NEW Summary of the SIMSUP program | RV El Puma | Hydroacoustic

Between 1993 and 1998 we worked with Dr. Carlos Robinson and Dr. Virgilio Arenas along with their staff from Instituto de Ciencias del Mar y Limnología de la Universidad Nacional Autonoma de Mexico sampling along the west coast of Baja California, Mexico. We have been recording the sound scattering layer (SSL) using an echosounder SIMRAD EY-200 (single beam) to detect fish schools and zooplankton patchiness. I'm particularly interested on the euphausiids, but we also studied other componets of the SSL like the Pacific sardine Sardinops sagax, the anchovy Engraulis mordax, and the swarms of the Galatheidae crustacean called "langostilla" Pleuroncodes planipes".

All the oceanographic surveys have been carried out aboard the RV El Puma (see below).We study the circadian and seasonal variability of the mesozooplanktonic biomass, and the sound scattering layer in three coastal zones along the west coast of Baja California. Currenly we have been carried out 13 oceanographic surveys (Oct 1994, Mar, Jul, and Oct 1995, Mar, Jun, and Dec 1996, Aug, Dec 1997 and March, Jul and Dec 1998). Three zones were studied: Ensenada-Punta Baja (31-30°N, 116-117°W), Punta Eugenia-Punta Abreojos (27-28°N, 114-116°W), and Bahía Magdalena (24-25°N, 112-113°W) Baja California, México. Zooplankton samples are taken using a Bongo net and opening-closing nets. Perpendicular transects were separated by 18 km, and with three oceanographic stations over the shelf, slope, and open ocean. Each transect was monitored for 24 hours obtaining about 6 replicates per station.

This sampling program provided us information about the behavior of the deep scattering layer and its components. For example, during July 1995 we recorded several local zooplankton patches that displayed a complete vertical migration (24 h) over the continental shelf. These aggregations have a close interaction with the ocean bottom during daytime and display a vertical migration rising near to the surface at night where a progressive horizontal dispersion then occurs. The DSL measured about 3.5 to 6.0 km horizontally during the night. The ascent and descent migration of the deep scattering layers were significantly fitted to a polynomial function of the second order having diferent speed upward and downward. This last suggests the euphausiid swarms have different swimming behavior during these two processes. Vertical migrations of the DSL were between 60 and 90 m. During the descent migration, maximum instantaneous speeds reached about 0.7 cm s-1, and during the ascent, maximum instantaneous speeds were about 0.3 cm s-1. These coastal euphausiid species, along with other macrozooplankton and nektonic organisms, have a daily close interaction with the ocean bottom and also with the pelagic environment, suggesting they play an important role providing food for demersal and pelagic organisms on the slope and shelf break in the upwelling region off the northwest coast of Baja California. Using an Isaacs-Kidd midwater net and Bongo nets, we inferred the DSL was composed mainly of juveniles and adults of the euphausiids Nyctiphanes simplex Hansen and Euphausia pacifica Hansen.

Figure 1. Series of echograms showing the deep scattering layer in a diel cycle in transect one during 2 to 3 July 1995. The continuous white line is the bottom. Colors represent echo intensity. Yellow and red represent 1480 millivolts, green and yellow 920, blue and green 560. This last interval represents approximately the target strength used to quantify abundance and to describe the movements of the DSL (-53 to -57 dB). White color means areas with closely packed organisms. (Figure from Robinson and Gomez, 1998, J. Plankt. Res 20:2009-2016).

Dense swarms of the euphausiid Nyctiphanes simplex are commonly recorded along the west coast of Baja California and Gulf of California. This euphausiid species play an important role in the throphodynamics of this region.