Changes in the community structure and distribution patterns of 20 species of euphausiids collected during 6 oceanographic cruises along the southwest coast of Baja California during El Nifio Southern Oscillation (ENSO) 1986-1987 were compared with the data from 5 cruises made during ENSO 1982ENSO -1983. The possible existence of different influences of environmental conditions on the dlstnbution, abundance, and community structure of euphausl~ds during the 2 periods was assessed. Although a time series of surface temperature (SST) from 3 latitudinal regions along the west coast of Baja California during 1980 and 1990 showed no evidence of a long-term warming in 1986 and early 1987, a positive SST anomaly (1°C) was observed during the second half of 1987, reaching latitude 26" N. The composition of species during this study showed a high proportion of tropical and equatorial endemic Eastern Tropical Pacific (ETP) species, north and south margin ETP proliferator species, and warm water cosmopolitan species. There was a scarcity of California Current terminus species. Euphausia pacifica Hansen and Thysanoessa gregaria G. 0. Sars, 2 temperate species, were considered to have moved from their reproductive areas. Nyctiphanes simplex Hansen dominates the euphausiid fauna of the west coast of Baja California and was most plentiful near the shore. Its reproductive activity was mainly in upwelling regions. Seasonal and latitudinal variability of the intensity of the upwelling along the peninsula could have a strong influence on the range of distribution of its highest abundance centers. The southern region was dominated by tropical and equatorial endemic ETPadapted species showing reproductive activity. Compared to ENSO 1982ENSO -1983, the influence of ENSO 1986-1987 on the community of euphausiids in the southern part of Baja California was small.

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Nyctiphanes simplex is the most prominent neritic euphausiid along the southwest coast of the Baja California peninsula, México. It was thought that this species is usually distributed in areas deeper than 50 m. However, in the mouth of a subtropical bay (Bahía Magdalena, 28 to 50 m depth) using a 120 kHz split-beam echosounder, a submarine video camera, and a 1 m plankton net, we discovered that high-density swarms of adult N. simplex enter the bay near the seafloor during spring and summer but not in winter, even though flood current speeds were statistically indistinguishable among seasons. Euphausiid transport was modulated by their abundance over the continental shelf, their daily vertical migration, and the semidiurnal tidal currents. From 6 series of 48 h sampling runs along a 18 km long transect located through the mouth of the bay, we estimated a set of acoustic parameters to identify euphausiid swarms and distinguish them from aggregations of the abundant local micronektonic galatheid Pleuroncodes planipes. The euphausiid swarms were vertically segregated from the dense aggregations of P. planipes. Euphausiids that entered the bay were completely dispersed within strong spring tidal currents (>125 cm s -1 ) and subsequently formed dense aggregations inside the bay, where current speed decreased substantially. Euphausiid distribution inside the bay never extended farther than the continental-shelf water mass delineated by the tidal front. In the bay, adult N. simplex remained close to the sea-bottom day and night. We hypothesized that the large euphausiid and red crab biomass transported from the continental shelf into Bahía Magdalena may contribute significantly to the trophodynamics of this eutrophic subtropical bay.

ABSTRACT. A retrospeciive anaiysis oi zooplankton volumes (1951 to 1996) was performed for the area between Punta Baja (30°N) and Punta Abreojos (26 ?ON) in relatlon to the warming anomaly that has taken place in the California Current System during the last 2 decades. The seasonal cycle of median standing stock of zooplankton In this area showed a moderate alternation between h~g h values from June to October (median monthly volumes between 86 and 108 m1/1000 m") and low values from Kovember to May (58 to 77 m1/1000 m3). The quarterly long-term means of zooplankton volumes were the lowest in winter, as were wind speeds. The standard deviations associated with the long-term means indicated interannual variability was higher than seasonal variability. The time series showed an interval of high zooplankton volume between 1952 and 1957. Following the strong ENSO (El Nifio Southern Oscillation) of 1957-1958, a period of low values occurred which extended into the early 1960s. There was a slow recovery of zooplankton biomass through the rest of the 1960s, but it did not reach the earlier high values. Available data suggest the increasing trend reached a peak in 1975. Subsequently, from 1976 to the ENSO of 1982ENSO of -1983, the biomass decreased. For the remainder of the 1980s, the few existing data showed an erratic behavior of the biomass. In the 1990s, there has been a decrease to values even lower than those observed durlng the 1957-1958 ENSO. Nonseasonal anomalles for zoop1.ankton and environmental variables were significantly different (p < 0.001) anlong decades but not between the northern (30" to 28" N ) and southern (28" to 26" N ) areas. The decrease in zooplankton volume in this region over the last 2 decades is less than that reported for the Southern California Bight. This may be partly caused by seasonal northward movements of tropical zooplankton species along the Baja California coast, Mexico.

ABSTRACT. Growth production of the euphausiid Nyctiphanes sirnplex Hansen was examined off the southwest coast of the peninsula of Baja California, Mexico (Bahia Magdalena, 24 to 25" N ) , as a function of seasonal changes of temperature, zooplankton biomass, upwelling index, and regional ocean circulation. The data were collected during 4 oceanographic surveys from June to November 1986. High densities of N. sirnplex were found in the shallow coastal waters (c300 m). High production is maintained during late spring and summer, decreasing during autumn when stratification of the water column develops. We estimate the production attributable to growth of N. sirnplex on the coastal shelf was 273.42 mg m-? yr.' and showed a strong seasonal variation. This value was about '4 the value previously reported at