Hydroacoustical Evidence of Autumn Inshore Residence of the Pelagic Crab Pleuroncodes Planipes at Punta Eugenia, Baja California, Mexico.

Abstract

A previously published conceptual model of the life history of the red crab Pleuroncodes planipes (Galatheidae) (Stimpson, 1860) reports that the center of distribution of this micronektonic crustacean is located at Bahía Magdalena, on the southwest coast of Baja California (25 to 24°20’ N). During spring, the highest abundance of mature females is found nearshore with abundance peaks of larval stages during February and March. Larval drift offshore is associated with a strong Ekman transport system, and the virtual disappearance of the adult pelagic and benthic population from the neritic region at Bahía Magdalena in late summer and autumn can be explained by an inshore-offshore migration. However, huge benthic-pelagic concentrations of this crustacean occur nearshore during autumn at Punta Eugenia (27 to 29° N). Circadian cycles of the spatial distribution of these aggregations 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 crab aggregations using hydroacoustics. The target strength of the red crab ranged between –50 and –54 dB. The largest aggregations were found in Bahía Magdalena during summer (June 1995 and 1996; 24 to 25°N) and nearshore at Punta Eugenia (27 to 28° N) during autumn and spring (March 1995 and 1996, October 1994 and 1995). These observations suggest that the conceptual model of inshoreoffshore adult population migration is valid for Bahía Magdalena, but apparently not for Punta Eugenia at least Turing autumn. At Punta Eugenia an abrupt change in the orientation of the shoreline enhances upwelling events and promotes semipermanent eddies, thus supporting the inshore residence of this adaptive omnivorous-detritivorous crustacean.