Hydroacoustical evidence of autumn inshore residence of the pelagic red crab Pleuroncodes planipes at Punta Eugenia, Baja California, Mexico

A previously published conceptual model of the Life history of the red crab Pleuroncodes planipes (Galatheidae) (Stimpson, 1860) reports that the cent er of distribution of this micronektonic crustacean is located at Bahia Mag dalena, on the southwest coast of Baja California (25 to 24 degrees 20'N). During spring, the highest abundance of mature females is found nearshore w ith abundance peaks of larval stages during February and March. Larval drif t offshore is associated with a strong Ekman transport system, and the virt ual disappearance of the adult pelagic and benthic population from the neri tic region at Bahia Magdalena in late summer and autumn can be explained by an inshore-offshore migration. However, huge benthic-pelagic concentration s of this crustacean occur nearshore during autumn at Punta Eugenia (27 to 29 degrees N). Circadian cycles of the spatial distribution of these aggreg ations were recorded with a single beam echosounder Simrad EY-200 (200 kHz) along Baja California during October 1994, March, June, and October 1995, and March and June 1996. Data obtained from Isaacs-Kidd midwater net trawls during October 1994 along Baja California were used to identify the red cr ab aggregations using hydroacoustics. The target strength of the red crab r anged between -50 and -54 dB. The largest aggregations were found in Bah's Magdalena during summer (June 1995 and 1996; 24 to 25 degrees N) and nearsh ore at Punts Eugenia (27 to 28 degrees N) during autumn and spring (March 1 995 and 1996, October 1994 and 1995). These observations suggest that the c onceptual model of inshore-offshore adult population migration is valid for Bahia Magdalena, but apparently not for Punts Eugenia at least during autu mn. At Punta Eugenia an abrupt change in the orientation of the shoreline e nhances upwelling events and promotes semipermanent eddies, thus supporting the inshore residence of this adaptive omnivorous-detritivorous crustacean .