[Demography and fishery of the sea urchin Loxechinus albus (Echinodermata: Echinidae) in south-austral Chile region].

In the Magellan Region of southern Chile (52 degrees 20'S - 55 degrees 30'S), the edible urchin Loxechinus albus is collected by 1200 artisanal fishermen, of whom 450 are divers. About 360 small fishing boats and 54 transport vessels carry the fresh product to 16 processing plants. Landings of about 27 000 tons were recorded between January and December 1995. Test diameters of urchins harvested monthly were measured for a total of 119 239 specimens, and 36 406 specimens were individually weighed; sex determination was carried out on 2 314 specimens. Field data indicate that the harvest was about 6.6 x 106 dozen urchins (this is a measuring method employed by fishermen in the region), with an extractive effort of 14 753 diver/days. The fisheries yield ranged from an annual minimum of 235 DUDD (dozen urchins per diver/day) to a maximum of 660 DUDD. In overall terms, the lowest average yields were between January and April (415-427 DUDD), and the highest yields between May and December (456-510 DUDD). Mean sizes increased from June to November and decreased from December to June. Size frequency of males and females were polymodal, with the most relevant modes at 72-84 mm in males, and at 79-88 mm in the females. The percentage of individuals below the minimum legal size (70 mm) did not exceed 4.9% for males and 3.6% for females. The size-weight records fit a power model which suggested that this species has a negative allometric growth (b = 2.007). Regarding weight, urchins in the size range from 80.0 to 84.9 mm were those with the maximum contribution to the regional landings. The highest values recorded for the utilized condition factor were: Average Condition Factor (ACF) = May to July, and November; Isometric (or Cubic) Condition Factor (ICF) = July; and Allometric Condition Factor (ACF) = June. Spawning occurred mainly between August and September, and ended by the end of October. Exploitation of this species represents one of the main sources of employment for the artisanal fisheries sector in the Magellan Region. The main difficulty observed in this fishery was obtaining a sufficient supply of urchins with a yellow-gold colored gonadic material, which forms the basis for demand of this urchin by the international market.