California Sea Grant College Program

The Sea Palm, Postelsia palmaeformis, is an intertidal kelp of the Order Laminariales, has a heteromorphic life history, and is endemic to the wave-exposed rocky shorelines of the Northeast Pacific. Postelsia is also among the most valued of seaweeds collected for the health- and wild-foods industry, and it is collected commercially in Oregon and California. When collectors cut fronds leaving the meristem intact they will regrow, allowing multiple collections per season to be made from the same individuals. Commercial collection takes place in California with minimal management or regulation, despite the fact that Postelsia’s life history characteristics make it especially vulnerable to overexploitation. Though many California collectors advocate and use this cutting method and maintain that it is sustainable, there is no scientific evidence to support this claim. We experimentally mimicked the frond-cutting method used by commercial collectors and explored the effect of the frequency and timing of collection on Postelsia survivorship, growth and reproduction. Our experiments were done in two areas of Postelsia’s biogeographic range: at the southern range limit and near the center of its distribution. Results showed that frond trimming has an immediate effect on Postelsia’s growth and reproductive output, and though fronds trimmed early in the season are largely able to regrow and eventually produce spores, they are somewhat delayed relative to untrimmed plants. Timing of trimming was found to be more important than frequency, because plants trimmed once late in the summer responded similarly to those trimmed twice during the year. Variation between sites studied was limited to general size and reproductive capabilities of plants; the effect of treatment was essentially the same at both sites. Based on these results, we recommend that commercial collectors take fronds, preserving the meristem, only once in the spring. The findings of this study are ideal for consideration in the implementation of an improved and appropriately designed management strategy, balancing conservation with human exploitation.