Spider crab (Maja brachydactyla) juveniles inhabit shallow coastal areas until they reach maturity and start a mating migration toward deeper waters. Small-scale spatial distribution was analyzed for a shallow-water local population using data from a monthly trap sampling study conducted from December 1997 to November 1999. The trap arrangement formed a grid covering a total area of 3.2 km2. Catch per unit of effort (CPUE) for each biological category (defined by sex and maturity status) was modeled using geostatistics to analyze the spatial structure and to map distribution. Juveniles showed persistent aggregations roughly 150–300 m in diameter and no large-scale movements. Juvenile patches were highly stable in time. During summer, some of the juveniles attained sexual maturity and the aggregation broke up. It was reestablished in September and October, when new juveniles recruited. Adults found in shallow waters did not show a clear spatial structure. The relation variance per mean CPUE was used as a dispersion index (DI). Juveniles were highly aggregated (DI » 1) throughout the whole year, except during the period when the terminal molt took place. Adults were almost randomly distributed (DI ≈ 0–1). In the case of juveniles, the DI was significantly and positively correlated with CPUE, and was negatively correlated with size, but did not show any significant correlation either with the percentage of individuals undergoing molt or with temperature.
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