Mauricio Braun

Chlorophyll-a distribution and associated oceanographic conditions in the upwelling region off northern Chile during the winter and spring 1993

The mesoscale distributions of chlorophyll-a (Chl-a) and associated physical and chemical variables off the northern coast of Chile are described using quasi-synoptic data obtained during the austral winter (August–September) and spring (November–December) of 1993. Surface temperature, salinity and chlorophyll-a data were collected in the region bounded by 18–24°S and 70–72°W. An area of persistent coastal upwelling (19–22°S, out to 74 km from the coast) was selected to examine the vertical distribution (0–100 m) of these variables together with dissolved oxygen (O2) and nutrient concentrations (phosphate, nitrate and nitrite). Surface temperature indicated that the occurrence of coastal upwelling was not well defined during the winter compared to the spring, but the vertical distribution of temperature, density, dissolved Oz and nutrients within the selected area confirmed that there was activity during both seasons. Low dissolved O2 ( 2 μM within the upper 100 m, together with the vertical salinity distribution, suggested that the main subsurface component of the upwelling water within the selected area was water of equatorial subsurface origin. Surface Chl-a distribution in the region was mostly heterogeneous during the winter, with higher concentrations ( > 1 mg m−3) both inshore and offshore, while during the spring these values were confined to within 37 km of the coast, in association with the upwelling band. Surface concentrations, however, were not significantly different between the two seasons over the whole of the region, and maximum values (> 10 mg m−3) were found in the same zone, inshore off Iquique (20°S), on both occasions. Within the selected upwelling area, most of the Chl-a was present in the upper 25 m layer during both seasons, and surface values were correlated with Chl-a integrated over this layer. Water column Chl-a concentrations in this area were significantly higher during the spring, and, during both periods, the highest Chl-a concentrations ( > 2 mg m−3) were located inshore (<37 km from the coast), with maximum values off Iquique. The predominance of net-phytoplankton was mostly associated with the highest total Chl-a concentration, and it increased towards the coast except inshore off Iquique and offshore off Arica-Iquique. These results are discussed in terms of their implications for the overall production of the pelagic system and the important fisheries that it sustains.

Anchovy larval distribution in the coastal zone off northern Chile: the effect of low dissolved oxygen concentrations and of a cold-warm sequence (1990-95)

Este estudio analiza los patrones de distribucion y abundancia de larvas de anchoveta (Engraulis ringens) en la zona norte de Chile (18°-24°S), dentro de la zona costera (0-100 mn), y su relacion con las condiciones oceanograficas en la capa superior (0-200 m de profundidad). Los datos utilizados se derivaron de cruceros realizados entre 1990 y 1995, durante cada invierno (julio-septiembre), la epoca de desove principal y de mayor abundancia de larvas. Durante el crucero de invierno de 1995 se realizo, ademas, un muestreo adicional con arrastres verticales a profundidades menores (<100 m) para analizar el efecto de la concentracion de oxigeno sobre la distribucion vertical de las larvas de anchoveta. Las distribuciones superficiales de temperatura y salinidad indicaron que este estudio cubrio una secuencia de condiciones normales frias (1990) a calidas tipo El Nino (1992), y luego un retorno a condiciones frias (1995). Durante el periodo calido, la base de la termoclina (isoterma de 15°C) se profundizo notoriamente a lo largo de toda la costa. Durante las condiciones frias, el limite superior de la capa de minimo oxigeno (ca. 1.0 ml 1-1) se localizo a baja profundidad (<50m) en la parte norte del area de estudio (Arica-Iquique) y las aguas sobre la termoclina estuvieron significativamente mas oxigenadas, aunque el cambio con la profundidad fue mayor, que en los periodos calidos. La abundancia promedio integrada de larvas de anchoveta fue significativamente diferente durante el periodo invierno 1990 - invierno 1995 (rango: 230-5300 larvas en 10 m2), con los mayores valores de abundancia observados durante 1993 y 1994. Las mayores abundancias se presentaron en las estaciones mas costeras (1-5 mn) durante todos los cruceros, aunque valores altos tambien ocurrieron en las estaciones oceanicas, especialmente durante el periodo 1993-95. La comparacion de las densidades promedios (larvas m-3), entre las diferentes profundidades muestreadas (0-25, 0-50 y/o 0-100 m), sugiere la ocurrencia de una barrera a las distribucion vertical de las larvas de anchoveta, localizada en las proximidades de concentraciones de oxigeno de 0,75 ml 1-1. Las condiciones calidas iniciadas durante 1992 parecen haber proporcionado un ambiente mas favorable para la sobrevivencia y alimentacion de las larvas de anchoveta en el area, sin embargo, los cambios en la distribucion de bajas concentraciones de oxigeno durante el periodo de estudio no parecen haber tenido una influencia directa sobre las variaciones en la abundancia y distribucion de estas larvas.

Chlorophyll- a distribution and mesoscale physical processes in upwelling and adjacent oceanic zones off northern Chile (summer–autumn 1994)

(HumboldtCurrent System), during the austral summer (February-March) and autumn (May) of 1994, were studied in the region bounded by 18-24`S and 70-72°W (out to 200 km from the coast ; 0-100 in depth) ; within this region, nutrients were measured in an area of persistent coastal upwelling (^-19-22`S, out to 80 km from the coast) . Temperature and salinity distributions, as well as nutrient concentrations, indicated the occurrence of active upwelling during both cruises . Also, and together with maps of geopotential anomaly (0/200 dbar) and depth of the thermocline (15°C isotherm), their distribution suggested the presence of a mainly equatorward flow, anticyclonic eddy-like structures, and intrusions of warm (>19 -C).high salinity (> 35 .0 psu), subtropical water towards the coast . A tongue of cooler and lower salinity water, and of lower flow fields, extended from the coast towards the offshore zone during both sampling periods, in association with higher chl-a concentrations (> 1 mg m -3, > 20 mg m -2 between 0 and 25 in depth) and predominance of netphytoplankton (> 20µm) . The comparison of these results with those for the winter and spring of 1993 in the same area suggest a relatively weak seasonal signal in chl-a concentration during the 1993-1994 period, with higher water column concentrations during the summer and spring periods in the selected upwelling area, though surface chl-a concentrations for the whole of the area did not vary significantly .

Descriptions of larvae of Merluccius australis, Macruronus magellanicus, and observations on a larva of Micromesistius australis from southern Chile (Pisces: Gadiformes)

Abstract Larvae of two commercially important fish species, namely southern hake (Merluccius australis) and hoki (Macruronus magellanicus), are described for the first time from material collected in southern Chile. Observations on a third species, blue whiting (Micromesistius australis), are also provided. The three species demonstrate larval characteristics typical of gadiform fishes, including a short gut, and pigmentation distributed in blotches and bands. Characteristics of the larvae of M. australis from Chile are similar to those observed in sketches from New Zealand, such that the descriptions may be used to identify the larvae from other regions. The similarity of characteristics between M. magellanicus larvae from Chile and Argentina and larvae of M. novaezelandiae from Tasmanian and New Zealand waters demonstrate the taxonomic affinity between these species. The presence of three dorsal fins in a late postflexion Micromesistius australis larva allowed it to be easily recognised.