Because of their large size and well developed eyes most adult euphausiids are able to detect and avoid nets, especially during the day (e.g. Pillar, 1984b). If nets are to be used to estimate euphausiid abundance or biomass, they should be of large mouth area and wide mesh diameter (0.5 mm or above), and be towed obliquely at night. Diurnal estimates of abundance will be further biased by the fact that many species of euphausiid display pronounced DVM and occur just above, or on, the sea bed during the day. To avoid underestimating abundance, therefore, nets should be towed from as close to the bottom as possible. Multiple opening-closing nets (e.g. RMT, MOCNESS) should be used where possible because these not only yield information on biomass but also on vertical distribution. The younger developmental stages of euphausiids do not have the same ability to escape nets as adults (although this will vary with stage) and can be confidently caught throughout the day. Smaller stages can be collected using standard vertical mesozooplankton hauls with nets of smaller mouth area and narrower mesh diameter.
Biomass estimates calculated from net tows are typically lower than those computed from acoustic methods, often by an order of magnitude (Pakhomov et al., unpublished data). High frequency echo-sounders (Everson et al., 1990; Everson and Pasternak, 1993) and Acoustic Doppler Current Profilers (e.g. Zhou et al., 1994;) have been used successfully to determine euphausiid abundances in a variety of pelagic environments. Not only do they yield real-time information on biomass but their fine resolution allows the calculation of packing densities (inter-individual distances) and so imparts information on aggregation structure (Barange et al., 1993; Miller et al., 1993). If used around the clock, acoustic methods obviously also allow observations on DVM. The one drawback with remote sensing methods such as these is that, although they can provide information on the size structure of aggregations, they do not allow the specific identification of targets, so supplementary net hauls are necessary.
Although a discourse on process orientated methodology falls outside the main scope of this chapter, a few pointers and cautionary words are felt important because of the growing emphasis on this type of work and the fact that continual advances are being made.