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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002 

Red Crab and Soft-Bottom Fishes along
the Pacific Shelf off the Western Coast
of Baja California Sur during
El Niño 1997-98

Eduardo F. Balart, Johatht Laudino
Santillán, David Sánchez Aguilar,
Lucía Campos Dávila,
Edgar Amador Silva

Centro de Investigaciones Biológicas del Noroeste,
S.C. Mar Bermejo 195, Col. Playa Palo de Santa Rita.
Apdo. Postal 128. La Paz, Baja California Sur,
México 23090, E-mail:

Introduction & Objectives

The western coast off Baja California Sur (BCS), México, is a biogeographic transitional area with high primary and secondary productivity. The presence of upwellings and the seasonal influence of the cold California current and the tropical North Equatorial current provide this area with a rich and diverse fish population (Hubbs, 1960; Castro-Aguirre et al, 1993; Galván-Magaña et al, 2000). This area is dominated by the overwhelming abundance of the red crab Pleuroncodes planipes, a key species linking the plankton and detritus food chain to higher trophic levels (Longhurst et al., 1967; Smith et al., 1975; Balart and Castro-Aguirre, 1995; Pérez-Flores and Aurioles-Gamboa, 1995; Aurioles-Gamboa and Pérez-Flores, 1997).

This work describes the variations in fish composition and red crab biomass associated with softbottoms as evidenced by shrimp-net trawling in three research cruises (April-May 1997, March-April and September-October 1998) along the western coast of BCS between 23° and 26.5° LN (11 latitudinal transects each with four stations at 15, 30, 45 and 60 fathoms depth). Analysis of stomach content in the dominant fish species, Prionotus stephanophrys, was carried out in order to look for trophic changes in their diet associated with El Niño.

Results & Discussion

Average bottom temperature increased by two and three degrees during the event, changing from 15.38°C during the April-May 1998 cruise to 17.74°C and 18.34°C in March-April and September-October 1998 respectively. Surface temperature, however, showed a greater and progressive increment in average temperature, changing from 17.33°C in the April-May 1998 cruise (average of 42 stations) to 20.55°C and 24.18°C during the 1998 cruises.

The recorded frequency and biomass of the bentho-pelagic red crab do not differ from the expected values considering the studies of Aurioles-Gamboa (1992; 1995a, b). According to the latter author the average biomass for April-May (average of three cruises) is about 40 ton/km2 in the 51-100 m depth stratum, a value close to our 35 ton/km2 averaged over 42 stations during the April-May 1997 cruise. A similar situation was documented for March-April and September-October 1998, when average biomass declined to 14.14 and 1.69 ton/km2. However, red crab biomass does not follow a simple south-north biomass trend since around the mouth of Magdalena-Almejas Bay there is always a "swarm" present. The observed biomass of the dominant invertebrate in the western coast of BCS follows the pattern described by Aurioles-Gamboa (1992 and 1995), that is, distribution and biomass are driven by bottom temperature, with maximum values around 13-15°C and declining up- and downward, and by depth. Because the distribution of the pelagic phase of the red crab results from the prevalent current patterns, there is a northward migration during ENSO events reaching California waters (e.g., Glyn, 1961).

Unfortunately, there is almost no published data about the abundance and biomass of the soft-bottom fishes in the study area. For this reason we inter-compared the values obtained during the research cruises. Fish average biomass was similar between cool cruises (April-May 1997 and March-April 1998) but these values were nearly half of those found during the warmer cruise (September-October 1998). Total fish species and mean number of fish species were quite different among cruises; during April-May 1998 the total number of fish species was half of that recorded during the other cruises (68 compared to 148 and 145 respectively) and the average number of fish species per station was three and four times lower than that of the March-April and September-October 1998 cruises (6.89 c.f. 20.88 and 28.50 respectively). As expected, fish species composition was greatly affected by El Niño 1997-98, with the presence of many southern "warmer" species and absence or declining abundance of northern "cool-water" species (e.g., Monolene asaedae) being recorded. The case of September-October 1998 includes the "normal" summer shift of fish diversity plus the remaining El Niño driven species. Many fish species extended their distribution ranges northward (e.g., Raja velezi, Ophidion iris, Scorpaena sonorae, Prionotus ruscarius, Conodon serrifer, Haemulopsis axillaris, Lutjanus guttatus, Larimus pacificus, Engyophrys sanctilaurentia, Etropus peruvianus) and others were recorded for first time in mainland México (i.e., Serranus huascarii, S. socorroensis).

The analysis of stomach contents showed that the importance of the main known preys, the euphausiid Nyctiphanex simplex and the red crab P. planipes (Shmitter-Soto, 1992; Schmitter-Soto and Castro-Aguirre, 1996) changed. The latter authors found N. simplex was the main prey (85.92% IRI) followed by the red crab (13.30% IRI). During the event we found values of 29.34% IRI for N. simplex and 41.03% IRI for the red crab, in addition to other emerging important prey species (i.e., Pasiphea chacei, mysids, etc.). The shift in importance of the two main preys seems to mirror the shift in abundance and frequency of the most available non-infaunal invertebrates, since P. stephanophrys is an opportunistic carnivore here as in other regions (Schmitter-Soto and Castro-Aguirre, 1996; Meléndez, 1987). As shown above, P. planipes maintained a "normal" biomass during the El Niño 1997-98 and N. simplex probably moved northward as documented in similar events (i.e., Hayward, 2000).

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