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Commercial take of Postelsia palmaeformis (hereafter Postelsia), an annual kelp found only on wave-exposed, rocky shores of the Northeast Pacific, is increasing rapidly in California where regulation of the edible seaweed 'fishery' is minimal. Many commercial collectors use a frond trim- ming method they claim is sustainable and allows for multiple collections per year. Unlike cutting at the stipe, which is lethal and can drive populations to extinction, frond trimming preserves the meris- tem, allowing fronds to regrow. To evaluate the ecological consequences of biomass loss and sustain- ability of this commercial take method we conducted 2 field experiments. We trimmed fronds at dif- ferent frequencies and times and then measured: (1) frond regrowth and reproductive output and (2) population recruitment. We explored the potential for geographic variation by replicating the first experiment near the center and southern limit of Postelsia's biogeographic range. Fronds trimmed in April-June were able to regrow and eventually produce viable spores, albeit at somewhat reduced rates. However, spore production was sharply reduced when fronds were trimmed after the onset of sporogenesis (end of July), whether trimmed once or twice. These effects were similar across the geo- graphic range examined but varied in magnitude. Recruitment was 38% greater in populations not subjected to trimming and population sizes were reduced by 40 to 50% when trimmed. A precaution- ary approach to management should: (1) mandate the frond trimming method, (2) limit collection to once a year and (3) close the commercial season before the onset of reproduction.

Population consequences of biomass loss due to commercial collection of the wild seaweed Postelsia palmaeformis