Population biology and long-term mariculture studies in the brown alga Lessonia trabeculata in Atacama, Chile

Lessonia trabeculata is one of the most valuable seaweeds in Chile, especially in the northern zone where its harvest has been going on for decades. We carried out population dynamics studies in the Atacama Desert coast (Bahia Chasco), in order to assess its productivity under natural and harvesting scenarios. We found very slow but consistent growth (1.98 cm month(-1)) and density (3-4 individuals m(-2) with no monthly variation) during 18 months of observations in an undisturbed subpopulation. However, after total harvesting, L. trabeculata exhibited different responses. Its recruitment was season-specific, with exceedingly high values in autumn (ca. 80 individuals m(-2) in 5 months) and a dramatic reduction of recruits in summer (1-5 individuals m(-2) in 7 months, with many areas with no recruitment). Gradually, density values tended to stabilize to growth rates under un-altered conditions. In parallel, pruning systems at three different thallus levels (frond meristem base cuts, removal of half and total canopy) were all inefficient and harmful: (i) Biomass takes longer to be harvested; (ii) pruned individuals die off; and (iii) do not detach easily from the substrata, delaying the recovery by potentially emerging L. trabeculata juveniles. Some of these results agreed with our culture experiments, where 26 months were needed to obtain up to 100 cm long thalli with shrub-like morphology. We conclude that management of L. trabeculata beds must be improved in order to guarantee survival of the industry, and we propose some practices that at some stage should involve the complete removal of older/senescent individuals.