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The effects of variable removal levels of the sea urchin, Strongylocentrotus franciscanus, on near-shore rocky communities in the traditional territory of the Hesquiat First Nation Mooney, Robert C.

Abstract

The shallow subtidal regions near Hot Springs Cove, Vancouver Island, British Columbia are characterised by large rocky areas dominated by the red sea urchin, Strongylocentrotus franciscanus. S. franciscanus were removed at three sites with four levels of urchin removal per site. Manipulations of urchin density were maintained throughout the experiment and monitored seasonally for two seasons pre-treatment and seven seasons post-treatment. The manipulations resulted in increased gonad indices of remaining red sea urchins and caused the conversion of urchin dominated subtidal regions into kelp dominated communities with greater fish abundance. As well, a feeding experiment illustrated that the depressed gonad indices of field-collected urchins resulted from limited food resources in areas of high urchin density. The removal of sea urchins at all levels resulted in a rapid increase in the presence of laminarialean algal species (kelps). Study plots where all urchins were removed developed a dense understory and seasonal canopy of kelps with little bare rock remaining. Intermediate levels of removal resulted in a mosaic of smaller urchin-dominated and kelp-dominated patches. Control plots tended to maintain the urchin-dominated barrens-state throughout the study period. Sea urchins that were fed M. integrifolia, during the feeding experiment, showed 2.9 and 2.4 times greater gonadal development by weight than urchins collected from the field before and after the feeding trial, respectively. These results suggest that food limitation is an important factor in the gonadal development of this urchin population. Findings suggest that with supplemental, feeding, the resource base of sea urchins could be expanded to include barren habitats. The removal of S. franciscanus at all levels resulted in an increase in fecundity (measured as gonad index) for the sea urchins that remained, as well as for the urchins that reinvaded the total removal plots. All study plots showed an increase in gonad index over time, but the increase in gonad index was statistically greater for urchins in removal plots versus the control plots. The results indicate that small isolated urchin removals can have measurable effects on the fishery value of nearby urchins. Of the seven fish species monitored, pile perch, striped seaperch, kelp perch and black rockfish were most associated with kelp forest habitat. Kelp greenling, lingcod and copper rockfish showed no association with kelp forest habitat. The experimental approach taken indicates that sea urchin removal, and subsequent kelp growth, determined relative fish abundance. High densities of S. franciscanus appear to be responsible for the absence of kelp forest habitat in the region, the depressed fecundity (gonad index) of S. franciscanus, and the abundance of some fish species. The effect of urchin removal is discussed in regards to the implications for ecological theory as well as sea urchin fisheries management.

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