POPULATIONS OF SYMPATRIC SCULPINS, COTTUS ALEUTICUS 



AND COTTUS ASPER, IN FOUR ADJACENT SALMONPRODUCING 



COASTAL STREAMS ON VANCOUVER ISLAND, B.C. 



J. C. Masoni and S. Machidori^^ 



ABSTRACT 



General life history, distribution and abundance, age structure, and growth and survival are 

 documented for sympatric populations of two cottid fishes. Stream obstructions may largely determine 

 the distributional limits for both cottids with Cottus aleuticus penetrating farthest upstream. Biomass 

 density and size of individual fish increased with distance upstream, largest individuals living at the 

 upstream borders of their species ranges. Both sculpins were numerically most abundant in their lower 

 ranges, reflecting the common estuarine origin of benthic young. From 69 to 74% of their combined 

 biomass in the upper estuaries were C. asper while 75-100% was C. aleuticus in the upper stream 

 zone. Cottus asper grew more rapidly and mortality rates were similar, but the oldest C aleuticus was 

 age 8 and 145 mm in length, compared with age 6 and 144 mm for C asper. The length-weight 

 relation was similar for both species. The community role of these sculpins is explored with primary 

 focus on possible competition with the stream-dwelling salmonids, and recommendations are made 

 which might lead to increased production of salmonid smolts to the sea. 



As part of a general study of the fish community 

 of Lymn Creek, populations of the sympatric 

 sculpins, Cottus aleuticus and C. asper, were 

 examined during 1968 with regard to population 

 structure, annual growth and mortality, and gen- 

 eral distribution and abundance in the system. In 

 addition, three adjacent streams (Cabin, Chef, 

 and Waterloo) were sampled in the fall of 1968 to 

 provide a comparative basis for interpreting the 

 findings at Lymn Creek. The present communica- 

 tion deals primarily with population characteris- 

 tics of sculpins in relation to life history. Their 

 role in the community, including possible compe- 

 tition with salmonids, is examined with a view of 

 enhancing salmonid production. 



THE STUDY AREA 



The four streams studied are neighboring sys- 

 tems emptying into the Strait of Georgia on the 

 east coast of Vancouver Island. They are small 

 streams (drainage area <20 km^, minimum sum- 

 mer flow <7 m^/min. Table 1), having similar 

 gradients and streambed materials, but Cabin 

 Creek is considerably smaller than the others. 

 Their watersheds are forested at a similar stage 



'Department of the Environment, Fisheries and Marine Ser- 

 vice, Research and Development Directorate, Pacific Biological 

 Station, Nanaimo, B.C. V9R 5K6, Canada. 



^Fisheries Agency of Japan, Far Seas Research Laboratory, 

 1000 Orido, Shimizu 424, Japan. 



of second-growth conifers, primarily Douglas fir. 

 Lymn and Waterloo creeks closely resemble each 

 other, although the latter stream has fewer major 

 obstructions (logjams) hindering the upstream 

 migration of salmon. Lymn Creek differs from 

 the other three streams in having a swampy 

 sloughlike area resulting from beaver activities 

 near the estuary. Both Lymn and Chef creeks 

 course through some 200 m of intertidal meadow, 

 but Cabin and Waterloo creeks empty directly 

 onto the open beach. Extensive intertidal zones in 

 all four streams result at low tide when nearly 

 the entire zone is exposed to freshwater flow. 



Unlike the other systems. Chef Creek is subject 

 to flow extremes, rapid runoff during freshets 

 and, during the late summer and early fall, 

 intermittent flow and isolated pools in the lower 

 reaches. 



Cutthroat trout, Salmo clarki; coho salmon, 

 Oncorhynchus kisutch; three-spined stickleback, 

 Gasterosteus aculeatus; coastrange sculpin, C. 

 aleuticus; and prickly sculpin, C. asper, reside in 



Table l. — Some physical characteristics of the four study 



streams. 



'Within the sculpin zone. 



Manuscript accepted April 1975. 



FISHERY BULLETIN: VOL. 74, NO. 1, 1976. 



131 



