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The Effects of Small-Scale Kelp Harvesting on Giant Kelp Surface Canopy Dynamics in the Ed Ricketts Underwater Park Region

Donnellan, M.D. and M.S. Foster (1999)

Final Report to the Monterey Bay National Marine Sanctuary and the Cities of Monterey and Pacific Grove, 33p.


The effects of hand-harvesting giant kelp (Macrocystis pyrifera) on the surface canopy of kelp forests offshore of Monterey and Pacific Grove were investigated. This study was stimulated by concerns over the possible negative biological effects of recent intensive kelp harvesting by local aquaculture firms on giant kelp, rockfish, and sea otter populations, particularly when harvesting activities are concentrated within a small local area during the winter. We used aerial photographs dating to 1976 and kelp harvest records to address only the question of important harvest effects on giant kelp canopies, not the general effects of kelp harvesting. Three different periods of kelp harvesting relative intensity in this area have occurred since the early 1970s. a period of "moderate harvest" from 1972 - 1985, a period of "low harvest" from 1986 - 1995, and a period of "high harvest" from 1996 to present. Eight surveys were available from each of the periods of 1972 - 1985 and 1986 - 1995, and two surveys were available from the period of 1996 - 1998.

For the years in which aerial kelp surveys were available, hardcopy maps of kelp forest canopies were derived from aerial survey slides taken during the fall period of maximum canopy extent. The resultant maps were scanned into a computer, and image analysis software was used to measure kelp canopy surface area (the actual surface area of kelp canopy fronds floating on the surface), kelp forest spatial extent (the total surface area of a kelp forest canopy contained within its outer perimeter), and kelp canopy density (an index of canopy abundance within a given kelp forest ranging from "dense" to "sparse"). Using a modified Before - After - Control - Impact (BACI) study design and analysis. of variance (ANOVA) statistical testing, these variables were compared between three harvested areas and one unharvested control area (Hopkins Marine Life Refuge) during each of the three periods of relative harvesting intensity. Since the period of interest (from 1996-1998) had a sample size of only two, we could not make rigorous, meaningful comparisons between this period and the other two periods. However, we were able to use a BACI approach (with two-sample t-tests) to test for a harvesting effect between the period of "moderate harvest" and the period of "low harvest." These comparisons were not directly applicable to the "high harvest" period of interest, but the results may suggest patterns to expect if the current level of kelp harvesting intensity continues. Regression analyses of the amount of kelp harvested during the winter versus the maximum amount of giant kelp surface canopy present during the following year were also performed.

No statistically significant differences in kelp canopies among periods were found using ANOVA analyses, and results of regression analyses were also statistically non-significant. No effects of current kelp harvesting practices on giant kelp canopies were detectable, but statistical power to detect an effect was low given the small sample size (n = 2) of the "high harvest" period and the inherent natural variability of kelp canopies. Therefore, these results do not necessarily indicate that there was not a harvesting effect, only that such an effect was undetectable given the available data. Separate comparisons of kelp canopies between " moderate harvest" and " low harvest ' periods (NOT the period in question) showed significant differences between the two periods at one of the three harvested study sites, but these results may have been confounded by kelp harvesting immediately prior to the aerial surveys. If these results were not confounded, they suggest that harvesting concentrated within a relatively small area may negatively affect giant kelp canopies. Continued yearly aerial surveys during the period of fall maximum kelp canopy are needed in order to resolve this issue.

Reviewed: April 11, 2024
Web Site Owner: National Ocean Service

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