Fish larvae
Sampling

Systematic sampling of fish eggs and larvae was initiated by the German planktologist Victor Hensen (1895) who introduced the Hensen plankton net. Quantitative sampling of zooplankton has always posed such problems as: clogging of mesh, avoidance by large zooplankton, extrusion of small specimens through the mesh aperture, etc. In order to standardize quantitative sampling of zooplankton, UNESCO organized working groups to develop standard gear, and published a manual on Zooplankton Sampling (1968). One of the suggestions of these groups was the introduction of conical-cylindrical nets. Experimental study showed that the cylindrical part of the net substantially reduced the problem of clogging (Tranter and Smith, 1968).

McGowan and Brown (1966) developed the slow-speed Bongo net, which is towed obliquely from the surface to near bottom and back to the surface. It minimizes bias resulting from uneven filtration per unit depth, the bridle-free mouth reduces net dodging by the organisms, and two parallel nets permit calibration of the extrusion of small organisms. This net has since become the international standard sampling gear for ichthyoplankton studies (Smith and Richardson, 1977). Sampling is normally made from the surface to 200 m depth, or to near the bottom, by double oblique haul.

The double oblique haul of the Bongo net integrates organisms of the entire water column; however, in order to assess the vertical distribution of plankton, we need to collect organisms from different strata without contamination from other layers. To attain this objective researchers have deviced different kinds of closing net samplers. Among several types of such nets are: the multiple opening-closing nets system (MOCNESS) (Wiebe et al., 1976; 1985), and the rectangular midwater trawl (RMT; Roe and Shale, 1979). Nets are opened and closed from shipboard during the tow. Hydrographic data, flow-meter readings, and the angle of net mouth are transmitted to an on-board microcomputer via conducting cables. A modified version of the MOCNESS was used to investigate the vertical distribution of ichthyoplankton in the southwest Atlantic by the R/V Meteor (Alheit, et al., 1991; Matsuura et al., 1992; Matsuura and Kitahara, 1995), and an RMT was used in the Benguela region by Olivar (1990).