A GROWTH STUDY OF ST MARY'S RAPIDS SLIMY SCULPIN
(Cottus cognatus)

Frank Allen Blissett III
Department of Biology and Chemistry
Lake Superior State University
Sault Ste. Marie, MI 49783




Abstract
	The Cottus cognatus (Slimy Sculpin) is a small scaleless fish of streams and rivers, ranging through much
of Canada and into New England and the Great Lakes Basin. The C. cognatus feeds primarily upon small
invertebrates and is in turn fed upon by larger fish such as Lake Trout, Walleye, and Burbot. Still, little is
recorded about the lift history of C. cognatus. I was therefore interested in doing a simple growth study of a
local C. cognatus population, namely those of the St. Mary's rapids.

	On June 9th, 1995 the aquatics lab of Lake Superior State University shocked and collected fish from the
St. Mary's rapids and on my request preserved several C. cognatus in alcohol for better preservation of otoliths.
Otoliths ("ear stones") were chosen to age the specimens due to the fact that C. cognatus is scaleless. The
lengths were measured to the millimetre and a sagitta otoliths dissected out of 4l sculpin. These were glued to a
specific position on a specific slide so I could later tell which fish were which length. The otoliths were then
aged with the help of a dissecting scope. I saw quite a lot of seasonal rings, but only on rare occasions were the
rings too cluttered to read.

	Only two age cohorts were found, age II and age III fish, age I fish possibly slipping through the net and
age IV+ fish being comparatively rare. Age II C. cognatus ranged from 31 to 56 millimetres, but smaller age II
fish may also have slipped through the nets. Age III fish were found to be larger, with a range of 57 to 73
millimetres, one measuring 53 millimetres. The mean length ofthe age II and age II cohorts were 43 and 64
millimetres respectively.

	A chi-square test for goodness of fit was performed for the age cohorts and both were found not to be
normally distributed at an alpha of .95. This result may be partially explained by a lack of smaller age II fish, but this
result for age II fish was unexpected. Additionally, since there was some cross over between age II and age III
fish, length should probably not be used as more than a rough means for age determination.





Selected References

Nielsen, L.A., D.L. Johnson. 1983. Fisheries Techniques. Am. Fish. Soc.

Page. L.M., B.M. Burr. 1991. A field Guide to Freshwater Fishes. Houghton Mifflin Co.