, as well as the isolation of the species at great depths within these lakes.

 

Habitat

The deepwater sculpin is a bottom-dwelling species only found in cold, deep, highly oxygenated lakes throughout their range. Within these lakes, deepwater sculpin occupy the deeper regions. However, as latitude increases, deepwater sculpin tend to occupy shallow depths as well.

 

Biology

Little is known of the biology of deepwater sculpin. A maximum age of seven has been reported for deepwater sculpin. Age at maturity is three years for females and two years for males. Diporeia spp. and Mysis relicta make up the vast majority of the diet of deepwater sculpin throughout their range. Deepwater sculpins are an important component of the diet of piscivores, such as lake trout (Salvelinus namaycush) and burbot (Lota lota). There is virtually no potential for migration or dispersal between inland lakes, although drift of larvae has been shown to occur from Lake Huron to Lake Erie.

 

Population Sizes and Trends

Population data on deepwater sculpin throughout their range are limited to presence/absence data that must be interpreted with caution. Deepwater sculpins are known to occur in 62 lakes throughout Canada. In a range-wide survey during 2004, deepwater sculpin were captured in 16 of 23 lakes where they were previously reported. They were not found in seven lakes where they were previously found, and found in four lakes where they were not previously reported. Thirty lakes where deepwater sculpin have been reported have only been sampled sporadically and the current status of populations in these lakes is unknown. Long-term index netting programs in the upper Great Lakes confirm that deepwater sculpin are abundant in the deep waters of Lake Michigan and are widespread in lakes Superior and Huron, although they are present in lower densities in the latter. In the lower Great lakes, deepwater sculpin are rarely seen, with a significant reappearance in 1996 in Lake Ontario, while larvae have been reported from Lake Erie, most likely due to drift from Lake Huron.

 

Limiting Factors and Threats

Lakes where deepwater sculpin occur must fall within the former boundaries of proglacial lakes, as the present distribution of the species indicates no secondary dispersal from glacial lake boundaries throughout Canada. Deepwater sculpin may be sensitive to shifts in species composition or pollution within these lakes. For example, temporal trends in the abundance of deepwater sculpin in Lake Michigan are best explained by alewife and burbot predation. Also, a recent decline of Diporeia spp. (possibly related to zebra mussel invasion) in the lower Great Lakes may represent a threat to deepwater sculpin populations. Finally, deepwater sculpin may be adversely affected by the eutrophication of lakes, resulting in low oxygen levels where they typically occur at the bottom of lakes.

 

Special Significance of the Species

Deepwater sculpins are an important component in the diet of deepwater piscivores in lakes where they occur. In the Great Lakes, the species is an excellent indicator of the well-being of the deepwater fish community and habitat. Its 1996 reappearance in Lake Ontario signalled a series of changes in the open-water fish community and a possible reduction in the predatory effects of smelt and alewife. Finally, deepwater sculpin are of particular interest to those studying zoogeography and post-glacial dispersal within Canada.

 

Existing Protection

COSEWIC designated the deepwater sculpin as Threatened within the Great Lakes in 1987. The habitat sections of the federal Fisheries Act generally protect the habitat of the deepwater sculpin. Populations found in Upper Waterton Lake in Waterton Lakes National Park are partially protected by the National Parks Act.