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Squid paralarvae were collected on four oceanographic cruises during winter and summer of 1998 to 1999 by the IMECOCAL Program. All paralarvae were sorted from zooplankton samples collected with standard bongo net tows. The region surveyed included the marine area off the west coast of the Baja California peninsula from Ensenada to Punta Abreojos, Mexico. Paralarvae were identified and counted, basic community structure was characterized through ecological indexes (richness, diversity and evenness), and squid distributions were related to prevailing water masses in the study area. Species richness for three of the four cruises analyzed was high, registering a maximum of 24 taxa. The exception was the winter cruise 1999 when richness decreased by 66%, and only 8 taxa were recorded. Diversity had no significant variations with 2.3 to 2.9 Bits/ind. The summer cruises showed low values of evenness (0.5 and 0.6) which were related to high abundances of ommastrephid and gonatid squids. During the winter cruises, evenness was relatively high (0.7 and 0.8) with more uniform specific abundances. The area off Punta Eugenia registered the highest values for both species richness and diversity. Although biogeographic affinities of paralarvae were established based on the distribution of adult squids, most species of paralarvae had a high correlation with corresponding water masses. Taxa with tropical affinities decreased progressively in abundance (from 31% to 8%) from the winter of 1998 which was at the end of the 1997-1998 El Niño event. Most tropical taxa were distributed at stations with Transitional-Subtropical Surface water (TStSW). Temperate taxa were dominant throughout the study period, with a maximum relative abundance in the winter of 1999. The distributions of temperate taxa were mostly linked to the effect of the 1999-2001 La Niña event and the progression of the Subarctic Water mass (SAW) along the west coast of the Baja California peninsula. Cosmopolitan taxa were present in samples only from the 1998 winter and 1999 summer cruises but they were distributed throughout the study area.

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