GROWTH AND EXPANSION OF AN EXPLOITED POPULATION OF THE SQUAT LOBSTER (PLEURONCODES MONODON) AFTER 3 YEARS WITHOUT HARVESTING

In 1991, after 3 years of fishery prohibition, the most important surviving population of squat lobster (Pleuroncodes monodon, Galatheidae, Decapoda) on the continental shelf of central Chile, the Achira population (36 degrees S, 73 degrees W), reached the highest biomass recorded since the start of population assessments in 1979 (34 084-43 910 t, 95% confidence interval), and expanded its distribution by 56 km to the north. The age composition of the population was characterised by two strong year-classes (1988 and 1989, born at the end of the previous fishing period and at the start of the prohibition respectively) and a mixture of older year-classes, less important in terms of numerical abundance. Population expansion had a clear spatial structure. The 1989 year class (mean size 23.5 mm carapace length) populated the traditional southern population area, while the 1988 and older year-classes (mean carapace length 33.5 mm and 39.5 mm, respectively) populated the areas to the north which were vacant prior to the fishery closure in 1989. We interpret this spatial structure as the result of size-dependent dispersal capacity, with recruitment of the 1989 year-class providing the impetus for expansion of the individuals of the 1988 and older year-classes. Using the simple age structure of the Achira population, we estimated an average rate of population increase of 10 323 t year(-1) during the period without harvesting, and a rate of expansion of 20 km year(-1). Most of the reproductive potential (eggs km(-2)) was transported into the expansion zone, which may create new nursery grounds towards the north. This in turn may alter the spatial distribution of size, if recruitment is successful in the northern areas, with a potential effect on expansion dynamics.