Short-term variation of abundance of the purple sea urchin, Paracentrotus lividus (Lamarck, 1816), subject to harvesting in northern Portugal. Paracentrotus lividus is a common and intensely harvested sea urchin at several European locations, including the Mediterranean Sea and the Atlantic coast of the Iberian Peninsula. The increasing human pressure on this resource due to the growing demand and market value of sea urchin gonads as seafood raises concerns on the ecological sustainability of present fisheries, which are showing a technological improvement and an expansion towards previously non-harvested areas. We examined the abundance of P. lividus of both commercial and non-commercial size before, during and after the harvesting season (from October to April) in the rocky shallow subtidal habitat along the northern Portuguese coast. The abundance of commercial (≥50 mm in test diameter) P. lividus individuals increased in the harvesting season, but drastically dropped by about 90% in the after-harvesting period. Such a pattern was consistent among three rocky shores spanning about 65 km of coast. The multivariate population structure and most size classes of non-commercial sea urchins did not differ depending on the period. The only exception was Class 4 (test diameter between 30 and 40 mm), which was more abundant in the harvesting than in the before- and, further, the after-harvesting period, but only at one shore. Very small (Class 1, test diameter below 10 mm) urchins were never found. The present findings suggest that human harvesting may cause considerable reductions in the abundance of target P. lividus, but that such an effect would not be evident concomitantly with harvesting, but in the subsequent period. Even if just under a precautionary principle, protection strategies focused on sea urchin populations and the harvesting period are advisable to contribute to maintain a sustainable local fishery of P. lividus populations that are likely to be negatively affected also by other natural and anthropogenic perturbations.