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Comparative ecology of two sympatric tidepool fishes, Oligocottus maculosus (Girard) and Oligocottus snyderi (Greeley) Nakamura, Royden


There are about twelve species of cottid fishes which inhabit tidepools at Port Renfrew, Vancouver Island, British Columbia. Two of the more abundant species, Ollgocottus maculosus (Girard) and Oligocottus snyderi (Greeley), display vertically zoned microdistrlbutions. Ollgocottus maculosus is very abundant in the upper tidepools while O. snyderi occurs only in lower tidepools. A comparative study of the ecology of these species was made to assess the role of environmental parameters in determining their microdistrlbutions. Environmental parameters considered were depth, substrate and cover, food, interspecific interactions, salinity and temperature. The relationships between the field data, behavioral and physiological laboratory experiments were used to assess the role of the various factors. The results indicate that the influence of the environmental factors considered varied with species. The tide-pool distribution of O. maculosus is influenced but not wholly determined by its high tide depth preferences. The high tide depth preferences of O. snyderi is not related to its tidepool distribution. The interspecific interactions between the two fish appear to affect the depth distribution of O. maculosus while that of O. snyderi is not affected. Substrate-cover affinities of O. maculosus do not limit the occurrence of this species to tidepools or tidepool areas of a specific type, whereas Ollgocottus snyderi is rigidly bound to pools with dense macrophytic cover. This factor, however, does not account for the absence of O. snyderi from upper tidepools with dense vegetal cover. Interspecific interactions between the two fish do not appear to grossly influence their respective microhabitat preferences. The possibility of interspecific competition for food appears remote as both species, though feeding on the same basic items, display no sign of "interactive segregation" or "separation of food spectra" as evidenced in competitive situations among some other species of fishes. However, O. maculosus does consume a greater variety of food. Field observations indicate the common occurrence of major food items irrespective of season or vertical location of the tidepools. For both Ollgocottus maculosus and O. snyderi, resistance to low salinities of the magnitude encountered in tidepools is substantial enough to allow either species to survive most hyposaline conditions found within the intertidal zone. However, the occurrence of O. maculosus beyond the upper boundary of the intertidal may be limited by hyposaline extremes. Ollgocottus snyderi generally shows a lesser resistance to low salinities than does O. maculosus. The tidepool distribution of O. maculosus is not limited by tidepool temperatures as this species displays a relative resistance and tolerance to temperature extremes at least as great as those occurring in the highest tidepools. Ollgocottus snyderi, however, is unable to tolerate and/or resist extreme temperatures occurring in upper tidepools and is therefore limited to lower pools. It appears that the tidepool distribution of O. maculosus is not dominated by any single factor with the possible exception of turbulence (open, exposed areas) and population density. Thus it is a versatile species, behaviorally as well as physiologically, and is able to exploit virtually all tide-pools in the intertidal zone. The tidepool distribution of O. snyderi is affected by substrate and cover as well as temperature. Of these, temperature appears to be the limiting factor within the intertidal zone since O. snyderi cannot survive in upper tidepools which are characterized by more extreme and unstable temperatures.

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