POPULATION-STRUCTURE OF DAYTIME SURFACE SWARMS OF NYCTIPHANES-SIMPLEX (CRUSTACEA, EUPHAUSIACEA) IN THE GULF OF CALIFORNIA, MEXICO

Daytime surface swarms of the euphausiid Nyctiphanes simplex were observed during 9 spring seasons between 1984 and 1992 along the east coast of Baja California in the southwest part of the Gulf of California. Analyses of sex and maturity of surface patch samples showed that 57 % of the euphausiids were in breeding condition; males possessed developed spermatophores while females were carrying eggs and metanauplii. Sex ratios of female: male were between 1 : 1 and 2 : 1. Swarm samples contained high numbers of eggs and metanauplii. Both results suggest that this swarming phenomenon is related to reproductive activity. However, analysis of one particular swarm sample of N. simplex showed that only 8 % were in breeding condition. The highest biomass found was 32.6 g wet wt m-3, which is correlated with the high trophic activity observed in the sample area.